European XFEL Publication Database: Articles
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Fermi condensation near van Hove singularities within the Hubbard Model on the triangular lattice
http://xfel.tind.io/record/1733
The proximity of the Fermi surface to van Hove singularities drastically enhances interaction effects and leads to essentially new physics. In this work we address the formation of flat bands (“Fermi condensation”) within the Hubbard model on the triangular lattice and provide a detailed analysis from an analytical and numerical perspective. To describe the effect we consider both weak-coupling and strong-coupling approaches, namely the renormalization group and dual fermion methods. It is shown that the band flattening is driven by correlations and is well pronounced even at sufficiently high temperatures, of the order of 0.1–0.2 of the hopping parameter. The effect can therefore be probed in experiments with ultracold fermions in optical lattices.Yudin, DmitryTue, 17 Jul 2018 08:47:28 GMThttp://xfel.tind.io/record/1733American Physical Society2014Measuring the Dzyaloshinskii–Moriya interaction in a weak ferromagnet
http://xfel.tind.io/record/1732
Magnetism—the spontaneous alignment of atomic moments in a material—is driven by quantum mechanical exchange interactions that operate over interatomic distances. Some magnetic interactions cause, or are caused by, a twisting of arrangements of atoms. This can lead to the magnetoelectric effect, predicted to play a prominent role in future technology, and to the phenomenon of weak ferromagnetism, governed by the so-called Dzyaloshinskii–Moriya interaction. Here we determine the sign of the latter interaction in iron borate $(FeBO_{3})$ by using synchrotron radiation. We present a novel experimental technique based on the interference between two X-ray scattering processes, where one acts as a reference wave. Our experimental results are validated by state-of-the-art $\it{ab}$ $\it{initio}$ calculations. Together, our experimental and theoretical approaches are expected to open up new possibilities for exploring, modelling and exploiting novel magnetic and magnetoelectric materials.Dmitrienko, V. E.Tue, 17 Jul 2018 08:40:42 GMThttp://xfel.tind.io/record/1732Nature2014First-principles modeling of magnetic excitations in $Mn_{12}$
http://xfel.tind.io/record/1731
We have developed a fully microscopic theory of magnetic properties of the prototype molecular magnet $Mn_{12}$. First, the intramolecular magnetic properties have been studied by means of first-principles density functional based methods, with local correlation effects being taken into account within the local density approximation plus $U$ (LDA+$U$) approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results we predict a distortion of ferrimagnetic ordering due to DM interactions. Further, we use an exact diagonalization approach allowing one to work with as large a Hilbert space dimension as $10^{8}$ without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.Mazurenko, V. V.Tue, 17 Jul 2018 08:34:42 GMThttp://xfel.tind.io/record/1731American Physical Society2014Magnetic anisotropy energy and effective exchange interactions in Co intercalated graphene on Ir(1 1 1)
http://xfel.tind.io/record/1730
The electronic structure, magnetic moments, effective exchange interaction parameter and the magnetic anisotropy energy of [monolayer Co]/Ir(1 1 1) and Co intercalated graphene on Ir(1 1 1) are studied making use of the first-principles density functional theory calculations. A large positive magnetic anisotropy of 1.24 meV/Co is found for [monolayer Co]/Ir(1 1 1), and a high Curie temperature of 1190 K is estimated. These findings show the Co/Ir(1 1 1) system is a promising candidate for perpendicular ultra-high density magnetic recording applications. The magnetic moments, exchange interactions and the magnetic anisotropy are strongly affected by graphene. Reduction of the magnetic anisotropy and the Curie temperature are found for graphene/[monolayer Co]/Ir(1 1 1). It is shown that for graphene placed in the hollow-hexagonal positions over the monolayer Co, the magnetic anisotropy remains positive, while for the placements with one of the C atoms on the top of Co it becomes negative. These findings may be important for assessing the use of graphene for magnetic recording and magnetoelectronic applications.Shick, A BTue, 17 Jul 2018 08:18:04 GMThttp://xfel.tind.io/record/1730IOP Publishing2014Collective charge excitations of strongly correlated electrons, vertex corrections, and gauge invariance
http://xfel.tind.io/record/1729
