Papers by W. Von Der Linden
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1999
The subtraction of the X-ray background in a PIXE spectrum has been the subject of many investiga... more The subtraction of the X-ray background in a PIXE spectrum has been the subject of many investigations and dierent techniques have been developed. These techniques vary from ®ltering to ®tting polynomial functions. The promising Bayesian Statistics technique has been used in this study to eliminate the background from the spectrum in a rigorous and self-consistent manner. We compare the results of the Bayesian background subtraction method to that obtained by stripping and the``rolling ball''; method.
Multimodal structures in the sampling density (e.g. two competing phases) can be a serious proble... more Multimodal structures in the sampling density (e.g. two competing phases) can be a serious problem for traditional Markov Chain Monte Carlo (MCMC), because correct sampling of the different structures can only be guaranteed for infinite sampling time. Samples may not decouple from the initial configuration for a long time and autocorrelation times may be hard to determine. We analyze a suitable modification [1] of the simulated tempering idea [2], which has orders of magnitude smaller autocorrelation times for multimodal sampling densities and which samples all peaks of multimodal structures according to their weight. The method generates exact, i. e. uncorrelated, samples and thus gives access to reliable error estimates. Exact tempering is applicable to arbitrary (continuous or discreet) sampling densities and moreover presents a possibility to calculate integrals over the density (e.g. the partition function for the Boltzmann distribution), which are not accessible by usual MCMC.
Journal of Electron Spectroscopy and Related Phenomena, 1997
High-resolution electron energy loss spectroscopy (HREELS) has been used to study stretch vibrati... more High-resolution electron energy loss spectroscopy (HREELS) has been used to study stretch vibrations of CO chemisorbed at low coverage on PtxNi1−x(111). Bayesian probability theory along with the entropic prior (Maxent) has been employed to deconvolve the apparatus function and to improve the apparent energy resolution. Maxent has proven very successful in a wide range of inversion problems. Here the resolution
Maximum Entropy and Bayesian Methods, 1996
ABSTRACT Soft X-ray tomography has become a standard diagnostic equipment to investigate plasma p... more ABSTRACT Soft X-ray tomography has become a standard diagnostic equipment to investigate plasma profiles. Due to limitations in viewing-access and detector numbers the reconstruction of the two-dimensional emissivity profile constitutes a highly underdetermined inversion problem. We discuss the principle features of the tomography problem from the Bayesian point of view in various stages of sophistication. The approach is applied to real-world data obtained from the Wendelstein 7AS stellerator.
Physical Review B, 2001
We have studied the electronic properties of the ladder compound ␣Ј-NaV 2 O 5 , adopting a joint ... more We have studied the electronic properties of the ladder compound ␣Ј-NaV 2 O 5 , adopting a joint experimental and theoretical approach. The momentum-dependent loss function was measured using electron energy-loss spectroscopy in transmission. The optical conductivity derived from the loss function by a Kramers-Kronig analysis agrees well with our results from local spin density approximation ͑LSDA͒ ϩU band-structure calculations upon application of an antiferromagnetic alignment of the V 3d xy spins along the legs and an on-site Coulomb interaction U between 2 and 3 eV. The decomposition of the calculated optical conductivity into contributions from transitions between selected energy regions of the density of states reveals the origin of the observed anisotropy of the optical conductivity. In addition, we have investigated the plasmon excitations related to transitions between the vanadium states within an effective 16-site vanadium cluster model. Good agreement between the theoretical and experimental loss functions was obtained using the hopping parameters derived from the tight-binding fit to the band-structure and moderate Coulomb interactions between the electrons within the ab plane.
Physical Review B, 2001
ABSTRACT We present a joint experimental and theoretical investigation of the electronic structur... more ABSTRACT We present a joint experimental and theoretical investigation of the electronic structure of CuGeO3. The momentum dependent loss function was measured using electron energy-loss spectroscopy in transmission. For momentum transfers parallel to the crystallographic c direction (along the CuO2 chains) the loss function agrees well with calculations based upon a Cu5O12 cluster model. The detailed analysis of the origin of the features below 7 eV in the calculated spectra reveals two distinct energy ranges in which transitions occur either into localized or delocalized states. A shift of spectral weight towards the low energy region can be observed when the coupling between adjacent CuO4 plaquettes is increased in our model. Above 2 eV the optical conductivity derived from the experimental loss function agrees well with results from optical measurements as well as with those from calculations based on our cluster model. The small spectral weight observed at 1.8 eV in the experiment has no counterpart in the theoretical loss function and thus cannot be assigned to charge transfer transitions involving Cu and O states. The loss functions with momentum transfers parallel to the crystallographic b direction show spectral weight at 6.5 eV that shifts to higher energies with increasing momentum transfer. This can probably be ascribed to transitions involving unoccupied Ge 4s and 4p states.
