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Publications (10 of 215) Show all publications
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
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
Gang, S.-g., Adam, R., Plötzing, M., von Witzleben, M., Weier, C., Parlak, U., . . . Oppeneer, P. M. (2018). Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation. Physical Review B, 97(6), Article ID 064412.
Open this publication in new window or tab >>Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 6, article id 064412Article in journal (Refereed) Published
Abstract [en]

We studied femtosecond spin dynamics in NixPd1-x magnetic thin films by optically pumping the system with infrared (1.55 eV) laser pulses and subsequently recording the reflectivity of extreme ultraviolet (XUV) pulses synchronized with the pump pulse train. XUV light in the energy range from 20 to 72 eV was produced by laser high-harmonic generation. The reflectivity of XUV radiation at characteristic resonant energies allowed separate detection of the spin dynamics in the elemental subsystems at the M-2,M-3 absorption edges of Ni (68.0 and 66.2 eV) and N-2,N-3 edges of Pd (55.7 and 50.9 eV). The measurements were performed in transversal magneto-optical Kerr effect geometry. In static measurements, we observed a magnetic signature of the Pd subsystem due to an induced magnetization. Calculated magneto-optical asymmetries based on density functional theory show close agreement with the measured results. Femtosecond spin dynamics measured at the Ni absorption edges indicates that increasing the Pd concentration, which causes a decrease in the Curie temperature T-C, results in a drop of the demagnetization time tau(M), contrary to the tau(M) similar to 1/T-C scaling expected for single-species materials. This observation is ascribed to the increase of the Pd-mediated spin-orbit coupling in the alloy.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-349353 (URN)10.1103/PhysRevB.97.064412 (DOI)000425492100005 ()
Funder
German Research Foundation (DFG), SCHN 353/17-1Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2015.0060Swedish National Infrastructure for Computing (SNIC)
Available from: 2018-05-02 Created: 2018-05-02 Last updated: 2018-05-02Bibliographically approved
Suzuki, M.-T., Ikeda, H. & Oppeneer, P. M. (2018). First-principles Theory of Magnetic Multipoles in Condensed Matter Systems. Journal of the Physical Society of Japan, 87(4), Article ID 041008.
Open this publication in new window or tab >>First-principles Theory of Magnetic Multipoles in Condensed Matter Systems
2018 (English)In: Journal of the Physical Society of Japan, ISSN 0031-9015, E-ISSN 1347-4073, Vol. 87, no 4, article id 041008Article in journal (Refereed) Published
Abstract [en]

The multipole concept, which characterizes the spacial distribution of scalar and vector objects by their angular dependence, has already become widely used in various areas of physics. In recent years it has become employed to systematically classify the anisotropic distribution of electrons and magnetization around atoms in solid state materials. This has been fuelled by the discovery of several physical phenomena that exhibit unusual higher rank multipole moments, beyond that of the conventional degrees of freedom as charge and magnetic dipole moment. Moreover, the higher rank electric/magnetic multipole moments have been suggested as promising order parameters in exotic hidden order phases. While the experimental investigations of such anomalous phases have provided encouraging observations of multipolar order, theoretical approaches have developed at a slower pace. In particular, a materials' specific theory has been missing. The multipole concept has furthermore been recognized as the key quantity which characterizes the resultant configuration of magnetic moments in a cluster of atomic moments. This cluster multipole moment has then been introduced as macroscopic order parameter for a noncollinear antiferromagnetic structure in crystals that can explain unusual physical phenomena whose appearance is determined by the magnetic point group symmetry. It is the purpose of this review to discuss the recent developments in the first-principles theory investigating multipolar degrees of freedom in condensed matter systems. These recent developments exemplify that ab initio electronic structure calculations can unveil detailed insight in the mechanism of physical phenomena caused by the unconventional, multipole degree of freedom.

