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  • 1.
    Alekhin, A.
    et al.
    CNRS, Inst Mol & Mat Le Mans, UMR 6283, F-72085 Le Mans, France.
    Razdolski, I.
    CNRS, Inst Mol & Mat Le Mans, UMR 6283, F-72085 Le Mans, France;Max Planck Gesell, Fritz Haber Inst, D-14195 Berlin, Germany.
    Berritta, Marco
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Buerstel, D.
    Univ Duisburg Essen, Fac Chem, Univ Str 5, D-45141 Essen, Germany.
    Temnov, V.
    CNRS, Inst Mol & Mat Le Mans, UMR 6283, F-72085 Le Mans, France.
    Diesing, D.
    Univ Duisburg Essen, Fac Chem, Univ Str 5, D-45141 Essen, Germany.
    Bovensiepen, U.
    Univ Duisburg Essen, Fac Phys, Lotharstr 1, D-47057 Duisburg, Germany;Univ Duisburg Essen, Ctr Nanointegrat CENIDE, Lotharstr 1, D-47057 Duisburg, Germany.
    Woltersdorf, G.
    Martin Luther Univ Halle Wittenberg, Inst Phys, Von Danckelmann Pl 3, D-06120 Halle, Germany.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Melnikov, A.
    Max Planck Gesell, Fritz Haber Inst, D-14195 Berlin, Germany;Martin Luther Univ Halle Wittenberg, Inst Phys, Von Danckelmann Pl 3, D-06120 Halle, Germany.
    Magneto-optical properties of Au upon the injection of hot spin-polarized electrons across Fe/Au(001) interfaces2019Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, nr 12, artikel-id 124002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate a novel method for the excitation of sizable magneto-optical effects in Au by means of the laser-induced injection of hot spin-polarized electrons in Au/Fe/MgO(001) heterostructures. It is based on the energy- and spin-dependent electron transmittance of Fe/Au interface which acts as a spin filter for non-thermalized electrons optically excited in Fe. We show that after crossing the interface, majority electrons propagate through the Au layer with the velocity on the order of 1 nm fs(-1) (close to the Fermi velocity) and the decay length on the order of 100nm. Featuring ultrafast functionality and requiring no strong external magnetic fields, spin injection results in a distinct magneto-optical response of Au. We develop a formalism based on the phase of the transient complex MOKE response and demonstrate its robustness in a plethora of experimental and theoretical MOKE studies on Au, including our ab initio calculations. Our work introduces a flexible tool to manipulate magneto-optical properties of metals on the femtosecond timescale that holds high potential for active magneto-photonics, plasmonics, and spintronics.

  • 2.
    Ali, Ehesan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Datta, Sambhu N.
    Influence of Solute-Solvent Hydrogen Bonding on Intramolecular Magnetic Exchange Interaction in Aminoxyl Diradicals: A QM/MM Broken-Symmetry DFT Study2009Ingår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 113, nr 16, s. 5545-5548Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have investigated the effect of nitroxide radical-water hydrogen bonding (NO center dot center dot center dot center dot H2O) on the intramolecular magnetic exchange interaction (J) for biologically relevant aminoxyl diradicals. We adopt a combination of broken-symmetry density functional theory and the quantum mechanics/molecular mechanics (QM/MM) approach. We find that the presence of hydrogen bonding reorients the radical spin density on -NO center dot. This phenomenon reduces the effective distance between the two interacting localized spin centers that eventually increases the intramolecular magnetic exchange interaction. We have also investigated the functional variation of the magnetic exchange interaction, using various GGA (BLYP, PBE, HCTH407), meta-GGA (TPSS, VXSC), and hybrid (O3LYP, B3LYP, B3P86, B3PW91, and PBE0) functionals.

  • 3.
    Ali, Ehesan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Sanyal, Biplab
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Tuning the Magnetic Interaction between Manganese Porphyrins and Ferromagnetic Co Substrate through Dedicated Control of the Adsorption2009Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, nr 32, s. 14381-14383Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report, on the basis of density-functional theory+U (DFT+U) calculations that metalloporphyrins can adsorb on ferromagnetic metal surfaces in two distinct configurations. Two separate adsorption minima are obtained for manganese porphyrin (MnP) on Co from our DFT+U total energy calculations, which correspond to strong and weak adsorption strengths, respectively. By steering the nature of adsorption, we find that distinct chemical interactions as well as magnetic exchange interactions between the metalloporphyrin and the metal surface can be realized. We furthermore show that a switching of the MnP molecule's spin state can occur even for the weakly adsorbed case. This new discovery opens up prospects for engineering the chemical and magnetic exchange interaction in new functionalized spintronic materials.

  • 4. Ali, Md Ehesan
    et al.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Influence of Noncovalent Cation/Anion-pi Interactions on the Magnetic Exchange Phenomenon2011Ingår i: The Journal of Physical Chemistry Letters, ISSN 1948-7185, Vol. 2, nr 9, s. 939-943Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The role of noncovalent ion-pi interactions in controlling the intramolecular magnetic exchange interaction in 1,3-phenylene-based bis(aminoxyl) diradical has been investigated computationally through deploying an external ion in the vicinity of the pi-cloud of the phenylene coupler. Using spin-polarized hybrid density functional theory-based calculations, we observe that the anions drastically enhance the magnetic exchange interaction for distances below the equilibrium distance. The phenomenon could be understood by two simultaneously occurring effects, which influence the intramolecular magnetic exchange interaction. The first one is the enhancement of the paratropic current density on the aryl couplers due to a small amount of charge transfer. The other one is the attainment of magnetization density on the anionic species due to such charge transfer, favorably altering the magnetic exchange pathway. The achieved understanding provides prospects of a completely new strategy of enhancing the intramolecular ferromagnetic coupling through the assistance of external ionic species inserted in molecular crystals.

  • 5.
    Ali, Md. Ehesan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Unraveling the Electronic Structure, Spin States, Optical and Vibrational Spectra of Malaria Pigment2015Ingår i: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 21, nr 23, s. 8544-8553Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A detailed knowledge of the electronic structure and magnetic and optical properties of hemozoin, the malaria pigment, is essential for the design of effective antimalarial drugs and malarial diagnosis. By employing state-of-the-art electronic structure calculations, we have performed an in-depth investigation of the malaria pigment. Specifically, molecular bond lengths and spin states of the two Fe-III heme centers and their exchange interaction, the UV/Vis absorption spectrum, and the IR vibrational spectra were calculated and compared with available experimental data. Our density functional theory (DFT)-based calculations predict a singlet ground spin state that stems from an S=5/2 spin state on each of the Fe heme centers with a very weak antiferromagnetic exchange interaction between them. Our theoretical UV/Vis and IR spectra provide explanations for various spectroscopic studies of hemozoin and -hematin (a synthetic analogue of hemozoin). A good comparison of calculated and measured properties demonstrates the convincing unveiling of the electronic structure of the malaria pigment. Based on the predicted vibrational spectra, we propose a unique spectral band from the nuclear resonance vibrational spectroscopy (NRVS) results that could be used as a key fingerprint for malarial detection.