Physical Review B, 2003
Cluster perturbation theory in combination with the Lanczos method is used to compute the one-ele... more Cluster perturbation theory in combination with the Lanczos method is used to compute the one-electron spectral function of the Holstein polaron in one and two dimensions. It is shown that the method allows reliable calculations using relatively small clusters, and at the same time significantly reduces finite-size effects. Results are compared with exact data and the relation to existing work is discussed. We also use a strong-coupling perturbation theory-equivalent to the Hubbard I approximation-to calculate the spectral function of the quarter-filled Holstein model of spinless fermions, starting from the exact atomic-limit Green function. The results agree well with previous calculations within the many-body coherent potential approximation.
Solid State Communications, 1987
The electron self-energy of unoccupied states is investigated taking into account dynamical scree... more The electron self-energy of unoccupied states is investigated taking into account dynamical screening and nonlocal exchange. To obtain agreement with experiment it is crucial to go beyond the framework of the homogeneous electron gas and include the nonlocal exchange with electrons in valence and core shells. This contribution of atomic origin gives rise to a considerable, almost linear energy dependence of the self-energy over a wide energy range in agreement with experimental findings for many substances and in disagreement with the local density approximation. Quantitative results are presented for Ag.
Physical review. B, Condensed matter, 1990
ABSTRACT We calculate the order parameter m in the infinite-range isotropic-interaction quantum m... more ABSTRACT We calculate the order parameter m in the infinite-range isotropic-interaction quantum model of antiferromagnetism due to Lieb and Mattis (LM). The Hamiltonian H1 in this model contains pure Heisenberg interactions. m is the mean value of the z component msz of the staggered spin per particle ms in the ground state of H1 plus a symmetry-breaking term HB due to an infinitesimal staggered field B in the thermodynamic limit (TL). We find a value of √3 for the ratio r=m/m0, m0 being the root-mean-square value of msz for B==0 in the TL. m and m0 measure, respectively, the spontaneous symmetry breaking and the amount of long-range order in the symmetry-unbroken state. This value is expected on the basis of our recent argument that r= √3 if ms behaves in the TL as a classical vector with no magnitude fluctuations—behavior that we show holds for the Lieb-Mattis model. The addition to H1 of a small easy-axis anisotropic term is also discussed briefly. These studies allow calculation of the size dependence of quantities that we have recently shown to become lower bounds on m in the TL (bounds that are valid for short-range-interaction models as well). In the LM model, low-lying energy eigenstates with excitation energies of order N-1, where N is the number of spins, form a continuum (with no gap) in the TL. These are closely related to similar states shown to exist for the nearest-neighbor isotropic Heisenberg antiferromagnet. We show that this continuum has a density of states that is O(N0); i.e., it is not extensive.
Physical review. B, Condensed matter, 1990
... Title, Monte Carlo study of superconductivity in the three-band Emery model. ... Description/... more ... Title, Monte Carlo study of superconductivity in the three-band Emery model. ... Description/Abstract, We have examined the three-band Hubbard model for the copper oxide planes in high-temperature superconductors using the projector quantum Monte Carlo method. ...
Physical review. B, Condensed matter, Jan 15, 1995
The interplay of electron-phonon coupling and strong electronic correlations is studied in the fr... more The interplay of electron-phonon coupling and strong electronic correlations is studied in the frame of the two-dimensional Hubbard-Holstein model. Static and dynamic properties are determined by quantum Monte Carlo simulations and by Migdal-Eliashberg theory. The comparison allows us to assess the diagrammatic approach. The competition between the phonon-mediated electron-electron attraction and the local Coulomb repulsion leads to a rich phase
A formalism is proposed that allows an efficient computation of the exchange-correlation function... more A formalism is proposed that allows an efficient computation of the exchange-correlation functional including the nonlocal exchange and dynamical screening correctly. In combination with LDA band structures this functional leads to remarkably good results for semiconductors and even wide gap insulators. The model furthermore provides a qualitative understanding of electronic many-body effects.