Place, publisher, year, edition, pages
PHYSICAL SOC JAPAN, 2018
National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-352468 (URN)10.7566/JPSJ.87.041008 (DOI)000429108000006 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)Knut and Alice Wallenberg Foundation, 2015.0060
Available from: 2018-06-05 Created: 2018-06-05 Last updated: 2018-06-05Bibliographically approved
Hosen, M. M., Dimitri, K., Aperis, A., Maldonado, P., Belopolski, I., Dhakal, G., . . . Neupane, M. (2018). Observation of gapless Dirac surface states in ZrGeTe. Physical Review B, 97(12), Article ID 121103.
Open this publication in new window or tab >>Observation of gapless Dirac surface states in ZrGeTe
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2018 (English)In: Physical Review B, Vol. 97, no 12, article id 121103Article in journal (Refereed) Published
Abstract [en]

The experimental discovery of the topological Dirac semimetal establishes a platform to search for various exotic quantum phases in real materials. ZrSiS-type materials have recently emerged as topological nodal-line semimetals where gapped Dirac-like surface states are observed. Here, we present a systematic angle-resolved photoemission spectroscopy (ARPES) study of ZrGeTe, a nonsymmorphic symmetry protected Dirac semimetal. We observe twoDirac-like gapless surface states at the same <overline> X point of the Brillouin zone. Our theoretical analysis and first-principles calculations reveal that these are protected by crystalline symmetry. Hence, ZrGeTe appears as a rare example of a naturally fine tuned system where the interplay between symmorphic and nonsymmorphic symmetry leads to rich phenomenology and thus opens up opportunities to investigate the physics of Dirac semimetallic and topological insulating phases realized in a single material.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-346602 (URN)10.1103/PhysRevB.97.121103 (DOI)000426779100001 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2018-03-19 Created: 2018-03-19 Last updated: 2018-05-15Bibliographically approved
Reid, A. H., Shen, X., Maldonado, P., Chase, T., Jal, E., Granitzka, P. W., . . . Dürr, H. A. (2018). Publisher Correction: Beyond a phenomenological description of magnetostriction. Nature Communications, 9, Article ID 1035.
Open this publication in new window or tab >>Publisher Correction: 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 1035Article in journal (Other academic) Published
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-355826 (URN)10.1038/s41467-018-03389-4 (DOI)000426899700003 ()29515124 (PubMedID)
Note

WoS title: Beyond a phenomenological description of magnetostriction (vol 9, 388, 2018)

Correction to: Nature Communications, vol. 9, article number: 388. DOI: 10.1038/s41467-017-02730-7

Available from: 2018-07-06 Created: 2018-07-06 Last updated: 2018-07-06Bibliographically approved
Balaz, P., Zonda, M., Carva, K., Maldonado, P. & Oppeneer, P. M. (2018). Transport theory for femtosecond laser-induced spin-transfer torques. Journal of Physics: Condensed Matter, 30(11), Article ID 115801.
Open this publication in new window or tab >>Transport theory for femtosecond laser-induced spin-transfer torques
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2018 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 11, article id 115801Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
ultrafast demagnetization, spin transfer torque, magnetization dynamics, spin current, spin transport
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350487 (URN)10.1088/1361-648X/aaad95 (DOI)000425996100001 ()
Funder
Swedish National Infrastructure for Computing (SNIC)Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)EU, Horizon 2020, 737709Knut and Alice Wallenberg Foundation, 2015.0060
Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-05-17Bibliographically approved
Chen, C., Tao, Z., Carr, A., Matyba, P., Szilvasi, T., Emmerich, S., . . . Murnane, M. (2017). Distinguishing attosecond electron-electron scattering and screening in transition metals. Proceedings of the National Academy of Sciences of the United States of America, 114(27), E5300-E5307
Open this publication in new window or tab >>Distinguishing attosecond electron-electron scattering and screening in transition metals
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2017 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 114, no 27, p. E5300-E5307Article in journal (Refereed) Published
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.

Keywords
attosecond science, high harmonic generation, ARPES, electron-electron interactions
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-330001 (URN)10.1073/pnas.1706466114 (DOI)000404576100006 ()
Funder
Swedish Research CouncilMarcus Wallenbergs Foundation for International Scientific Collaboration, 2015.0060
Available from: 2017-10-13 Created: 2017-10-13 Last updated: 2017-10-13Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-9069-2631

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