We consider the collective, long-wavelength charge excitations in correlated media in presence of short- and long-range forces. As an example for the case of a short-range interaction, we examine the two-dimensional Hubbard model within dynamical mean-field theory (DMFT). It is shown explicitly that the DMFT susceptibility including vertex corrections respects the Ward identity and yields a manifestly gauge-invariant response in finite dimensions. For computing the susceptibility, we use a different expression and establish its formal equivalence to the standard DMFT formula. It allows for a more stable analytical continuation. We find a zero-sound mode expected for short-range forces. The relation between the vertex corrections, gauge invariance, and the appearance of the collective modes is discussed. Long-range forces are treated within extended dynamical mean-field theory. In order to obtain a gauge-invariant response, it is necessary to additionally incorporate some nonlocal vertex corrections into the polarization. In doing so, we obtain plasmons in the three-dimensional Hubbard model. The plasma frequency is determined by the (single-particle) density distribution as a consequence of gauge invariance. We compare this result with the plasma frequency extracted from the analytical continuation of the susceptibility. It is in good agreement with the prediction from the gauge-invariance condition.Hafermann, HartmutTue, 17 Jul 2018 08:14:13 GMThttp://xfel.tind.io/record/1729American Physical Society2014Plasmons in strongly correlated systems: spectral weight transfer and renormalized dispersion
http://xfel.tind.io/record/1728
We study the charge-density dynamics within the two-dimensional extended Hubbard model in the presence of long-range Coulomb interaction across the metal-insulator transition point. To take into account strong correlations we start from self-consistent extended dynamical mean-field theory and include nonlocal dynamical vertex corrections through a ladder approximation to the polarization operator. This is necessary to fulfill charge conservation and to describe plasmons in the correlated state. The calculated plasmon spectra are qualitatively different from those in the random-phase approximation: they exhibit a spectral density transfer and a renormalized dispersion with enhanced deviation from the canonical $√q$ behavior. Both features are reminiscent of interaction induced changes found in single-electron spectra of strongly correlated systems.van Loon, E. G. C. P.Tue, 17 Jul 2018 08:08:54 GMThttp://xfel.tind.io/record/1728American Physical Society2014Superconductivity, antiferromagnetism, and phase separation in the two-dimensional Hubbard model: A dual-fermion approach
http://xfel.tind.io/record/1727
The dual-fermion approach offers a way to perform diagrammatic expansion around the dynamical mean field theory. Using this formalism, the influence of antiferromagnetic fluctuations on the self-energy is taken into account through ladder-type diagrams in the particle-hole channel. The resulting phase diagram for the (quasi-)two-dimensional Hubbard model exhibits antiferromagnetism and $d$-wave superconductivity. Furthermore, a uniform charge instability, i.e., phase separation, is obtained in the low-doping regime around the Mott insulator. We also examine spin/charge density wave fluctuations including $d$-wave symmetry. The model exhibits a tendency towards an unconventional charge density wave, but no divergence of the susceptibility is found.Otsuki, JunyaTue, 17 Jul 2018 08:05:11 GMThttp://xfel.tind.io/record/1727American Physical Society2014Beyond extended dynamical mean-field theory: Dual boson approach to the two-dimensional extended Hubbard model
http://xfel.tind.io/record/1726
The dual boson approach [Rubtsov, Katsnelson, and Lichtenstein, Ann. Phys. 327, 1320 (2012)] provides a means to construct a diagrammatic expansion around the extended dynamical mean-field theory (EDMFT). In this paper, we present the numerical implementation of the approach and apply it to the extended Hubbard model with nearest-neighbor interaction $V$. We calculate the EDMFT phase diagram and study the effect of diagrams beyond EDMFT on the transition to the charge-ordered phase. Including diagrammatic corrections to the EDMFT polarization shifts the EDMFT phase boundary to lower values of $V$. The approach interpolates between the random phase approximation in the weak coupling limit and EDMFT for strong coupling. Neglecting vertex corrections leads to results reminiscent of the EDMFT+$GW$ approximation. We, however, find significant deviations from the dual boson results already for small values of the interaction, emphasizing the crucial importance of fermion-boson vertex corrections.van Loon, Erik G. C. P.Tue, 17 Jul 2018 08:00:31 GMThttp://xfel.tind.io/record/1726American Physical Society2014Probing of valley polarization in graphene via optical second-harmonic generation
http://xfel.tind.io/record/1725
Valley polarization in graphene breaks inversion symmetry and therefore leads to second-harmonic generation. We present a complete theory of this effect within a single-particle approximation. It is shown that this may be a sensitive tool to measure the valley polarization created, e.g., by polarized light and, thus, can be used for the development of ultrafast valleytronics in graphene.Wehling, T. O.Tue, 17 Jul 2018 07:56:52 GMThttp://xfel.tind.io/record/1725American Physical Society2015Structural evolution of colloidal crystal films in the process of melting revealed by Bragg peak analysis
