SwePub - search: WFRF:(Oppeneer Peter M.)

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1.	Lloyd-Hughes, J., et al. (author) The 2021 ultrafast spectroscopic probes of condensed matter roadmap 2021 In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:35 Research review (peer-reviewed)abstract In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light-matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.
2.	Henighan, T., et al. (author) Generation mechanism of terahertz coherent acoustic phonons in Fe 2016 In: PHYSICAL REVIEW B. - 2469-9950. ; 93:22 Journal article (peer-reviewed)abstract We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is similar to 100 fs. An effective electronic Gruneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.
3.	Seifert, T., et al. (author) Efficient metallic spintronic emitters of ultrabroadband terahertz radiation 2016 In: Nature Photonics. - 1749-4885 .- 1749-4893. ; 10:7, s. 483- Journal article (peer-reviewed)abstract Terahertz electromagnetic radiation is extremely useful for numerous applications, including imaging and spectroscopy. It is thus highly desirable to have an efficient table-top emitter covering the 1-30 THz window that is driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source that relies on three tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photoinduced spin currents, the inverse spin-Hall effect and a broadband Fabry-Perot resonance. Guided by an analytical model, this spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-30 THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz field amplitude, flexibility, scalability and cost.
4.	Emmel, M., et al. (author) Electronic properties of Co2FeSi investigated by X-ray magnetic linear dichroism 2014 In: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 368, s. 364-373 Journal article (peer-reviewed)abstract 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.
5.	Bernien, M., et al. (author) Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates 2009 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:4, s. 047202- Journal article (peer-reviewed)abstract 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.
6.	Hofherr, M., et al. (author) Speed and efficiency of femtosecond spin current injection into a nonmagnetic material 2017 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 96:10 Journal article (peer-reviewed)abstract We investigate femtosecond spin injection from an optically excited Ni top layer into an Au bottom layer using time-resolved complex magneto-optical Kerr effect (C-MOKE) measurements. Employing the C-MOKE formalism, we are able to follow layer-resolved demagnetization in Ni and the simultaneous spin injection into the adjacent Au film, both occurring within similar to 40 fs. We confirm the ballistic to diffusive propagation of the spin transfer process with ab initio theory and superdiffusive transport calculations. In particular, our combined experimental-theoretical effort does allow us to quantify the so far elusive amount of spin injection, and therefore the spin injection efficiency at the interface.
7.	Legut, D., et al. (author) Influence of the crystal structure of thin Co films on X-ray magnetic linear dichroism-Comparison of ab initio theory and reflectometry experiments 2014 In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:17, s. 17E132- Journal article (peer-reviewed)abstract We report an investigation of the influence of the crystal structure of Co thin films on the X-ray magnetic linear dichroism (XMLD) spectrum. We compare XMLD spectra measured in reflection at the 3p-edges for two distinct orientations of the magnetization in the crystalline Co film with ab initio calculated spectra. The latter was computed for the face-centered cubic as well as the hexagonal-close packed crystal structures of Co. We find that the XMLD signal is strongly dependent on the magnetization direction with respect to the crystal axes as well as strongly influenced by the crystal structure. (C) 2014 AIP Publishing LLC.
8.	Mertins, H-Ch., et al. (author) Magneto-optical reflection spectroscopy on graphene/Co in the soft x-ray range 2017 In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 903 Journal article (peer-reviewed)abstract The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.
9.	Rudolf, Dennis, et al. (author) Element Selective Investigation of Spin Dynamics in Magnetic Multilayers 2015 In: Ultrafast Magnetism I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 307-309 Conference paper (peer-reviewed)abstract Our understanding of ultrafast switching processes in novel spin-based electronics depends on our detailed knowledge of interactions between spin, charge and phonons in magnetic structures. We present element-selective studies, using extreme ultraviolet (XUV) light, to gain insight into spin dynamics in exchange coupled magnetic multilayers on the femtosecond time scale.
10.	Rudolf, Dennis, et al. (author) Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current 2012 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 3, s. 1037- Journal article (peer-reviewed)abstract Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferroor antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers.