  • 6. Ali, Md. Ehesan
    et al.
    Sanyal, Biplab
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Electronic Structure, Spin-States, and Spin-Crossover Reaction of Heme-Related Fe-Porphyrins: A Theoretical Perspective2012Ingår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 116, nr 20, s. 5849-5859Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

  • 7.
    Ali, Md Ehesan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Staemmler, Volker
    Illas, Francesc
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Designing the Redox-Driven Switching of Ferro- to Antiferromagnetic Couplings in Organic Diradicals2013Ingår i: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 9, nr 12, s. 5216-5220Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Switching of the magnetic exchange coupling from ferro- to antiferromagnetic or vice versa in a single molecule is an appealing but rarely occurring phenomenon in molecular magnetism. Here, we report this for an unprecedented pure organic system, computationally designed by tailoring a conformationally restricted, known nitroxide-diradical (Rajca et al. J. Am. Chem. Soc. 2007, 129, 10159). This ferro- to antiferromagnetic coupling switching of an "m-phenylene" based diradical is governed by a stereoelectronic effect and controlled by a redox-driven chemical reaction.

  • 8.
    Amft, Martin
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Burkert, T
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Sanyal, Biplab
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    First-principles calculations of optical and magneto-optical properties of Ga1-xMnxAs and MnAs2009Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 404, nr 20, s. 3782-3788Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a first-principles density-functional investigation of the optical and magneto-optical properties of MnGa1-xAs (with x = 0,0.0625, and 1) systems. Our calculated dielectric function, magneto-optical Kerr effect, and magnetic circular dichroism spectra agree reasonably well with existing experimental results. A comparison of the optical and magneto-optical spectra of MnAs in the naturally occurring hexagonal phase and in the cubic zinc-blende structure is made. The differences in the spectral properties of these two phases could aid detection of MnAs in the zinc-blende structure. (C) 2009 Elsevier B.V. All rights reserved.

  • 9.
    Amft, Martin
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Fysiska institutionen.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Fysiska institutionen.
    Calculated magneto-optical Kerr spectra of the half-Heusler compounds AuMnX (X = In, Sn, Sb)2007Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, nr 31, s. 315216-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The ferromagnetic ground states of the half-Heusler compounds AuMnX (X ≤ In, Sn, Sb) have been calculated in the framework of the local spin-density approximation (LSDA) to density functional theory (DFT). AuMnSn is computed to be a half-metallic ferromagnet, whereas AuMnIn and AuMnSb are not half-metallic, due to their different band filling. The computed relativistic electronic structures served as inputs to calculate the magneto-optical Kerr rotations and ellipticities for all three materials. In the case of AuMnSn the largest, zero-temperature, polar Kerr rotation has been found to be -0.45° at about 1eV photon energy. The computed MOKE spectra of AuMnSn are in qualitative agreement with recent experiments. The largest Kerr rotations of AuMnIn and AuMnSb have been calculated to be +0.64° at 4.3eV and -0.85° at 0.9eV, respectively.

  • 10.
    Aperis, Alex
    et al.
    Department of Physics, National Technical University of Athens, Athens, Greece.
    Kotetes, P.
    Varelogiannis, G.
    Oppeneer, Peter
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Small-q phonon-mediated unconventional superconductivity in the iron pnictides2011Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, nr 9, s. 092505-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report self-consistent calculations of the gap symmetry for iron-based high-temperature superconductors using realistic small-q phonon-mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless s± state that was first encountered in a spin-fluctuation mechanism picture. Nodal s± as well as other gap structures such as dx2−y2, s±+dx2−y2, and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase-sensitive experiments reporting a gap changing sign, attributed previously only to a nonphononic mechanism, and isotope effect measurements proving the involvement of phonons in the pairing.

  • 11.
    Aperis, Alex
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Maldonado, Pablo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB22015Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 92, nr 5, artikel-id 054516Artikel i tidskrift (Refereegranskat)
  • 12.
    Aperis, Alex
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high-Tc superconductivity in FeSe/SrTiO32018Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 97, artikel-id 060501(R)Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We examine the impact of interfacial phonons on the superconducting state of FeSe/SrTiO3 developing a material's specific multiband, full bandwidth, and anisotropic Eliashberg theory for this system. Our self-consistent calculations highlight the importance of the interfacial electron-phonon interaction, which is hidden behind the seemingly weak-coupling constant λm=0.4, in mediating the high Tc, and explain other puzzling experimental observations, such as the s-wave symmetry and replica bands. We discover that the formation of replica bands has a Tc decreasing effect that is nevertheless compensated by deep Fermi-sea Cooper pairing which has a Tc enhancing effect. We predict a strong-coupling dip-hump signature in the tunneling spectra due to the interfacial coupling.

  • 13.
    Arruda, Lucas M.
    et al.
    Free Univ Berlin, Inst Expt Phys, Arnimallee 14, D-14195 Berlin, Germany;Minist Educ Brazil, CAPES Fdn, BR-70040020 Brasilia, DF, Brazil.
    Ali, Md. Ehesan
    Inst Nano Sci & Technol, Phase 10,Sect 64, Mohali 160062, Punjab, India.
    Bernien, Matthias
    Free Univ Berlin, Inst Expt Phys, Arnimallee 14, D-14195 Berlin, Germany.
    Nickel, Fabian
    Free Univ Berlin, Inst Expt Phys, Arnimallee 14, D-14195 Berlin, Germany.
    Kopprasch, Jens
    Free Univ Berlin, Inst Expt Phys, Arnimallee 14, D-14195 Berlin, Germany.
    Czekelius, Constantin
    Heinrich Heine Univ Dusseldorf, Inst Organ Chem & Makromol Chem, Univ Str 1, D-40225 Dusseldorf, Germany.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Kuch, Wolfgang
    Free Univ Berlin, Inst Expt Phys, Arnimallee 14, D-14195 Berlin, Germany.
    Modifying the Magnetic Anisotropy of an Iron Porphyrin Molecule by an on-Surface Ring-Closure Reaction2019Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, nr 23, s. 14547-14555Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The magnetic properties of adsorbed metalloporphyrin molecules can be altered or tuned by the substrate, additional axial ligands, or changes to the molecules' macrocycle. These modifications influence the electronic configuration of the fourfold-coordinated central metal ion that is responsible for the metalloporphyrins' magnetic properties. We report a substantial increase in the effective spin moment obtained from sum-rule analysis of X-ray magnetic circular dichroism for an iron metalloporphyrin molecule on Au(111) through its conversion from iron(II)-octaethylporphyrin to iron(II)-tetrabenzoporphyrin in a surface-assisted ring-closure ligand reaction. Density functional theory calculations with additional strong Coulomb correlation (DFT+U) show that the on-surface reaction alters the conformation of the molecule, increasing its planarity and the ion-surface distance. A spin-Hamiltonian fit of the magnetization as a function of field reveals a substantial increase in the intra-atomic magnetic dipole term (T-z) and a decrease in the magnitude of the easy-plane anisotropy upon ring closure. This consequence of the ring closure demonstrates how new magnetic properties can be obtained from on-surface reactions, resulting here in significant modifications to the magnetic anisotropy of the Fe ion, and sheds light onto the molecule-substrate interaction in these systems.