http://xfel.tind.io/record/1724
In situ X-ray diffraction studies of structural evolution of colloidal crystal films formed by polystyrene spherical particles upon incremental heating are reported. The Bragg peak parameters, such as peak position, integrated intensity, and radial and azimuthal widths were analyzed as a function of temperature. A quantitative study of colloidal crystal lattice distortions and mosaic spread as a function of temperature was carried out using Williamson–Hall plots based on mosaic block model. The temperature dependence of the diameter of polystyrene particles was obtained from the analysis of Bragg peaks, and the form factor contribution extracted from the diffraction patterns. Four stages of structural evolution in a colloidal crystal upon heating were identified. Based on this analysis, a model of the heating and melting process in the colloidal crystal film is suggested.Sulyanova, Elena A.Tue, 17 Jul 2018 07:52:57 GMThttp://xfel.tind.io/record/1724American Chemical Society2015Exchange parameters of strongly correlated materials: Extraction from spin-polarized density functional theory plus dynamical mean-field theory
http://xfel.tind.io/record/1723
In this paper we present an accurate numerical scheme for extracting interatomic exchange parameters $(J_{ij})$ of strongly correlated systems, based on first-principles full-potential electronic structure theory. The electronic structure is modeled with the help of a full-potential linear muffin-tin orbital method. The effects of strong electron correlations are considered within the charge self-consistent density functional theory plus dynamical mean-field theory. The exchange parameters are then extracted using the magnetic force theorem; hence all the calculations are performed within a single computational framework. The method allows us to investigate how the $(J_{ij})$ parameters are affected by dynamical electron correlations. In addition to describing the formalism and details of the implementation, we also present magnetic properties of a few commonly discussed systems, characterized by different degrees of electron localization. In bcc Fe, treated as a moderately correlated metal, we found a minor renormalization of the $(J_{ij})$ interactions once the dynamical correlations are introduced. However, generally, if the magnetic coupling has several competing contributions from different orbitals, the redistribution of the spectral weight and changes in the exchange splitting of these states can lead to a dramatic modification of the total interaction parameter. In NiO we found that both static and dynamical mean-field results provide an adequate description of the exchange interactions, which is somewhat surprising given the fact that these two methods result in quite different electronic structures. By employing the Hubbard-I approximation for the treatment of the $4f$ states in hcp Gd we reproduce the experimentally observed multiplet structure. The calculated exchange parameters result in being rather close to the ones obtained by treating the $4f$ electrons as noninteracting core states.Kvashnin, Y. O.Tue, 17 Jul 2018 07:47:09 GMThttp://xfel.tind.io/record/1723American Physical Society2015Phonon-pump extreme-ultraviolet-photoemission probe in graphene: Anomalous heating of Dirac carriers by lattice deformation
http://xfel.tind.io/record/1722
We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane $E_1u$ lattice vibration at $6.3 \mu m$. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme-ultraviolet (XUV) pulses, we measure the response of the Dirac electrons near the $K$ point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predict dramatic band structure changes when the $E_{1u}$ vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.Gierz, IsabellaFri, 13 Jul 2018 14:49:38 GMThttp://xfel.tind.io/record/1722American Physical Society2015From Hubbard bands to spin-polaron excitations in the doped Mott material $Na_{x}CoO_{2}$
http://xfel.tind.io/record/1721
We investigate the excitation spectrum of strongly correlated sodium cobaltate within a realistic many-body description $ \it {beyond}$ dynamical mean-field theory (DMFT). At lower doping around $x=0.3$, rather close to Mott-critical half-filling, the single-particle spectral function of $Na_{x}CoO_{2}$ displays an upper Hubbard band which is captured within DMFT. Momentum-dependent self-energy effects beyond DMFT become dominant at higher doping. Around a doping level of $x\sim 0.67$, the incoherent excitations give way to finite-energy spin-polaron excitations in close agreement with optics experiments. These excitations are a direct consequence of the formation of bound states between quasiparticles and paramagnons in the proximity of in-plane ferromagnetic ordering.Wilhelm, AljoschaFri, 13 Jul 2018 14:42:18 GMThttp://xfel.tind.io/record/1721American Physical Society2015Theoretical study of electronic damage in single-particle imaging experiments at x-ray free-electron lasers for pulse durations from 0.1 to 10 fs
