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Schrodi, F., Aperis, A. & Oppeneer, P. M. (2019). Increased performance of Matsubara space calculations: A case study within Eliashberg theory. Physical Review B, 99(18), Article ID 184508.
Open this publication in new window or tab >>Increased performance of Matsubara space calculations: A case study within Eliashberg theory
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 18, article id 184508Article in journal (Refereed) Published
Abstract [en]

We present a method to considerably improve the numerical performance for solving Eliashberg-type coupled equations on the imaginary axis. Instead of the standard practice of introducing a hard numerical cutoff for treating the infinite summations involved, our scheme allows for the efficient calculation of such sums extended formally up to infinity. The method is first benchmarked with isotropic Migdal-Eliashberg theory calculations and subsequently applied to the solution of the full-bandwidth, multiband, and anisotropic equations focusing on the FeSe/SrTiO3 interface as a case study. Compared to the standard procedure, we reach similarly well converged results with less than one fifth of the number of frequencies for the anisotropic case, while for the isotropic set of equations we spare approximately ninety percent of the complexity. Since our proposed approximations are very general, our numerical scheme opens the possibility of studying the superconducting properties of a wide range of materials at ultralow temperatures.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-387593 (URN)10.1103/PhysRevB.99.184508 (DOI)000469055500010 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-06-26Bibliographically approved
Reddy, I. R., Oppeneer, P. M. & Tarafder, K. (2019). Interfacial Spin Manipulation of Nickel-Quinonoid Complex Adsorbed on Co(001) Substrate. MAGNETOCHEMISTRY, 5(1), Article ID 2.
Open this publication in new window or tab >>Interfacial Spin Manipulation of Nickel-Quinonoid Complex Adsorbed on Co(001) Substrate
2019 (English)In: MAGNETOCHEMISTRY, ISSN 2312-7481, Vol. 5, no 1, article id 2Article in journal (Refereed) Published
Abstract [en]

We studied the structural, electronic, and magnetic properties of a recently synthesized Ni(II)-quinonoid complex upon adsorption on a magnetic Co(001) substrate. Our density functional theory +U (DFT+U) calculations predict that the molecule undergoes a spin-state switching from low-spin S = 0 in the gas phase to high-spin S approximate to 1 when adsorbed on the Co(001) surface. A strong covalent interaction of the quinonoid rings and surface atoms leads to an increase of the Ni-O(N) bond lengths in the chemisorbed molecule that support the spin-state switching. Our DFT+U calculations show that the molecule is ferromagnetically coupled to the substrate. The Co surface-Ni center exchange mechanism was carefully investigated. We identified an indirect exchange interaction via the quinonoid ligands that stabilizes the molecule's spin moment in ferromagnetic alignment with the Co surface magnetization.

Keywords
spin-state switching, interfacial spin-manipulation, DFT+U theory
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-392066 (URN)10.3390/magnetochemistry5010002 (DOI)000477013300002 ()
Funder
Swedish Research Council, SB/FTP/PS-032/2014Knut and Alice Wallenberg Foundation, 2015.0060
Available from: 2019-09-09 Created: 2019-09-09 Last updated: 2019-09-09Bibliographically approved
Alekhin, A., Razdolski, I., Berritta, M., Buerstel, D., Temnov, V., Diesing, D., . . . Melnikov, A. (2019). Magneto-optical properties of Au upon the injection of hot spin-polarized electrons across Fe/Au(001) interfaces. Journal of Physics: Condensed Matter, 31(12), Article ID 124002.
Open this publication in new window or tab >>Magneto-optical properties of Au upon the injection of hot spin-polarized electrons across Fe/Au(001) interfaces
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2019 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 12, article id 124002Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
spin current, time-resolved MOKE, nonlinear magneto-optical spectroscopy, Drude model
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-377198 (URN)10.1088/1361-648X/aafd06 (DOI)000457442600001 ()30625433 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2015.0060EU, Horizon 2020, 737709
Available from: 2019-02-25 Created: 2019-02-25 Last updated: 2019-02-25Bibliographically approved
Arruda, L. M., Ali, M. E. E., Bernien, M., Nickel, F., Kopprasch, J., Czekelius, C., . . . Kuch, W. (2019). Modifying the Magnetic Anisotropy of an Iron Porphyrin Molecule by an on-Surface Ring-Closure Reaction. The Journal of Physical Chemistry C, 123(23), 14547-14555
Open this publication in new window or tab >>Modifying the Magnetic Anisotropy of an Iron Porphyrin Molecule by an on-Surface Ring-Closure Reaction
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2019 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 23, p. 14547-14555Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-390210 (URN)10.1021/acs.jpcc.9b03126 (DOI)000471834000049 ()
Funder
Swedish Research CouncilGerman Research Foundation (DFG), SFB 658
Available from: 2019-08-09 Created: 2019-08-09 Last updated: 2019-08-09Bibliographically approved
Shi, X., You, W., Zhang, Y., Tao, Z., Oppeneer, P. M., Wu, X., . . . Murnane, M. (2019). Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe2 mediated by mode-selective electron-phonon coupling. Science Advances, 5(3), Article ID eaav4449.
Open this publication in new window or tab >>Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe2 mediated by mode-selective electron-phonon coupling
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2019 (English)In: Science Advances, E-ISSN 2375-2548, Vol. 5, no 3, article id eaav4449Article in journal (Refereed) Published
Abstract [en]