  • 14.
    Balaz, Pavel
    et al.
    Charles Univ Prague, Fac Math & Phys, Dept Condensed Matter Phys, Ke Karlovu 5, CZ-12116 Prague, Czech Republic..
    Zonda, Martin
    Charles Univ Prague, Fac Math & Phys, Dept Condensed Matter Phys, Ke Karlovu 5, CZ-12116 Prague, Czech Republic..
    Carva, Karel
    Charles Univ Prague, Fac Math & Phys, Dept Condensed Matter Phys, Ke Karlovu 5, CZ-12116 Prague, Czech Republic..
    Maldonado, Pablo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Transport theory for femtosecond laser-induced spin-transfer torques2018Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, nr 11, artikel-id 115801Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser pulse is accompanied by a nonthermal spin-polarized current of hot electrons. These spin currents are studied here theoretically in a spin valve with noncollinear magnetizations. To this end, we introduce an extended model of superdiffusive spin transport that enables the treatment of noncollinear magnetic configurations, and apply it to the perpendicular spin valve geometry. We show how spin-transfer torques arise due to this mechanism and calculate their action on the magnetization present, as well as how the latter depends on the thicknesses of the layers and other transport parameters. We demonstrate that there exists a certain optimum thickness of the out-of-plane magnetized spin-current polarizer such that the torque acting on the second magnetic layer is maximal. Moreover, we study the magnetization dynamics excited by the superdiffusive spin-transfer torque due to the flow of hot electrons employing the Landau-Lifshitz-Gilbert equation. Thereby we show that a femtosecond laser pulse applied to one magnetic layer can excite small-angle precessions of the magnetization in the second magnetic layer. We compare our calculations with recent experimental results.

  • 15. Ballav, Nirmalya
    et al.
    Waeckerlin, Christian
    Siewert, Dorota
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Jung, Thomas A.
    Emergence of On-Surface Magnetochemistry2013Ingår i: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 4, nr 14, s. 2303-2311Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The control of exchange coupling across the molecule-substrate interface is a key feature in molecular spintronics. This Perspective reviews the emerging field of on-surface magnetochemistry, where coordination chemistry is applied to surface-supported metal porphyrins and metal phthalocyanines to control their magnetic properties. The particularities of the surface as a multiatomic ligand or "surface ligand'' are introduced. The asymmetry involved in the action of a chemical ligand and a surface ligand on the same planar complexes modifies the well-established ''trans effect'' to the notion of the ''surface-trans effect". As ad-complexes on ferromagnetic substrates are usually exchange-coupled, the magnetochemical implications of the surface-trans effect are of particular interest. The combined action of the different ligands allows for the reproducible control of spin states in on-surface supramolecular architectures and opens up new ways toward building and operating spin systems at interfaces. Notably, spin-switching has been demonstrated to be controlled collectively via the interaction with a ligand (chemical selectivity) and individually via local addressing at the interface.

  • 16.
    Battiato, Marco
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Barbalinardo, G.
    Carva, Karel
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Beyond linear response theory for intensive light-matter interactions: Order formalism and ultrafast transient dynamics2012Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, nr 4, s. 045117-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recently constructed radiation sources deliver brilliant, ultrashort coherent radiation fields with which the material's response can be investigated on the femtosecond to attosecond time scale. Here, we develop a theoretical framework for the interaction of the material's electrons with such intensive, short radiation pulses. Our theory is based on the time evolution of the electron density matrix, as defined through the Liouville-von Neumann equation. The latter equation is solved here within the framework of the response theory, incorporating the perturbing field in higher orders. An analytical tool, called the order notation, is developed, which permits the explicit calculation of the arising nth-order operatorial convolutions. As examples of the formalism, explicit expressions for several optical phenomena are worked out. Through the developed theory presented here, two fundamental results are achieved: first, the perturbing field to higher than linear orders is included in an elegant and compact way, allowing to treat highly brilliant light, and, second, the complete transient time response on the subfemtosecond scale is analytically provided, thus dropping the adiabatic approximation commonly made in standard linear response theory.

  • 17.
    Battiato, Marco
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Barbalinardo, G.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Quantum theory of the inverse Faraday effect2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, nr 1, s. 014413-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We provide a quantum theoretical description of the magnetic polarization induced by intense circularly polarized light in a material. Such effect-commonly referred to as the inverse Faraday effect-is treated using beyond-linear response theory, considering the applied electromagnetic field as external perturbation. An analytical time-dependent solution of the Liouville-von Neumann equation to second order is obtained for the density matrix and used to derive expressions for the optomagnetic polarization. Two distinct cases are treated, the long-time adiabatic limit of polarization imparted by continuous wave irradiation, and the full temporal shape of the transient magnetic polarization induced by a short laser pulse. We further derive expressions for the Verdet constants for the inverse, optomagnetic Faraday effect and for the conventional, magneto-optical Faraday effect and show that they are in general different. Additionally, we derive expressions for the Faraday and inverse Faraday effects within the Drude-Lorentz theory and demonstrate that their equality does not hold in general, but only for dissipationless media. As an example, we perform initial quantum mechanical calculations of the two Verdet constants for a hydrogenlike atom and we extract the trends. We observe that one reason for a large inverse Faraday effect in heavy atoms is the spatial extension of the wave functions rather than the spin-orbit interaction, which nonetheless contributes positively.

  • 18.
    Battiato, Marco
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Carva, Karel
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization2010Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, nr 2, s. 027203-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose a semiclassical model for femtosecond laser-induced demagnetization due to spin-polarized excited electron diffusion in the superdiffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.

  • 19.
    Battiato, Marco
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Carva, Karel
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures2012Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, nr 2, s. 024404-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Femtosecond laser excitation of a ferromagnetic material creates energetic spin-polarized electrons that have anomalous transport characteristics. We develop a semiclassical theory that is specifically dedicated to capture the transport of laser-excited nonequilibrium (NEQ) electrons. The randomly occurring multiple electronic collisions, which give rise to electron thermalization, are treated exactly and we include the generation of electron cascades due to inelastic electron-electron scatterings. The developed theory can, moreover, treat the presence of several different layers in the laser-irradiated material. The derived spin-dependent transport equation is solved numerically and it is shown that the hot NEQ electron spin transport occurs neither in the diffusive nor ballistic regime, it is superdiffusive. As the excited spin majority and minority electrons in typical transition-metal ferromagnets (e.g., Fe, Ni) have distinct, energy-dependent lifetimes, fast spin dynamics in the femtosecond (fs) regime is generated, causing effectively a spin current. As examples, we solve the resulting spin dynamics numerically for typical heterostructures, specifically, a ferromagnetic/nonmagnetic metallic layered junction (i.e., Fe/Al and Ni/Al) and a ferromagnetic/nonmagnetic insulator junction (Fe or Ni layer on a large band-gap insulator as, e.g., MgO). For the ferromagnetic/nonmagnetic metallic junction where the ferromagnetic layer is laser-excited, the computed spin dynamics shows that injection of a superdiffusive spin current in the nonmagnetic layer (Al) is achieved. The injected spin current consists of screened NEQ, mobile majority-spin electrons and is nearly 90% spin-polarized for Ni and about 65% for Fe. Concomitantly, a fast demagnetization of the ferromagnetic polarization in the femtosecond regime is driven. The analogy of the generated spin current to a superdiffusive spin Seebeck effect is surveyed.