http://xfel.tind.io/record/1720
X-ray free-electron lasers (XFELs) may allow us to employ the single-particle imaging (SPI) method to determine the structure of macromolecules that do not form stable crystals. Ultrashort pulses of 10 fs and less allow us to outrun complete disintegration by Coulomb explosion and minimize radiation damage due to nuclear motion, but electronic damage is still present. The major contribution to the electronic damage comes from the plasma generated in the sample that is strongly dependent on the amount of Auger ionization. Since the Auger process has a characteristic time scale on the order of femtoseconds, one may expect that its contribution will be significantly reduced for attosecond pulses. Here we study the effect of electronic damage on the SPI at pulse durations from 0.1 to 10 fs and in a large range of XFEL fluences to determine optimal conditions for imaging of biological samples. We analyzed the contribution of different electronic excitation processes and found that at fluences higher than $10^{13}$–$10^{15} photons/ $$\mu m^{2}$ (depending on the photon energy and pulse duration) the diffracted signal saturates and does not increase further. A significant gain in the signal is obtained by reducing the pulse duration from 10 to 1 fs. Pulses below a duration of 1 fs do not give a significant gain in the scattering signal in comparison with 1-fs pulses. We also study the limits imposed on SPI by Compton scattering.Gorobtsov, O. Yu.Fri, 13 Jul 2018 14:26:15 GMThttp://xfel.tind.io/record/1720American Physical Society2015Long-lived nonequilibrium states in the Hubbard model with an electric field
http://xfel.tind.io/record/1719
We study the single-band Hubbard model in the presence of a large spatially uniform electric field out of equilibrium. Using the Keldysh nonequilibrium formalism, we solve the problem using perturbation theory in the Coulomb interaction $U$. We present numerical results for the charge current, the total energy of the system, and the double occupancy on an infinite-dimensional hypercubic lattice with nearest-neighbor hopping. The system is isolated from an external bath and is in the paramagnetic state. We show that an electric field pulse can drive the system to a steady nonequilibrium state, which does not evolve into a thermal state. We compare results obtained within second-order perturbation theory (SOPT), self-consistent SOPT, and iterated perturbation theory (IPT). We also discuss the importance of initial conditions for a system which is not coupled to an external bath.Joura, Alexander V.Fri, 13 Jul 2018 12:40:51 GMThttp://xfel.tind.io/record/1719American Physical Society2015Shifting the voltage drop in electron transport through a single molecule
http://xfel.tind.io/record/1718
A Mn-porphyrin was contacted on Au(111) in a low-temperature scanning tunneling microscope (STM). Differential conductance spectra show a zero-bias resonance that is due to an underscreened Kondo effect according to many-body calculations. When the Mn center is contacted by the STM tip, the spectrum appears to invert along the voltage axis. A drastic change in the electrostatic potential of the molecule involving a small geometric relaxation is found to cause this observation.Karan, SujoyFri, 13 Jul 2018 12:36:16 GMThttp://xfel.tind.io/record/1718American Physical Society2015Thermodynamic consistency of the charge response in dynamical mean-field based approaches
http://xfel.tind.io/record/1717
We consider the thermodynamic consistency of the charge response function in the (extended) Hubbard model. In dynamical mean-field theory (DMFT), thermodynamic consistency is preserved. We prove that the static, homogeneous DMFT susceptibility is consistent as long as vertex corrections obtained from the two-particle impurity correlation function are included. In the presence of a nonlocal interaction, the problem may be treated within extended DMFT (EDMFT), or its diagrammatic extension, the dual boson approach. We show that here maintaining thermodynamic consistency requires knowledge of three- and four-particle impurity correlation functions, which are typically neglected. Nevertheless, the dual boson approximation to the response is remarkably close to consistency. This holds even when two-particle vertex corrections are neglected. EDMFT is consistent only in the strongly correlated regime and near half-filling, where the physics is predominantly local.van Loon, Erik G. C. P.Fri, 13 Jul 2018 12:07:26 GMThttp://xfel.tind.io/record/1717American Physical Society2015Electronic structure and core-level spectra of light actinide dioxides in the dynamical mean-field theory
http://xfel.tind.io/record/1716