11.	Tesch, M. F., et al. (author) Magnetocrystalline anisotropy in x-ray magnetic linear dichroism at the 3 p edges of crystalline Fe thin films 2014 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:14, s. 140404- Journal article (peer-reviewed)abstract X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3 p absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the 3 p semicore states on an equal footing.
12.	Wende, H., et al. (author) Substrate-induced magnetic ordering and switching of iron porphyrin molecules 2007 In: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 6:7, s. 516-520 Journal article (peer-reviewed)abstract To realize molecular spintronic devices, it is important to externally control the magnetization of a molecular magnet. One class of materials particularly promising as building blocks for molecular electronic devices is the paramagnetic porphyrin molecule in contact with a metallic substrate. Here, we study the structural orientation and the magnetic coupling of in-situ-sublimated Fe porphyrin molecules on ferromagnetic Ni and Co films on Cu(100). Our studies involve X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments. In a combined experimental and computational study we demonstrate that owing to an indirect, superexchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferromagnetically. The Fe magnetic moment can be rotated along directions in plane as well as out of plane by a magnetization reversal of the substrate, thereby opening up an avenue for spin-dependent molecular electronics.
13.	Zusin, Dmitriy, et al. (author) 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 2018 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 97:2 Journal article (peer-reviewed)abstract 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%.
14.	Bekaert, J., et al. (author) Free surfaces recast superconductivity in few-monolayer MgB2 : Combined first-principles and ARPES demonstration 2017 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7:1 Journal article (peer-reviewed)abstract 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.
15.	Bekaert, J., et al. (author) Hydrogen-Induced High-Temperature Superconductivity in Two-Dimensional Materials : The Example of Hydrogenated Monolayer MgB2 2019 In: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 123:7 Journal article (peer-reviewed)abstract Hydrogen-based compounds under ultrahigh pressure, such as the polyhydrides H3S and LaH10, superconduct through the conventional electron-phonon coupling mechanism to attain the record critical temperatures known to date. Here we exploit the intrinsic advantages of hydrogen to strongly enhance phonon-mediated superconductivity in a completely different system, namely, a two-dimensional material with hydrogen adatoms. We find that van Hove singularities in the electronic structure, originating from atomiclike hydrogen states, lead to a strong increase of the electronic density of states at the Fermi level, and thus of the electron-phonon coupling. Additionally, the emergence of high-frequency hydrogen-related phonon modes in this system boosts the electron-phonon coupling further. As a concrete example, we demonstrate the effect of hydrogen adatoms on the superconducting properties of monolayer MgB2, by solving the fully anisotropic Eliashberg equations, in conjunction with a first-principles description of the electronic and vibrational states, and their coupling. We show that hydrogenation leads to a high critical temperature of 67 K, which can be boosted to over 100 K by biaxial tensile strain.
16.	Dakovski, Georgi L., et al. (author) Anomalous femtosecond quasiparticle dynamics of hidden order state in URu(2)Si(2) 2011 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:16, s. 161103- Journal article (peer-reviewed)abstract 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.
17.	Fan, Tingting, et al. (author) Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism 2015 In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 112:46, s. 14206-14211 Journal article (peer-reviewed)abstract We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 mu m, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N-4,N-5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.
18.	Fan, T., et al. (author) Generation of Bright Soft X-ray Harmonics with Circular Polarization for X-ray Magnetic Circular Dichroism 2016 In: 2016 Conference On Lasers And Electro-Optics (CLEO). - 9781943580118 Conference paper (peer-reviewed)abstract We present the first circularly polarized harmonics in the soft X-ray region and the physics underlying it. This source enables the first X-ray magnetic circular dichroism measurements in rare earth elements on tabletop.
19.	Frietsch, B., et al. (author) Disparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal 2015 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6 Journal article (peer-reviewed)abstract The Heisenberg-Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.
20.	Frietsch, B., et al. (author) The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals 2020 In: Science Advances. - : American Association for the Advancement of Science. - 2375-2548. ; 6:39 Journal article (peer-reviewed)abstract Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin-lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.