  • 20.
    Battiato, Marco
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Maldonado, Pablo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Treating the effect of interface reflections on superdiffusive spin transport in multilayer samples (invited)2014Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, nr 17, s. 172611-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Femtosecond laser-induced magnetization dynamics has recently been related to superdiffusive spin transport. With the aim to accurately compute spin superdiffusion in the complex geometries of layered heterostructures and free standing layers, we develop here a dedicated numerical scheme. We introduce a discretization technique to solve the superdiffusive equation numerically on a time and space grid. The discretization scheme facilitates an explicit treatment of the total reflection at the vacuum-material surfaces as well as of partial reflections at the interfaces between two different materials. The advantages of the numerical technique are discussed. (C) 2014 AIP Publishing LLC.

  • 21.
    Bekaert, J.
    et al.
    Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Aperis, Alex
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Partoens, B.
    Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Milosevic, M. V.
    Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of FeB42018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, nr 1, artikel-id 014503Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an advanced method to study spin fluctuations in superconductors quantitatively and entirely fromfirst principles. This method can be generally applied to materials where electron-phonon coupling and spinfluctuations coexist. We employ it here to examine the recently synthesized superconductor iron tetraboride(FeB4) with experimentalTc∼2.4K[H.Gouet al.,Phys.Rev.Lett.111,157002(2013)]. We prove thatFeB4is particularly prone to ferromagnetic spin fluctuations due to the presence of iron, resulting in a largeStoner interaction strength,I=1.5 eV, as calculated from first principles. The other important factor is itsFermi surface that consists of three separate sheets, among which two are nested ellipsoids. The resultingsusceptibility has a ferromagnetic peak aroundq=0, from which we calculated the repulsive interaction betweenCooper pair electrons using the random phase approximation. Subsequently, we combined the electron-phononinteraction calculated from first principles with the spin fluctuation interaction in fully anisotropic Eliashbergtheory calculations. We show that the resulting superconducting gap spectrum is conventional, yet very stronglydepleted due to coupling to the spin fluctuations. The critical temperature decreases from Tc=41 K, if they arenot taken into account, toTc=1.7 K, in good agreement with the experimental value.

  • 22. Bekaert, J.
    et al.
    Aperis, Alex
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Partoens, B.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Milošević, M. V.
    Evolution of multigap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB22017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, artikel-id 094510Artikel i tidskrift (Refereegranskat)
  • 23.
    Bekaert, J.
    et al.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Bignardi, L.
    Univ Groningen, Zernike Inst Adv Mat, Nijenborgh 4, NL-9747 AG Groningen, Netherlands.; Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.
    Aperis, Alex
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    van Abswoude, P.
    Univ Groningen, Zernike Inst Adv Mat, Nijenborgh 4, NL-9747 AG Groningen, Netherlands.
    Mattevi, C.
    IOM CNR, Lab TASC, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.; Imperial Coll London, Dept Mat, Exhibit Rd, London SW7 2AZ, England.
    Gorovikov, S.
    Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.; Canadian Light Source Inc, 44 Innovat Blvd, Saskatoon, SK S7N 2V3, Canada.
    Petaccia, L.
    Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.
    Goldoni, A.
    Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.
    Partoens, B.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Peeters, F. M.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Milošević, M. V.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Rudolf, P.
    Univ Groningen, Zernike Inst Adv Mat, Nijenborgh 4, NL-9747 AG Groningen, Netherlands.
    Cepek, C.
    IOM CNR, Lab TASC, Str Statale 14 Km 163-5, I-34149 Trieste, Italy .
    Free surfaces recast superconductivity in few-monolayer MgB2: Combined first-principles and ARPES demonstration2017Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, nr 1, artikel-id 14458Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two-dimensional materials are known to harbour properties very different from those of their bulk counterparts. Recent years have seen the rise of atomically thin superconductors, with a caveat that superconductivity is strongly depleted unless enhanced by specific substrates, intercalants or adatoms. Surprisingly, the role in superconductivity of electronic states originating from simple free surfaces of two-dimensional materials has remained elusive to date. Here, based on first-principles calculations, anisotropic Eliashberg theory, and angle-resolved photoemission spectroscopy (ARPES), we show that surface states in few-monolayer MgB2 make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. As a proof of principle, we predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB2. These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.

  • 24. Bernien, M.
    et al.
    Miguel, J.
    Weis, C.
    Ali, Md. E.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Kurde, J.
    Krumme, B.
    Panchmatia, P. M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Sanyal, B.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Piantek, M.
    Srivastava, P.
    Baberschke, K.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Eriksson, Olle
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Kuch, W.
    Wende, H.
    Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates2009Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, nr 4, s. 047202-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional theory+U calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.

  • 25.
    Brena, Barbara
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Puglia, Carla
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Yt- och gränsskiktsvetenskap.
    de Simone, Monica
    Coreno, Marcello
    Tarafder, Kartick
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Feyer, Vitaly
    Banerjee, Rudra
    Gothelid, Emmanuelle
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Yt- och gränsskiktsvetenskap.
    Sanyal, Biplab
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Eriksson, Olle
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Valence-band electronic structure of iron phthalocyanine: An experimental and theoretical photoelectron spectroscopy study2011Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, nr 7, s. 074312-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic structure of iron phthalocyanine (FePc) in the valence region was examined within a joint theoretical-experimental collaboration. Particular emphasis was placed on the determination of the energy position of the Fe 3d levels in proximity of the highest occupied molecular orbital (HOMO). Photoelectron spectroscopy (PES) measurements were performed on FePc in gas phase at several photon energies in the interval between 21 and 150 eV. Significant variations of the relative intensities were observed, indicating a different elemental and atomic orbital composition of the highest lying spectral features. The electronic structure of a single FePc molecule was first computed by quantum chemical calculations by means of density functional theory (DFT). The hybrid Becke 3-parameter, Lee, Yang and Parr (B3LYP) functional and the semilocal 1996 functional of Perdew, Burke and Ernzerhof (PBE) of the generalized gradient approximation (GGA-) type, exchange-correlation functionals were used. The DFT/B3LYP calculations find that the HOMO is a doubly occupied pi-type orbital formed by the carbon 2p electrons, and the HOMO-1 is a mixing of carbon 2p and iron 3d electrons. In contrast, the DFT/PBE calculations find an iron 3d contribution in the HOMO. The experimental photoelectron spectra of the valence band taken at different energies were simulated by means of the Gelius model, taking into account the atomic subshell photoionization cross sections. Moreover, calculations of the electronic structure of FePc using the GGA+U method were performed, where the strong correlations of the Fe 3d electronic states were incorporated through the Hubbard model. Through a comparison with our quantum chemical calculations we find that the best agreement with the experimental results is obtained for a U-eff value of 5 eV.

  • 26.
    Butorin, Sergei M.
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Molekyl- och kondenserade materiens fysik.
    Modin, Anders
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Molekyl- och kondenserade materiens fysik.
    Vegelius, Johan R.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Molekyl- och kondenserade materiens fysik.
    Suzuki, Michi-To
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Molekyl- och kondenserade materiens fysik. RIKEN Ctr Emergent Matter Sci, 2-1 Hirosawa, Wako, Saitama 3510198, Japan..
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Andersson, David A.
    Los Alamos Natl Lab, Div Mat Sci & Technol, Mat Sci Radiat & Dynam Extremes, Los Alamos, NM 87545 USA..
    Shuh, David K.
    Lawrence Berkeley Natl Lab, Div Chem Sci, MS 70A1150,One Cyclotron Rd, Berkeley, CA 94720 USA..
    Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides2016Ingår i: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 88, nr 8, s. 4169-4173Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A systematic X-ray absorption study at actinide N-6,(7) (4f -> 6d transitions) edges was performed for light-actinide oxides including data obtained for the first time for NpO2, PuO2 and UO3. The measurements were supported by ab initio calculations based on local-density approximation. with added 5f-5f Coulomb interaction (LDA+U). Improved energy resolution compared to common experiments at actinide L-2,L-3 (2p -> 6d transitions) edges allowed us to resolve the major structures of the 13 unoccupied 6d density of states (DOS) and estimate the crystal-field 116) splittings in the 6d shell directly from the spectra of light-actinide dioxides. The measurements demonstrated an enhanced sensitivity of the N-6,N-7, spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO2-x, the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2 which is in agreement with the tentative Np-O phase diagram.