The local-density approximation combined with the dynamical mean-field theory (LDA+DMFT) is applied to the paramagnetic phase of light actinide dioxides: $UO_{2},N_{p}O_{2}, and PuO_{2}$. The calculated band gaps and the valence-band electronic structure are in very good agreement with the optical absorption experiments as well as with the photoemission spectra. The hybridization of the actinide $5f$ shell with the 2p states of oxygen is found to be relatively large; it increases the filling of the $5f$ orbitals from the nominal ionic configurations with two, three, and four electrons to nearly half-integer values 2.5, 3.4, and 4.4. The large hybridization leaves an imprint also on the core-level photoemission spectra in the form of satellite peaks. It is demonstrated that these satellites are accurately reproduced by the LDA+DMFT calculations.Kolorenč, JindřichFri, 13 Jul 2018 11:55:20 GMThttp://xfel.tind.io/record/1716American Physical Society2015Magnetic interactions in strongly correlated systems: Spin and orbital contributions
http://xfel.tind.io/record/1715
We present a technique to map an electronic model with local interactions (a generalized multi-orbital Hubbard model) onto an effective model of interacting ${ \it classical}$ spins, by requiring that the thermodynamic potentials associated to spin rotations in the two systems are equivalent up to second order in the rotation angles, when the electronic system is in a symmetry-broken phase. This allows to determine the parameters of relativistic and non-relativistic magnetic interactions in the effective spin model in terms of equilibrium Green’s functions of the electronic model. The Hamiltonian of the electronic system includes, in addition to the non-relativistic part, relativistic single-particle terms such as the Zeeman coupling to an external magnetic field, spin–orbit coupling, and arbitrary magnetic anisotropies; the orbital degrees of freedom of the electrons are explicitly taken into account. We determine the complete relativistic exchange tensors, accounting for anisotropic exchange, Dzyaloshinskii–Moriya interactions, as well as additional non-diagonal symmetric terms (which may include dipole–dipole interaction). The expressions of all these magnetic interactions are determined in a unified framework, including previously disregarded features such as the vertices of two-particle Green’s functions and non-local self-energies. We do not assume any smallness in spin–orbit coupling, so our treatment is in this sense exact. Finally, we show how to distinguish and address separately the spin, orbital and spin–orbital contributions to magnetism, providing expressions that can be computed within a tight-binding Dynamical Mean Field Theory.Secchi, A.Fri, 13 Jul 2018 11:20:30 GMThttp://xfel.tind.io/record/1715Elsevier2015Mechanisms of finite-temperature magnetism in the three-dimensional Hubbard model
http://xfel.tind.io/record/1714
We examine the nature of the transition to the antiferromagnetically ordered state in the half-filled three-dimensional Hubbard model using the dual-fermion multiscale approach. Consistent with analytics, in the weak-coupling regime we find that spin-flip excitations across the Fermi surface are important and that the strong-coupling regime is described by Heisenberg physics. In the intermediate-interaction, strong-correlation regime we find aspects of both local and nonlocal correlations. We analyze the critical exponents of the transition in the strong-coupling regime and find them to be consistent with Heisenberg physics down to an interaction of $U/t=10$.Hirschmeier, DanielFri, 13 Jul 2018 11:11:42 GMThttp://xfel.tind.io/record/1714American Physical Society2015Racah materials: role of atomic multiplets in intermediate valence systems
http://xfel.tind.io/record/1713
We address the long-standing mystery of the nonmagnetic insulating state of the intermediate valence compound $SmB_{6}$. Within a combination of the local density approximation (LDA) and an exact diagonalization (ED) of an effective discrete Anderson impurity model, the intermediate valence ground state with the f-shell occupation $〈n_{4f}〉 = 5.6$ is found for the Sm atom in SmB6. This ground state is a singlet, and the first excited triplet state $\sim3$ meV higher in the energy. $SmB_{6}$ is a narrow band insulator already in LDA, with the direct band gap of $\sim10$ meV. The electron correlations increase the band gap which now becomes indirect. Thus, the many-body effects are relevant to form the indirect band gap, crucial for the idea of “topological Kondo insulator" in $SmB_{6}$. Also, an actinide analog PuB6 is considered, and the intermediate valence singlet ground state is found for the Pu atom. We propose that $[Sm, Pu]B_{6}$ belong to a new class of the intermediate valence materials with the multi-orbital “Kondo-like" singlet ground-state. Crucial role of complex spin-orbital $f ^{n}–f ^{n+1}$ multiplet structure differently hybridized with ligand states in such Racah materials is discussed.Shick, A. B.Fri, 13 Jul 2018 09:51:02 GMThttp://xfel.tind.io/record/1713Nature2015Layer-resolved magnetic exchange interactions of surfaces of late $3d$ elements: Effects of electronic correlations