21.	Hosen, M. Mofazzel, et al. (author) Discovery of topological nodal-line fermionic phase in a magnetic material GdSbTe 2018 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8 Journal article (peer-reviewed)abstract Topological Dirac semimetals with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. A majority of discovered topological semimetals are nonmagnetic and conserve time reversal symmetry. Here we report the experimental discovery of an antiferromagnetic topological nodal-line semimetallic state in GdSbTe using angle-resolved photoemission spectroscopy. Our systematic study reveals the detailed electronic structure of the paramagnetic state of antiferromagnetic GdSbTe. We observe the presence of multiple Fermi surface pockets including a diamond-shape, and small circular pockets around the zone center and high symmetry X points of the Brillouin zone (BZ), respectively. Furthermore, we observe the presence of a Dirac-like state at the X point of the BZ and the effect of magnetism along the nodal-line direction. Interestingly, our experimental data show a robust Dirac-like state both below and above the magnetic transition temperature (TN = 13 K). Having a relatively high transition temperature, GdSbTe provides an archetypical platform to study the interaction between magnetism and topological states of matter.
22.	Hosen, M. Mofazzel, et al. (author) Observation of gapless Dirac surface states in ZrGeTe 2018 In: Physical Review B. - : American Physical Society. ; 97:12 Journal article (peer-reviewed)abstract 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 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.
23.	Hosen, M. Mofazzel, et al. (author) Tunability of the topological nodal-line semimetal phase in ZrSiX-type materials (X = S, Se, Te) 2017 In: Physical Review B. - 2469-9950 .- 2469-9969. ; 95:16 Journal article (peer-reviewed)abstract The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has invigorated the study of other members of this family. Here, we present a comparative electronic structure study of ZrSiX (where X = S, Se, Te) using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Our ARPES studies show that the overall electronic structure of ZrSiX materials comprises the diamond-shaped Fermi pocket, the nearly elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone center (Gamma) point, the M point, and the X point of the Brillouin zone, respectively. We also observe a small Fermi surface pocket along the M-Gamma-M direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe. Furthermore, our theoretical studies show a transition from nodal-line to nodeless gapped phase by tuning the chalcogenide from S to Te in these material systems. Our findings provide direct evidence for the tunability of the TNL phase in ZrSiX material systems by adjusting the spin-orbit coupling strength via the X anion.
24.	Jansing, C., et al. (author) X-ray natural birefringence in reflection from graphene 2016 In: PHYSICAL REVIEW B. - 2469-9950. ; 94:4 Journal article (peer-reviewed)abstract The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon 1s absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to 30 degrees, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to 17 degrees. Both graphene systems show resonantly enhanced effects at the pi* and sigma* energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of 140 degrees has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the pi* resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of pi* and sigma* bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced sigma* excitation energy and a strong tilt of 50% of the p(z) orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the p(z) orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting lambda/4 or lambda/2 plates in the design of future high-intensity light sources.
25.	Kampfrath, T., et al. (author) Terahertz spin current pulses controlled by magnetic heterostructures 2013 In: Nature Nanotechnology. - 1748-3387 .- 1748-3395. ; 8:4, s. 256-260 Journal article (peer-reviewed)abstract In spin-based electronics, information is encoded by the spin state of electron bunches(1-4). Processing this information requires the controlled transport of spin angular momentum through a solid(5,6), preferably at frequencies reaching the so far unexplored terahertz regime(7-9). Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins(10-12) from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter(13) based on the inverse spin Hall effect(14,15), which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states(16). Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters(7-9).
26.	Kampfrath, T., et al. (author) Ultrafast spin precession and transport controlled and probed with terahertz radiation 2015 In: Ultrafast Magnetism I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 324-326 Conference paper (peer-reviewed)abstract We present examples of how terahertz (THz) electromagnetic transients can be used to control spin precession in antiferromagnets (through the THz Zeeman torque) and to probe spin transport in magnetic heterostructures (through the THz inverse spin Hall effect), on femtosecond time scales.