  • 27.
    Buzzi, Michele
    et al.
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Makita, Mikako
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Howald, Ludovic
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Kleibert, Armin
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Vodungbo, Boris
    Ecole Polytech, CNRS, UMR 7639, Lab Opt Appl,ENSTA ParisTech, Chemin Huniere, F-91761 Palaiseau, France.;UPMC Univ Paris 06, Sorbonne Univ, CNRS, LCPMR, F-75005 Paris, France..
    Maldonado, Pablo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Raabe, Jörg
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Jaouen, Nicolas
    Synchrotron SOLEIL, BP 48, F-91192 Gif Sur Yvette, France..
    Redlin, Harald
    DESY, HASYLAB, Notkestr 85, D-22607 Hamburg, Germany..
    Tiedtke, Kai
    DESY, HASYLAB, Notkestr 85, D-22607 Hamburg, Germany..
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    David, Christian
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Nolting, Frithjof
    Paul Scherrer Inst, CH-5232 Villigen, Switzerland..
    Luning, Jan
    UPMC Univ Paris 06, Sorbonne Univ, CNRS, LCPMR, F-75005 Paris, France.;Synchrotron SOLEIL, BP 48, F-91192 Gif Sur Yvette, France..
    Single-shot Monitoring of Ultrafast Processes via X-ray Streaking at a Free Electron Laser2017Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, artikel-id 7253Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window. We used a single x-ray pulse to resolve the laser-induced ultrafast demagnetisation dynamics in a thin cobalt film over a time window of about 1.6 ps with an excellent signal to noise ratio. From one representative single shot measurement we extract a spin relaxation time of (130 +/- 30) fs with an average value, based on 193 single shot events of (113 +/- 20) fs. These results are limited by the achieved experimental time resolution of 120 fs, and both values are in excellent agreement with previous results and theoretical modelling. More generally, this new experimental approach to ultrafast x-ray spectroscopy paves the way to the study of non-repetitive processes that cannot be investigated using traditional repetitive pump-probe schemes.

  • 28.
    Carva, K
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Legut, D
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Influence of laser-excited electron distributions on the X-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni2009Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 86, nr 5, s. 57002-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In pump-probe experiments an intensive laser pulse creates non-equilibrium excited-electron distributions in the first few hundred femtoseconds after the pulse. The influence of nonequilibrium electron distributions caused by a pump laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is investigated theoretically here for the first time, considering electron distributions immediately after the pulse as well as thermalized ones, that are not in equilibrium with the lattice or spin systems. The XMCD signal is shown not to be simply proportional to the spin momentum in these situations. The computed spectra are compared to recent pump-probe XMCD experiments on Ni. We find that the majority of experimentally observed features considered to be a proof of ultrafast spin momentum transfer to the lattice can alternatively be attributed to non-equilibrium electron distributions. Furthermore, we find the XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid, even for the laser-induced non-equilibrium electron distributions. Copyright (C) EPLA, 2009

  • 29. Carva, Karel
    et al.
    Battiato, Marco
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Legut, D.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 18, s. 184425-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co, and fcc Ni. The Elliott-Yafet electron-phonon spin-flip scattering is computed from first principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Using the developed formalism we compute the phonon-induced spin lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated demagnetization rate is evaluated for laser-created thermalized and nonequilibrium electron distributions. Nonthermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the subpicosecond regime.

  • 30.
    Carva, Karel
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Battiato, Marco
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Legut, Dominik
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Theory of femtosecond laser-induced demagnetization2015Ingår i: ULTRAFAST MAGNETISM I, 2015, s. 111-115Konferensbidrag (Refereegranskat)
    Abstract [en]

    Using ab initio calculations we computed the ultrafast demagnetization that can be achieved by Elliott-Yafet electron-phonon spin-flip scatterings in laser-excited ferromagnets. Our calculations show that nonequilibrium laser-created distributions contribute mostly to the ultrafast demagnetization. Nonetheless, the total Elliott-Yafet contribution is too small to account for the fs-demagnetization.

  • 31.
    Carva, Karel
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Battiato, Marco
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Ab Initio Investigation of the Elliott-Yafet Electron-Phonon Mechanism in Laser-Induced Ultrafast Demagnetization2011Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, nr 20, s. 207201-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spin-flip (SF) Eliashberg function is calculated from first principles for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization. This is used to compute the SF probability and demagnetization rate for laser-created thermalized as well as nonequilibrium electron distributions. Increased SF probabilities are found for thermalized electrons, but the induced demagnetization rate is extremely small. A larger demagnetization rate is obtained for nonequilibrium electron distributions, but its contribution is too small to account for femtosecond demagnetization.

  • 32.
    Carva, Karel
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Battiato, Marco
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Is the controversy over femtosecond magneto-optics really solved?2011Ingår i: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 7, nr 9, s. 665-665Artikel i tidskrift (Refereegranskat)
  • 33.
    Chen, Cong
    et al.
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Tao, Zhensheng
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Carr, Adra
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Matyba, Piotr
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA.;Umea Univ, Dept Phys, SE-90187 Umea, Sweden..
    Szilvasi, Tibor
    Univ Wisconsin Madison, Dept Chem & Biol Engn, Madison, WI 53706 USA..
    Emmerich, Sebastian
    Univ Kaiserslautern, Dept Phys, Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Piecuch, Martin
    Univ Kaiserslautern, Dept Phys, Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Keller, Mark
    NIST, Boulder, CO 80305 USA..
    Zusin, Dmitriy
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Eich, Steffen
    Univ Kaiserslautern, Dept Phys, Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Rollinger, Markus
    Univ Kaiserslautern, Dept Phys, Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Youa, Wenjing
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Mathias, Stefan
    Univ Kaiserslautern, Dept Phys, Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany.;Georg August Univ Gottingen, Phys Inst, D-37077 Gottingen, Germany..
    Thumm, Uwe
    Kansas State Univ, Dept Phys, Cardwell Hall, Manhattan, KS 66506 USA..
    Mavrikakis, Manos
    Univ Wisconsin Madison, Dept Chem & Biol Engn, Madison, WI 53706 USA..
    Aeschlimann, Martin
    Univ Kaiserslautern, Dept Phys, Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Kapteyn, Henry
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Murnane, Margaret
    Univ Colorado, Dept Phys, JILA, Boulder, CO 80309 USA.;NIST, Boulder, CO 80309 USA..
    Distinguishing attosecond electron-electron scattering and screening in transition metals2017Ingår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 114, nr 27, s. E5300-E5307Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electron-electron interactions are the fastest processes in materials, occurring on femtosecond to attosecond timescales, depending on the electronic band structure of the material and the excitation energy. Such interactions can play a dominant role in light-induced processes such as nano-enhanced plasmonics and catalysis, light harvesting, or phase transitions. However, to date it has not been possible to experimentally distinguish fundamental electron interactions such as scattering and screening. Here, we use sequences of attosecond pulses to directly measure electron-electron interactions in different bands of different materials with both simple and complex Fermi surfaces. By extracting the time delays associated with photoemission we show that the lifetime of photoelectrons from the d band of Cu are longer by similar to 100 as compared with those from the same band of Ni. We attribute this to the enhanced electron-electron scattering in the unfilled d band of Ni. Using theoretical modeling, we can extract the contributions of electron-electron scattering and screening in different bands of different materials with both simple and complex Fermi surfaces. Our results also show that screening influences high-energy photoelectrons (approximate to 20 eV) significantly less than low-energy photoelectrons. As a result, high-energy photoelectrons can serve as a direct probe of spin-dependent electron-electron scattering by neglecting screening. This can then be applied to quantifying the contribution of electron interactions and screening to low-energy excitations near the Fermi level. The information derived here provides valuable and unique information for a host of quantum materials.