Quantum materials represent one of the most promising frontiers in the quest for faster, lightweight, energy-efficient technologies. However, their inherent complexity and rich phase landscape make them challenging to understand or manipulate. Here, we present a new ultrafast electron calorimetry technique that can systematically uncover new phases of quantum matter. Using time- and angle-resolved photoemission spectroscopy, we measure the dynamic electron temperature, band structure, and heat capacity. This approach allows us to uncover a new long-lived metastable state in the charge density wave material 1T-TaSe2, which is distinct from all the known equilibrium phases: It is characterized by a substantially reduced effective total heat capacity that is only 30% of the normal value, because of selective electron-phonon coupling to a subset of phonon modes. As a result, less energy is required to melt the charge order and transform the state of the material than under thermal equilibrium conditions.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-381840 (URN)10.1126/sciadv.aav4449 (DOI)000462564300067 ()30838333 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2015.0060
Available from: 2019-04-15 Created: 2019-04-15 Last updated: 2019-04-15Bibliographically approved
Bekaert, J., Aperis, A., Partoens, B., Oppeneer, P. M. & Milosevic, M. V. (2018). Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of FeB4. Physical Review B, 97(1), Article ID 014503.
Open this publication in new window or tab >>Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of FeB4
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 1, article id 014503Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-337870 (URN)10.1103/PhysRevB.97.014503 (DOI)000419229100004 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2018-01-05 Created: 2018-01-05 Last updated: 2018-02-14Bibliographically approved
Reid, A. H., Shen, X., Maldonado, P., Chase, T., Jal, E., Granitzka, P. W., . . . Dürr, H. A. (2018). Beyond a phenomenological description of magnetostriction. Nature Communications, 9, Article ID 388.
Open this publication in new window or tab >>Beyond a phenomenological description of magnetostriction
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2018 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 388Article in journal (Refereed) Published
Abstract [en]

Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction-the underlying magnetoelastic stress-can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the subpicosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-343793 (URN)10.1038/s41467-017-02730-7 (DOI)000423430900008 ()29374151 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2015.0060EU, Horizon 2020, 737709Swedish National Infrastructure for Computing (SNIC)
Note

Correction in: Nature Communications, vol. 9, article number: 1035. DOI: 10.1038/s41467-018-03389-4

Available from: 2018-03-05 Created: 2018-03-05 Last updated: 2018-07-06Bibliographically approved
Shalaby, M., Donges, A., Carva, K., Allenspach, R., Oppeneer, P. M., Nowak, U. & Hauri, C. P. (2018). Coherent and incoherent ultrafast magnetization dynamics in 3d ferromagnets driven by extreme terahertz fields. Physical Review B, 98(1), Article ID 014405.
Open this publication in new window or tab >>Coherent and incoherent ultrafast magnetization dynamics in 3d ferromagnets driven by extreme terahertz fields
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 1, article id 014405Article in journal (Refereed) Published
Abstract [en]

Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of approximate to 100 mJ/cm(2). We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting nonconducting materials for achieving coherent THz-magnetization reversal.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-360190 (URN)10.1103/PhysRevB.98.014405 (DOI)000437109000003 ()
Funder
EU, FP7, Seventh Framework Programme, 290605Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)Knut and Alice Wallenberg Foundation, 2015.0060
Available from: 2018-09-11 Created: 2018-09-11 Last updated: 2018-09-11Bibliographically approved
Tengdin, P., You, W., Chen, C., Shi, X., Zusin, D., Zhang, Y., . . . Murnane, M. M. (2018). Critical behavior within 20 fs drives the out-of-equilibrium laser-induced magnetic phase transition in nickel. Science Advances, 4(3), Article ID eaap9744.
Open this publication in new window or tab >>Critical behavior within 20 fs drives the out-of-equilibrium laser-induced magnetic phase transition in nickel
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2018 (English)In: Science Advances, ISSN 0036-8156, E-ISSN 2375-2548, Vol. 4, no 3, article id eaap9744Article in journal (Refereed) Published
Abstract [en]

It has long been known that ferromagnets undergo a phase transition from ferromagnetic to paramagnetic at the Curie temperature, associated with critical phenomena such as a divergence in the heat capacity. A ferromagnet can also be transiently demagnetized by heating it with an ultrafast laser pulse. However, to date, the connection between out-of-equilibrium and equilibrium phase transitions, or how fast the out-of-equilibrium phase transitions can proceed, was not known. By combining time-and angle-resolved photoemission with time-resolved transverse magneto-optical Kerr spectroscopies, we show that the same critical behavior also governs the ultrafast magnetic phase transition in nickel. This is evidenced by several observations. First, we observe a divergence of the transient heat capacity of the electron spin system preceding material demagnetization. Second, when the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: The spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, whereas demagnetization and the collapse of the exchange splitting occur on much longer, fluence-independent time scales of similar to 176 fs. Third, we find that the transient electron temperature alone dictates the magnetic response. Our results are important because they connect the out-of-equilibrium material behavior to the strongly coupled equilibrium behavior and uncover a new time scale in the process of ultrafast demagnetization.

Place, publisher, year, edition, pages
AMER ASSOC ADVANCEMENT SCIENCE, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-354526 (URN)10.1126/sciadv.aap9744 (DOI)000427892700027 ()29511738 (PubMedID)
Funder
Swedish Research CouncilWallenberg Foundations, 2015.0060EU, Horizon 2020, 737709
Available from: 2018-06-21 Created: 2018-06-21 Last updated: 2018-06-21Bibliographically approved
Zusin, D., Tengdin, P. M., Gopalakrishnan, M., Gentry, C., Blonsky, A., Gerrity, M., . . . Murnane, M. M. (2018). Direct measurement of the static and transient magneto-optical permittivity of cobalt across the entire M-edge in reflection geometry by use of polarization scanning. Physical Review B, 97(2), Article ID 024433.
Open this publication in new window or tab >>Direct measurement of the static and transient magneto-optical permittivity of cobalt across the entire M-edge in reflection geometry by use of polarization scanning
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 2, article id 024433Article in journal (Refereed) Published
Abstract [en]

The microscopic state of amagnetic material is characterized by its resonant magneto-optical response through the off-diagonal dielectric tensor component epsilon(xy). However, the measurement of the full complex epsilon(xy) in the extreme ultraviolet spectral region covering the M absorption edges of 3d ferromagnets is challenging due to the need for either a careful polarization analysis, which is complicated by a lack of efficient polarization analyzers, or scanning the angle of incidence in fine steps. Here, we propose and demonstrate a technique to extract the complex resonant permittivity epsilon(xy) simply by scanning the polarization angle of linearly polarized high harmonics to measure the magneto-optical asymmetry in reflection geometry. Because this technique is more practical and faster to experimentally implement than previous approaches, we can directly measure the full time evolution of epsilon(xy)(t) during laser-induced demagnetization across the entire M-2,M-3 absorption edge of cobalt with femtosecond time resolution. We find that for polycrystalline Co films on an insulating substrate, the changes in epsilon(xy) are uniform throughout the spectrum, to within our experimental precision. This result suggests that, in the regime of strong demagnetization, the ultrafast demagnetization response is primarily dominated by magnon generation. We estimate the contribution of exchange-splitting reduction to the ultrafast demagnetization process to be no more than 25%.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-343791 (URN)10.1103/PhysRevB.97.024433 (DOI)000423517300002 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2015.0060
Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2018-03-07Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-9069-2631

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