27.	Legut, D., et al. (author) Interference Effects in T-MOKE Spectra of Fe Thin Films at the 3p Edges - Theory and Experiment 2015 In: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 127:2, s. 466-468 Journal article (peer-reviewed)abstract We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.
28.	Mertins, H. -Ch., et al. (author) Giant magneto-optical Faraday effect of graphene on Co in the soft x-ray range 2018 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:6 Journal article (peer-reviewed)abstract Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon 1s edge of single-layer graphene on Co, reaching Faraday rotation angles of 2.9 x 10(5)deg/mm. This value is of the order of those observed at the Co 3p and 2p edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned n orbitals of carbon atoms. It is induced by hybridization with the Co 3d(z)2 orbitals while carbon a bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of 0.14 mu(B) is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon p(z) and metal 3d orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co 3d(z)( )(2)orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with 3d(xy) and 3d(yz) orbitals of neighboring Co atoms forming tilted p(z) bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.
29.	Modin, Anders, et al. (author) 5 f-Shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations 2015 In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:31 Journal article (peer-reviewed)abstract Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5f-shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.
30.	Neupane, Madhab, et al. (author) Observation of topological nodal fermion semimetal phase in ZrSiS 2016 In: PHYSICAL REVIEW B. - 2469-9950. ; 93:20 Journal article (peer-reviewed)abstract Unveiling new topological phases of matter is one of the current objectives in condensed matter physics. Recent experimental discoveries of Dirac and Weyl semimetals prompt the search for other exotic phases of matter. Here we present a systematic angle-resolved photoemission spectroscopy study of ZrSiS, a prime topological nodal semimetal candidate. Our wider Brillouin zone (BZ) mapping shows multiple Fermi surface pockets such as the diamond-shaped Fermi surface, elliptical-shaped Fermi surface, and a small electron pocket encircling at the zone center (Gamma) point, the M point, and the X point of the BZ, respectively. We experimentally establish the spinless nodal fermion semimetal phase in ZrSiS, which is supported by our first-principles calculations. Our findings evidence that the ZrSiS-type of material family is a new platform on which to explore exotic states of quantum matter; these materials are expected to provide an avenue for engineering two-dimensional topological insulator systems.
31.	Oppeneer, Peter M., et al. (author) A novel spectroscopy : X-ray magneto-optical spectroscopy of buried antiferromagnetic films-Theory and experiment 2002 In: Highlights of the Leibniz-Institute Dresden. ; , s. 7-10 Research review (pop. science, debate, etc.)abstract X-ray magneto-optical (MO) spectroscopies have developed into extremelypowerful tools for the investigation of both fundamental and applied magnetism.In particular the x-ray magnetic circular dichroism (XMCD) has become widelyused during the last decade. A disadvantage of the XMCD and related techniquesis that these can be applied only to study ferromagnetic (FM) materials. Thereexists currently an increasing interest in antiferromagnetic (AFM) materialswhich call for different x-ray spectroscopic techniques. A novel, versatilemagneto-x-ray spectroscopy has been developed, that can be applied to AFM’s.This magneto-x-ray spectroscopy may be measured in reflection and is relatedto the x-ray Voigt effect and the x-ray magnetic linear dichroism (XMLD), whichboth involve a transmission geometry. Using the newly developed technique,element-selective x-ray MO spectra of AFM materials have been measured forthe first time in reflection. The XMLD-type reflection spectroscopy has beenapplied to buried AFM layers, which was demonstrated by spectra obtained fromexchange-biased microstructures of current technological importance: NiO/Coand NiMn/Co, containing the insulating AFM NiO, and the metallic AFM NiMn,respectively. The measured spectrum provides information about the exchangesplitd-density of states of the AFM atom.
32.	Arruda, Lucas M., et al. (author) Modifying the Magnetic Anisotropy of an Iron Porphyrin Molecule by an on-Surface Ring-Closure Reaction 2019 In: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 123:23, s. 14547-14555 Journal article (peer-reviewed)abstract 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.