  • 34. Chylarecka, D.
    et al.
    Kim, T. K.
    Tarafder, Kartick
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Mueller, K.
    Goedel, K.
    Czekaj, I.
    Waeckerlin, C.
    Cinchetti, M.
    Ali, Md E.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Piamonteze, C.
    Schmitt, F.
    Wuestenberg, J. -P
    Ziegler, C.
    Nolting, F.
    Aeschlimann, M.
    Oppeneer, Peter
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Ballav, N.
    Jung, T. A.
    Indirect Magnetic Coupling of Manganese Porphyrin to a Ferromagnetic Cobalt Substrate2011Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, nr 4, s. 1295-1301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface.

  • 35.
    Conradson, Steven D.
    et al.
    Synchrotron Soleil, Orme Merisiers St Aubin, F-91192 Gif Sur Yvette, France..
    Gilbertson, Steven M.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Daifuku, Stephanie L.
    Univ Rochester, Dept Chem, Rochester, NY 14627 USA..
    Kehl, Jeffrey A.
    Univ Rochester, Dept Chem, Rochester, NY 14627 USA..
    Durakiewicz, Tomasz
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Andersson, David A.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Bishop, Alan R.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Byler, Darrin D.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Maldonado, Pablo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Valdez, James A.
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Neidig, Michael L.
    Univ Rochester, Dept Chem, Rochester, NY 14627 USA..
    Rodriguez, George
    Los Alamos Natl Lab, Los Alamos, NM 87545 USA..
    Possible Demonstration of a Polaronic Bose-Einstein(-Mott) Condensate in UO2(+x) by Ultrafast THz Spectroscopy and Microwave Dissipation2015Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, artikel-id 15278Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bose-Einstein condensates (BECs) composed of polarons would be an advance because they would combine coherently charge, spin, and a crystal lattice. Following our earlier report of unique structural and spectroscopic properties, we now identify potentially definitive evidence for polaronic BECs in photo-and chemically doped UO2(+x) on the basis of exceptional coherence in the ultrafast time dependent terahertz absorption and microwave spectroscopy results that show collective behavior including dissipation patterns whose precedents are condensate vortex and defect disorder and condensate excitations. That some of these signatures of coherence in an atom-based system extend to ambient temperature suggests a novel mechanism that could be a synchronized, dynamical, disproportionation excitation, possibly via the solid state analog of a Feshbach resonance that promotes the coherence. Such a mechanism would demonstrate that the use of ultra-low temperatures to establish the BEC energy distribution is a convenience rather than a necessity, with the actual requirement for the particles being in the same state that is not necessarily the ground state attainable by other means. A macroscopic quantum object created by chemical doping that can persist to ambient temperature and resides in a bulk solid would be revolutionary in a number of scientific and technological fields.

  • 36. Dakovski, Georgi L.
    et al.
    Li, Yinwan
    Gilbertson, Steve M.
    Rodriguez, George
    Balatsky, Alexander V.
    Zhu, Jian-Xin
    Gofryk, Krzysztof
    Bauer, Eric D.
    Tobash, Paul H.
    Taylor, Antoinette
    Sarrao, John L.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Riseborough, Peter S.
    Mydosh, John A.
    Durakiewicz, Tomasz
    Anomalous femtosecond quasiparticle dynamics of hidden order state in URu(2)Si(2)2011Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, nr 16, s. 161103-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    At T(0) = 17.5 K an exotic phase emerges from a heavy fermion state in URu(2)Si(2). The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at the HO transition, however, experiments and ab initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time-and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector q(< 110 >) = 0.56 +/- 0.08 angstrom(-1). Measurements of the temperature-time response reveal that, upon entering the HO, the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO.

  • 37.
    Dhara, Barun
    et al.
    IISER, Dept Chem, Pune 411008, Maharashtra, India..
    Tarafder, Kartick
    NITK, Dept Phys, Mangalore 575025, India..
    Jha, Plawan K.
    IISER, Dept Chem, Pune 411008, Maharashtra, India..
    Panja, Soumendra N.
    IISER, Dept Phys, Pune 411008, Maharashtra, India..
    Nair, Sunil
    IISER, Dept Phys, Pune 411008, Maharashtra, India..
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Ballav, Nirmalya
    IISER, Dept Chem, Pune 411008, Maharashtra, India..
    Possible Room-Temperature Ferromagnetism in Self-Assembled Ensembles of Paramagnetic and Diamagnetic Molecular Semiconductors2016Ingår i: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, nr 24, s. 4988-4995Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Owing to long spin-relaxation time and chemically customizable physical properties, molecule-based semiconductor materials like metal-phthalocyanines offer promising alternatives to conventional dilute magnetic semiconductors/oxides (DMSs/ DMOs) to achieve room-temperature (RT) ferromagnetism. However, air-stable molecule based materials exhibiting both semiconductivity and magnetic-order at RT have so far remained elusive. We present here the concept of supramolecular arrangement to accomplish possibly RT ferromagnetism. Specifically, we observe a clear hysteresis-loop (H-c approximate to 120 Oe) at 300 K in the magnetization versus field (M H) plot of the self-assembled ensembles of diamagnetic Zn-phthalocyanine having peripheral F atoms (ZnFPc; S = 0) and paramagnetic Fe-phthalocyanine having peripehral H atoms (FePc; S = 1). Tauc plot of the self-assembled FePc center dot center dot center dot ZnFPc ensembles showed an optical band gap of similar to 1.05 eV and temperature-dependent current-voltage (I-V) studies suggest semiconducting characteristics in the material. Using DFT+U quantum-chemical calculations, we reveal the origin of such unusual ferromagnetic exchange-interaction in the supramolecular FePc center dot center dot center dot ZnFPc system.