Physical Review B, 2014
We perform electronic structure calculations for the recently synthesized iron-based superconduct... more We perform electronic structure calculations for the recently synthesized iron-based superconductor LiFeO2Fe2Se2. In contrast to other iron-based superconductors, this material comprises two different iron atoms in 3d 5 and 3d 6 configurations. In band theory, both contribute to the low-energy electronic structure. Spin-polarized density functional theory calculations predict an antiferromagnetic metallic ground state with different moments on the two Fe sites. However, several other almost degenerate magnetic configurations exist. Due to their different valences, the two iron atoms behave very differently when local quantum correlations are included through the dynamical meanfield theory. The contributions from the half-filled 3d 5 atoms in the LiFeO2 layer are suppressed and the 3d 6 states from the FeSe layer restore the standard iron-based superconductor fermiology.
Physical Review B, 2014
ABSTRACT Electronic susceptibilities are a very popular tool to study electronic and magnetic pro... more ABSTRACT Electronic susceptibilities are a very popular tool to study electronic and magnetic properties of materials, both in experiment and theory. Unfortunately, the numerical evaluation of even the bare susceptibility, which depends on the computation of matrix elements and sums over energy bands, is very work-intensive and therefore various approximations have been introduced to speed up the calculations. We present a reliable and efficient implementation to compute static as well as dynamic bare susceptibilities based on full-potential density functional theory (DFT) calculations. Based on the exact results we will assess the accuracy of replacing the matrix elements with a constant and the impact of truncating the sum over the energy bands. Results will be given for representative and topical materials, such as Cr, a classical transition metal, as well as for FeSe and LaFeAsO, examples of iron-based superconductors.
ABSTRACT The Bayesian Formalism (BF) and the principle of Maximum Entropy (MaxEnt) are applied to... more ABSTRACT The Bayesian Formalism (BF) and the principle of Maximum Entropy (MaxEnt) are applied to techniques relevant to Ion-Beam Analysis (IBA). The BF is an efficient and theoretically sound method of information recovery, and when applied together with an entropic prior it has special applications in solving ill-posed inverse problems, e.g. the deconvolution of the detector response function from PIXE or RBS spectra. Results include the recovery of depth profiles in PIXE using a two-detector set-up. The deconvolution of RBS spectra using the BF is also compared to more conventional deconvolution techniques, namely Fourier Transforms and the non-linear Jansson method.
An introduction to the use of maximum entropy (ME) and Bayesian methods in ion-beam applications ... more An introduction to the use of maximum entropy (ME) and Bayesian methods in ion-beam applications is presented. The formalism is applied to the deconvolution of detector blurring functions in RBS and PIXE measurements. The resulting improvement in detector resolution can be as much as a factor of 5 when the detector functions are well known. The possibilities of further applications
Journal de Physique I, 1992
Plasma Physics and Controlled Fusion, 1996
... M Anton, H Weisen, MJ Dutch, W von der Linden, F Buhlmann, R Chavan, B Marletaz, P Mar... more ... M Anton, H Weisen, MJ Dutch, W von der Linden, F Buhlmann, R Chavan, B Marletaz, P Marmillod and P Paris Centre de Recherches en Physique des Plasmas, Association EURATOM Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne, CH ...
Physical Review Letters, 1993
ABSTRACT The experimental information about magnetic bands and their temperature dependence is di... more ABSTRACT The experimental information about magnetic bands and their temperature dependence is distorted by Fermi distribution and apparatus function. The recovery of the spectral density constitutes an ill-posed inversion problem. We have invoked the maximum entropy regularization to deconvolute spin-resolved inverse photoemission data of Ni(110). The effective energy resolution is hereby improved by a factor of 5 and structures below EF, which are generally lost in inverse photoemission, are recovered. The temperature-dependent data for Ni reveal clearly a pair of spin-split bands. The splitting vanishes on approaching the Curie temperature.
Physical Review Letters, 2013
We introduce a versatile method to compute electronic steady-state properties of strongly correla... more We introduce a versatile method to compute electronic steady-state properties of strongly correlated extended quantum systems out of equilibrium. The approach is based on dynamical mean-field theory (DMFT), in which the original system is mapped onto an auxiliary nonequilibrium impurity problem imbedded in a Markovian environment. The steady-state Green's function of the auxiliary system is solved by full diagonalization of the corresponding Lindblad equation. The approach can be regarded as the nontrivial extension of the exact-diagonalization-based DMFT to the nonequilibrium case. As a first application, we consider an interacting Hubbard layer attached to two metallic leads and present results for the steady-state current and the nonequilibrium density of states.
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Papers by W. Von Der Linden