33.	Arruda, Lucas M., et al. (author) Surface-orientation- and ligand-dependent quenching of the spin magnetic moment of Co porphyrins adsorbed on Cu substrates 2020 In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 22:22, s. 12688-12696 Journal article (peer-reviewed)abstract Porphyrin molecules are particularly interesting candidates for spintronic applications due to their bonding flexibility, which allows to modify their properties substantially by the addition or transformation of ligands. Here, we investigate the electronic and magnetic properties of cobalt octaethylporphyrin (CoOEP), deposited on copper substrates with two distinct crystallographic surface orientations, Cu(100) and Cu(111), with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). A significant magnetic moment is present in the Co ions of the molecules deposited on Cu(100), but it is completely quenched on Cu(111). Heating the molecules on both substrates to 500 K induces a ring-closure reaction with cobalt tetrabenzoporphyrin (CoTBP) as reaction product. In these molecules, the magnetic moment is quenched on both surfaces. Our XMCD and XAS measurements suggest that the filling of the dz(2)orbital leads to a non-integer valence state and causes the quench of the spin moments on all samples except CoOEP/Cu(100), where the molecular conformation induces variations to the ligand field that lift the quench. We further employ density functional theory calculations, supplemented with on-site Coulomb correlations (DFT+U), to study the adsorption of these spin-bearing molecules on the Cu substrates. Our calculations show that charge transfer from the Cu substrates as well as charge redistribution within the Co 3d orbitals lead to the filling of the Co minority spin dz(2)orbital, causing a 'turning off' of the exchange splitting and quenching of the spin moment at the Co magnetic centers. Our investigations suggest that, by this mechanism, molecule-substrate interactions can be used to control the quenching of the magnetic moments of the adsorbed molecules.
34.	Bekaert, J., et al. (author) Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations : The case of FeB4 2018 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:1 Journal article (peer-reviewed)abstract 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.
35.	Bekaert, J., et al. (author) Evolution of multigap superconductivity in the atomically thin limit : Strain-enhanced three-gap superconductivity in monolayer MgB2 2017 In: Physical Review B. - 2469-9950 .- 2469-9969. ; 96 Journal article (peer-reviewed)
36.	Butorin, Sergei M., et al. (author) Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides 2016 In: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 88:8, s. 4169-4173 Journal article (peer-reviewed)abstract 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.
37.	Chylarecka, D., et al. (author) Indirect Magnetic Coupling of Manganese Porphyrin to a Ferromagnetic Cobalt Substrate 2011 In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:4, s. 1295-1301 Journal article (peer-reviewed)abstract 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.
38.	Ciuciulkaite, Agne, MSc, 1991-, et al. (author) Magnetic and all-optical switching properties of amorphous TbxCo100-x alloys 2020 In: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 4:10 Journal article (peer-reviewed)abstract Amorphous TbxCo100-x magnetic alloys exhibit a list of intriguing properties, such as perpendicular magnetic anisotropy, high magneto-optical activity, and magnetization switching using ultrashort optical pulses. Varying the Tb:Co ratio in these alloys allows for tuning properties such as the saturation magnetic moment, coercive field, and the performance of light-induced magnetization switching. In this paper, we investigate the magnetic, optical, and magneto-optical properties of various TbxCo100-x thin-film alloy compositions. We report on the effect the choice of different seeding layers has on the structural and magnetic properties of TbxCo100-x layers. We also demonstrate that for a range of alloys, deposited on fused silica substrates, with Tb content of 24-30 at. %, helicity-dependent all-optical switching of magnetization can be achieved, albeit in a multishot framework. We explain this property to arise from the helicity-dependent laser-induced magnetization on the Co sublattice due to the inverse Faraday effect. Our paper provides an insight into material aspects for future potential hybrid magnetoplasmonic TbCo-based architectures.
39.	Conradson, Steven D., et al. (author) Possible Demonstration of a Polaronic Bose-Einstein(-Mott) Condensate in UO2(+x) by Ultrafast THz Spectroscopy and Microwave Dissipation 2015 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5 Journal article (peer-reviewed)abstract 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.