  • 38. Dreiser, Jan
    et al.
    Pedersen, Kasper S.
    Piamonteze, Cinthia
    Rusponi, Stefano
    Salman, Zaher
    Ali, Md Ehesan
    Schau-Magnussen, Magnus
    Thuesen, Christian Aa
    Piligkos, Stergios
    Weihe, Hogni
    Mutka, Hannu
    Waldmann, Oliver
    Oppeneer, Peter
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Bendix, Jesper
    Nolting, Frithjof
    Brune, Harald
    Direct observation of a ferri-to-ferromagnetic transition in a fluoride-bridged 3d-4f molecular cluster2012Ingår i: Chemical Science, ISSN 2041-6520, Vol. 3, nr 4, s. 1024-1032Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on the synthesis, crystal structure and magnetic characterisation of the trinuclear, fluoride-bridged, molecular nanomagnet [Dy(hfac)(3)(H2O)-CrF2(py)(4)-Dy(hfac)(3)(NO3)] (1) (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone, py = pyridine) and a closely related dinuclear species [Dy(hfac)(4)-CrF2( py)(4)]center dot 1/2 CHCl3 (2). Element-specific magnetisation curves obtained on 1 by X-ray magnetic circular dichroism (XMCD) allow us to directly observe the field-induced transition from a ferrimagnetic to a ferromagnetic arrangement of the Dy and Cr magnetic moments. By fitting a spin-Hamiltonian model to the XMCD data we extract a weak antiferromagnetic exchange coupling of j -0.18 cm(-1) between the Dy-III and Cr-III ions. The value found from XMCD is consistent with SQUID magnetometry and inelastic neutron scattering measurements. Furthermore, alternating current susceptibility and muon-spin relaxation measurements reveal that 1 shows thermally activated relaxation of magnetisation with a small effective barrier for magnetisation reversal of Delta(eff) 3 cm(-1). Density-functional theory calculations show that the Dy-Cr couplings originate from superexchange via the fluoride bridges.

  • 39. Dreiser, Jan
    et al.
    Waeckerlin, Christian
    Ali, Md Ehesan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Piamonteze, Cinthia
    Donati, Fabio
    Singha, Aparajita
    Pedersen, Kasper Steen
    Rusponi, Stefano
    Bendix, Jesper
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Jung, Thomas A.
    Brune, Harald
    Exchange Interaction of Strongly Anisotropic Tripodal Erbium Single-Ion Magnets with Metallic Surfaces2014Ingår i: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 8, nr 5, s. 4662-4671Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a comprehensive study of Er(trensal) single-ion magnets deposited in ultrahigh vacuum onto metallic surfaces. X-ray photoelectron spectroscopy reveals that the molecular structure is preserved after sublimation, and that the molecules are physisorbed on Au(111) while they are chemisorbed on a Ni thin film on 0(100) single-crystalline surfaces. X-ray magnetic circular dichroism (XMCD) measurements performed on Au(111) samples covered with molecular monolayers held at temperatures down to 4 K suggest that the easy axes of the strongly anisotropic molecules are randomly oriented. Furthermore XMCD indicates a weak antiferromagnetic exchange coupling between the single-ion magnets and the ferromagnetic Ni/Cu(100) substrate. For the latter case, spin-Hamiltonian fits to the XMCD M(H) suggest a significant structural distortion of the molecules. Scanning tunneling microscopy reveals that the molecules are mobile on Au(111) at room temperature, whereas they are more strongly attached on Ni/Cu(100). X-ray photoelectron spectroscopy results provide evidence for the chemical bonding between Er(trensal) molecules and the Ni substrate. Density functional theory calculations support these findings and, in addition, reveal the most stable adsorption configuration on Ni/Cu(100) as well as the Ni-Er exchange path. Our study suggests that the magnetic moment of Er(trensal) can be stabilized via suppression of quantum tunneling of magnetization by exchange coupling to the Ni surface atoms. Moreover, it opens up pathways toward optical addressing of surface-deposited single-ion magnets.

  • 40. Durakiewicz, T.
    et al.
    Riseborough, P. S.
    Batista, C. D.
    Yang, Y.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Joyce, J. J.
    Bauer, E. D.
    Graham, K. S.
    Quest for Band Renormalization and Self-Energy in Correlated f-Electron Systems2010Ingår i: Acta Physica Polonica. A, ISSN 0587-4246, E-ISSN 1898-794X, Vol. 117, nr 2, s. 264-267Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Coexisting low-energy scales are observed in f-electron materials. The information about some of low-energy scales is imprinted in the electron self-energy, which can be measured by angle-resolved photoemission (ARPES). Such measurements in d-electron materials over the last decade were based on high energy- and momentum-resolution ARPES techniques used to extract the self-energy information from measured spectra. Simultaneously, many-body theoretical approaches have been developed to find a link between the self-energy and many-body interactions. Here we show the transcription of such methods from d-electrons to f-electrons by presenting the first example of low energy scales in the f-electron material USb2, measured with synchrotron-based ARPES. The proposed approach will help in answering the fundamental questions about the complex nature of the heavy fermion state.

  • 41. Durakiewicz, T.
    et al.
    Riseborough, P. S.
    Olson, C. G.
    Joyce, J. J.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Elgazzar, S.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Bauer, E. D.
    Sarrao, J. L.
    Guziewicz, E.
    Moore, D. P.
    Butterfield, M. T.
    Graham, K. S.
    Observation of a kink in the dispersion of f-electrons2008Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 84, nr 3, s. 37003-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Strong interactions in correlated electron systems may result in the formation of heavy quasiparticles that exhibit kinks in their dispersion relation. Spectral weight is incoherently shifted away from the Fermi energy, but Luttinger's theorem requires the Fermi volume to remain constant. Our angle-resolved photoemission study of USb2 reveals a kink in a noncrossing 5f band, representing the first experimental observation of a kink structure in f-electron systems. The kink energy scale of 21 meV and the ultra-small peak width of 3 meV are observed. We propose the novel mechanism of renormalization of a point-like Fermi surface, and that Luttinger's theorem remains applicable.

  • 42. Durr, Hermann A.
    et al.
    Eimueller, Thomas
    Elmers, Hans-Joachim
    Eisebitt, Stefan
    Farle, Michael
    Kuch, Wolfgang
    Matthes, Frank
    Martins, Michael
    Mertins, Hans-Christoph
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Plucinski, Lukasz
    Schneider, Claus M.
    Wende, Heiko
    Wurth, Wilfried
    Zabel, Hartmut
    A Closer Look Into Magnetism: Opportunities With Synchrotron Radiation2009Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 45, nr 1, s. 15-57Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    The unique properties of synchrotron radiation, such as broad energy spectrum, variable light polarization, and flexible time structure, have made it an enormously powerful tool in the study of magnetic phenomena and materials. The refinement of experimental techniques has led to many new research opportunities, keeping up with the challenges put up by modern magnetism research. In this contribution, we review some of the recent developments in the application of synchrotron radiation and particularly soft X-rays to current problems in magnetism, and we discuss future perspectives.

  • 43.
    Elgazzar, S.
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Fysiska institutionen.
    Opahle, I.
    Richter, Manuel
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Fysiska institutionen.
    Electronic structure and Fermi surface of PrMIn5 (M = Co, Rh, and Ir) compounds2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr 12, s. 125105-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report density functional calculations of the electronic structure, Fermi surface, and de Haas-van Alphen (dHvA) quantities of the PrMIn5 (M=Co, Rh, and Ir) compounds. Our investigation is carried out within the framework of the local density approximation, using a relativistic, full-potential band-structure method. A critical analysis of the electronic structures and the de Haas-van Alphen quantities is performed, which shows that good agreement with recent measurements is obtained when we assume the Pr 4f states to be localized. The topology of the Fermi surface is calculated to be similar to that of non-4f reference compounds, e.g., LaRhIn5. The similarities of the Fermi surfaces and the dHvA extremal orbits among the compounds in the series are discussed. We, furthermore, compare our calculated effective masses with experimental measurements and discuss the differences between them.