http://xfel.tind.io/record/1712
We present the results of an $\it {ab}$ $\it {initio}$ study of the magnetic properties of Fe, Co, and Ni surfaces. In particular, we discuss their electronic structure and magnetic exchange interactions $(J_{ij})$, as obtained by means of a combination of density functional theory and dynamical mean-field theory. All studied systems have a pronounced tendency to ferromagnetism both for bulk and surface atoms. The presence of narrowband surface states is shown to enhance the magnetic moment as well as the exchange couplings. The most interesting results were obtained for the Fe surface where the atoms have a tendency to couple antiferromagnetically with each other. This interaction is relatively small when compared to interlayer ferromagnetic interaction, and it depends strongly on the lattice parameter. Local correlation effects are shown to lead to strong changes of the overall shape of the spectral functions. However, they seem not to play a decisive role in the overall picture of magnetic couplings studied here. We have also investigated the influence of correlations on the spin and orbital moments of bulklike and surface atoms. We found that dynamical correlations in general lead to enhanced values of the orbital moment.Keshavarz, S.Fri, 13 Jul 2018 09:42:13 GMThttp://xfel.tind.io/record/1712American Physical Society2015Tuning emergent magnetism in a Hund's impurity
http://xfel.tind.io/record/1711
The recently proposed concept of a Hund's metal—a metal in which electron correlations are driven by Hund's rule coupling—can be used to explain the exotic magnetic and electronic behaviour of strongly correlated electron systems of multi-orbital metallic materials. Tuning the abundance of parameters that determine these materials is, however, experimentally challenging. Here, we show that the basic constituent of a Hund's metal—a Hund's impurity—can be realized using a single iron atom adsorbed on a platinum surface, a system that comprises a magnetic moment in the presence of strong charge fluctuations. The magnetic properties can be controlled by using the tip of a scanning tunnelling microscope to change the binding site and degree of hydrogenation of the $3d$ transition-metal atom. We are able to experimentally explore a regime of four almost degenerate energy scales (Zeeman energy, temperature, Kondo temperature and magnetic anisotropy) and probe the magnetic excitations with the microscope tip. The regime of our Hund's impurity can be tuned from an emergent magnetic moment to a multi-orbital Kondo state, and the system could be used to test predictions of advanced many-body theories for non-Fermi liquids in quantum magnets or unconventional superconductors.Khajetoorians, A. A.Fri, 13 Jul 2018 09:37:02 GMThttp://xfel.tind.io/record/1711Nature2015Sorting algorithms for single-particle imaging experiments at X-ray free-electron lasers
http://xfel.tind.io/record/1710
Modern X-ray free-electron lasers (XFELs) operating at high repetition rates produce a tremendous amount of data. It is a great challenge to classify this information and reduce the initial data set to a manageable size for further analysis. Here an approach for classification of diffraction patterns measured in prototypical diffract-and-destroy single-particle imaging experiments at XFELs is presented. It is proposed that the data are classified on the basis of a set of parameters that take into account the underlying diffraction physics and specific relations between the real-space structure of a particle and its reciprocal-space intensity distribution. The approach is demonstrated by applying principal component analysis and support vector machine algorithms to the simulated and measured X-ray data sets.Bobkov, S. A.Fri, 13 Jul 2018 09:29:38 GMThttp://xfel.tind.io/record/1710International Union of Crystallography2015Comparing XMCD and DFT with STM spin excitation spectroscopy for Fe and Co adatoms on $Cu_{2} N / Cu (100)$
http://xfel.tind.io/record/1709
We report on the magnetic properties of Fe and Co adatoms on a $Cu_{2} N / Cu (100)−c (2×2)$ surface investigated by x-ray magnetic dichroism measurements and density functional theory (DFT) calculations including the local coulomb interaction. We compare these results with properties formerly deduced from STM spin excitation spectroscopy (SES) performed on the individual adatoms. In particular we focus on the values of the local magnetic moments determined by XMCD compared to the expectation values derived from the description of the SES data. The angular dependence of the projected magnetic moments along the magnetic field, as measured by XMCD, can be understood on the basis of the SES Hamiltonian. In agreement with DFT, the XMCD measurements show large orbital contributions to the total magnetic moment for both magnetic adatoms.Etzkorn, M.Fri, 13 Jul 2018 09:18:07 GMThttp://xfel.tind.io/record/1709American Physical Society2015