40.	Dhakal, Gyanendra, et al. (author) Observation of anisotropic Dirac cones in the topological material Ti2Te2P 2022 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:12 Journal article (peer-reviewed)abstract Anisotropic bulk Dirac (or Weyl) cones in three-dimensional systems have recently gained intense research interest as they are examples of materials with tilted Dirac (or Weyl) cones indicating the violation of Lorentz invariance. In contrast, the studies on anisotropic surface Dirac cones in topological materials which contribute to anisotropic carrier mobility have been limited. By employing angle-resolved photoemission spectroscopy and first-principles calculations, we reveal the anisotropic surface Dirac dispersion in a tetradymite material Ti2Te2P on the (001) plane of the Brillouin zone. We observe quasielliptical Fermi pockets at the (M) over bar point of the Brillouin zone forming the anisotropic surface Dirac cones. Our calculations of the Z(2) indices confirm that the system is topologically nontrivial with multiple topological phases in the same material. In addition, the observed nodal-line-like feature formed by bulk bands makes this system topologically rich.
41.	Durakiewicz, T., et al. (author) Observation of a kink in the dispersion of f-electrons 2008 In: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 84:3, s. 37003- Journal article (peer-reviewed)abstract 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. (author) A Closer Look Into Magnetism : Opportunities With Synchrotron Radiation 2009 In: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 45:1, s. 15-57 Research review (peer-reviewed)abstract 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.	Galanakis, I, et al. (author) Sign reversal of the orbital moment via ligand states - art. no. 172405 2001 In: PHYSICAL REVIEW B. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 6317:17, s. 2405- Journal article (peer-reviewed)abstract It is demonstrated that the coupling between spin and orbital moments in magnetic systems may-for certain materials-be reversed from antiparallel to parallel, via the influence of ligand states. This is exemplified by first-principles calculations for an
44.	Galanakis, I, et al. (author) Tuning the orbital moment in transition metal compounds using ligand states 2001 In: JOURNAL OF PHYSICS-CONDENSED MATTER. - : IOP PUBLISHING LTD. - 0953-8984. ; 13:20, s. 4553-4566 Journal article (peer-reviewed)abstract The influence of ligand states on the orbital magnetism of a 3d atom in a ferromagnetic compound has been studied using an ab initio technique. It is shown, using VAu4, MnAu4 and VPt3 as examples, that the large spin-orbit coupling of the 5d atom is respo
45.	Gang, Seung-gi, et al. (author) Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation 2018 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 97:6 Journal article (peer-reviewed)abstract 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.
46.	Hecker, Michael, et al. (author) Layer-selective magnetization reversal in GMR layer systems 2004 In: Physica B. ; 345, s. 173- Journal article (peer-reviewed)
47.	Hecker, Michael, et al. (author) Polarized soft-X-ray reflection spectroscopy of giant-magnetoresistive Co/Cu multilayers 2005 In: Physical Review B. ; :72, s. 054437- Journal article (peer-reviewed)
48.	Hecker, Michael, et al. (author) Soft X-ray magnetic reflection spectroscopy at the 3p absorption edges of thin Fe films 2005 In: J. Electron Spectrosc. Relat. Phen.. ; 144, s. 881- Journal article (peer-reviewed)
49.	Hosen, M. Mofazzel, et al. (author) Distinct multiple fermionic states in a single topological metal 2018 In: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9 Journal article (peer-reviewed)abstract Among the quantum materials that have recently gained interest are the topological insulators, wherein symmetry-protected surface states cross in reciprocal space, and the Dirac nodal-line semimetals, where bulk bands touch along a line in k-space. However, the existence of multiple fermion phases in a single material has not been verified yet. Using angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations, we systematically study the metallic material Hf2Te2P and discover properties, which are unique in a single topological quantum material. We experimentally observe weak topological insulator surface states and our calculations suggest additional strong topological insulator surface states. Our first-principles calculations reveal a one-dimensional Dirac crossing—the surface Dirac-node arc—along a high-symmetry direction which is confirmed by our ARPES measurements. This novel state originates from the surface bands of a weak topological insulator and is therefore distinct from the well-known Fermi arcs in semimetals.
50.	John, R., et al. (author) Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses 2017 In: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 7 Journal article (peer-reviewed)abstract Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

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