  • 44.
    Elgazzar, S
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Rusz, Jan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Amft, Martin
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och materialvetenskap.
    Mydosh, J. A.
    Hidden order in URu2Si2 originates from Fermi surface gapping induced by dynamic symmetry breaking2009Ingår i: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 8, nr 4, s. 337-341Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter driving the transition. The notion of a 'hidden order' has recently been used for a variety of materials where a clear phase transition occurs without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2, where a mysterious hidden-order transition occurs at 17.5 K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Here, we provide a microscopic explanation of the hidden order using density-functional theory calculations. We identify the Fermi surface 'hot spots' where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As the mechanism for the hidden order, we deduce spontaneous symmetry breaking through a dynamic mode of antiferromagnetic moment excitations.

  • 45. Elgazzar, S.
    et al.
    Rusz, Jan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Mydosh, J. A.
    Electronic structure and Fermi surface of paramagnetic and antiferromagnetic UPt2Si22012Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, nr 7, s. 075104-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report density functional theory-based calculations of the electronic structure and Fermi surface properties of the intermetallic uranium compound UPt2Si2, which orders antiferromagnetically at TN=32 K with a total magnetic moment of 2μB/U-atom and exhibits a moderate mass enhancement in the specific-heat coefficient. Our investigation is carried out using relativistic, full-potential band-structure methods within the framework of the local spin density approximation (LSDA), the LSDA with orbital polarization correction (LSDA+OPC), and the LSDA supplemented with an additional Hubbard U (LSDA+U). We find that the LSDA+OPC scheme predicts the total magnetic moment in best agreement with experiment; from this we infer that the 5f electrons in UPt2Si2 are orbitally polarized, mostly itinerant, and exhibit only a slight tendency toward localization. Our total energy calculations predict UPt2Si2 to form in the CaBe2Si2 (P4/nmm) structure, in contrast to URu2Si2 (ThCr2Si2: I4/mmm). The theoretical Fermi surfaces are also studied for the nonmagnetic and antiferromagnetic phases with the employed computational schemes and are found to be quasi-two-dimensional. At the antiferromagnetic transition, the Fermi surface is found to become more two-dimensional with small regions of gapping.

  • 46. Elgazzar, Saad
    et al.
    Opahle, Ingo
    Hayn, Roland
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Fysiska institutionen.
    Calculated de Haas-van Alphen quantities of CeMIn5 (M=Co, Rh, and Ir) compounds2004Ingår i: Phys. Rev. B, Vol. 69, s. 214510-Artikel i tidskrift (Refereegranskat)
  • 47. Elgazzar, Saad
    et al.
    Opahle, Ingo
    Hayn, Roland
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Fysiska institutionen.
    Electronic structure and de Haas-van Alphen quantities of CeMIn5 (M=Co, Rh, and Ir)2005Ingår i: J. Magn. Magn. Mater., Vol. 290, s. 388-Artikel i tidskrift (Refereegranskat)
  • 48.
    Elgazzar, Saad
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Rusz, Jan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi.
    Colineau, E.
    Griveau, J. -C
    Magnani, N.
    Rebizant, J.
    Caciuffo, R.
    Ab initio computational and experimental investigation of the electronic structure of actinide 218 materials2010Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 23, s. 235117-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report a comprehensive investigation of the electronic structure and magnetic properties of actinide 218 compounds, which crystallize in the tetragonal Ho2CoGa8 crystal structure. Specifically, we study experimentally the group of plutonium-based compounds Pu2MGa8 (with M=Rh, Co, and Fe), which are structurally related to the unconventional superconductors PuCoGa5 and PuRhGa5 and are measured to be nonmagnetic and nonsuperconducting down to 2 K, yet displaying relatively high linear specific-heat coefficients of 61 to 133 mJ/mol K-2. We perform density-functional theory based calculations, in which we apply three different approaches to access the tendency of 5f electron localization, the local spin-density approximation (LSDA) LSDA+U, and the 5f open-core approach. For comparison to the above-mentioned compounds we also investigate computationally the plutonium compounds with M=Ir and Pd, the uranium-based compounds U2MGa8 (with M=Co, Fe, Rh, and Ru), as well as Np2CoGa8, and Am2CoGa8. On the basis of ab initio LSDA calculations we optimize the equilibrium lattice parameters and the internal fractional coordinates within the Ho2CoGa8 crystal structure. The obtained lattice parameters are in relatively good agreement with experimental values, when we assume delocalized 5f states for all compounds except Am2CoGa8. We discuss the computed electronic structures and the theoretical Fermi surfaces. For the Pu-218 compounds we find that LSDA calculations, in which the 5f's are treated as delocalized, predict a magnetically ordered ground state, whereas LSDA+U calculations predict a nonmagnetic ground state in accordance with experiment. For the U-218 compounds the LSDA itinerant 5f approach predicts a nonmagnetic ground state, in accordance with available experimental data. For Am2CoGa8 our calculations are consistent with the scenario of localized 5f electrons. We find that, on account of the elongated tetragonal structure, most of the theoretical Fermi surfaces are quasi-two-dimensional.

  • 49. Emmel, M.
    et al.
    Alfonsov, A.
    Legut, D.
    Kehlberger, A.
    Vilanova, E.
    Krug, I. P.
    Gottlob, D. M.
    Belesi, M.
    Buechner, B.
    Klaeui, M.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Wurmehl, S.
    Elmers, H. J.
    Jakob, G.
    Electronic properties of Co2FeSi investigated by X-ray magnetic linear dichroism2014Ingår i: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 368, s. 364-373Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present experimental XMLD spectra measured on epitaxial (001)-oriented thin Co2FeSi films, which are rich in features and depend sensitively on the degree of atomic order and interdiffusion from capping layers. Al- and Cr-capped films with different degrees of atomic order were prepared by DC magnetron sputtering by varying the deposition temperatures. The local structural properties of the film samples were additionally investigated by nuclear magnetic resonance (NMR) measurements. The XMLD spectra of the different samples show clear and uniform trends at the L-3,L-2 edges. The Al-capped samples show similar behavior as previous measured XMLD spectra of Co2FeSi0.6Al0.4. Thus, we assume that during deposition Al atoms are being implanted into the subsurface of Co2FeSi. Such an interdiffusion is not observed for the corresponding Cr-capped films, which makes Cr the material of choice for capping Co2FeSi films. We report stronger XMLD intensities at the L-3,L-2 Co and Fe egdes for films with a higher saturation magnetization. Additionally, we compare the spectra with ab initio predictions and obtain a reasonably good agreement. Furthermore, we were able to detect an XMCD signal at the Si Ledge, indicating the presence of a magnetic moment at the Si atoms.

  • 50. Eschenlohr, A.
    et al.
    Battiato, Marco
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Maldonado, Pablo
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Pontius, N.
    Kachel, T.
    Holldack, K.
    Mitzner, R.
    Foehlisch, A.
    Oppeneer, Peter M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Materialteori.
    Stamm, C.
    Optical excitation of thin magnetic layers in multilayer structures Reply2014Ingår i: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 13, nr 2, s. 102-103Artikel i tidskrift (Refereegranskat)
12345 1 - 50 av 236
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