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W. Hergert

Publications and source records attributed to W. Hergert.

15 recordsLinked to original sources
arXiv2018

Ab-initio approach to the stability and the structural, electronic and magnetic properties of the (001) Znfe2O4 surface terminations

We present a Density Functional Theory (DFT) based study of the structural and magnetic properties of the (001) surface of the semiconducting oxide ZnFe2O4 (spinel structure). The calculations were performed using the DFT based ab initio plane wave and pseudopotential method as implemented in the Quantum Espresso code. The all electron Full-potential linearized-augmented-plane-wave method (FP-LAPW) was also employed to check the accuracy of plane wave method. In both calculations the DFT+U methodology was employed and different (001) surface terminations of ZnFe2O4 were studied: We find that the surface terminated in Zn is the stable one. For all the (001) surface terminations our calculations predict that the Zn-Fe cationic inversion (antisites), which are defects in bulk ZnFe2O4, becomes stable and an integral part of the surface. Also, a ferrimagnetic behavior is predicted for the case of antisites in the superficial layer. Our results for different properties of the surface of ZnFe2O4 are compared with those obtained in bulk samples and those reported in the literature.

cond-mat.mtrl-sci↗
arXiv2018

GTPack: A Mathematica group theory package for application in solid-state physics and photonics

We present the Mathematica group theory package GTPack providing about 200 additional modules to the standard Mathematica language. The content ranges from basic group theory and representation theory to more applied methods like crystal field theory, tight-binding and plane-wave approaches capable for symmetry based studies in the fields of solid-state physics and photonics. GTPack is freely available via http://GTPack.org. The package is designed to be easily accessible by providing a complete Mathematica-style documentation, an optional input validation and an error strategy. We illustrate the basic framework of the package and show basic examples to present the functionality. Furthermore, we give a complete list of the implemented commands including references for algorithms within the supplementary material.

cond-mat.mtrl-sci↗
arXiv2015

Study of electronic and magnetic properties and related x-ray absorption spectroscopy of ultrathin Co films on BaTiO$_3$

We present a first-principles study of electronic and magnetic properties of thin Co films on a BaTiO$_3$(001) single crystal. The crystalline structure of 1, 2, and 3 monolayer thick Co films was determined and served as input for calculations of the electronic and magnetic properties of the films. The estimation of exchange constants indicates that the Co films are ferromagnetic with a high critical temperature, which depends on the film thickness and the interface geometry. In addition, we calculated x-ray absorption spectra, related magnetic circular dichroism (XMCD) and linear dichroism (XLD) of the Co L$_{2,3}$ edges as a function of Co film thickness and ferroelectric polarization of BaTiO$_3$. We found characteristic features, which depend strongly on the magnetic properties and the structure of the film. While there is only a weak dependence of XMCD spectra on the ferroelectric polarization, the XLD of the films is much more sensitive to the polarization switching, which could possibly be observed experimentally.

cond-mat.mtrl-sci↗
arXiv2015

Study of the Negative Magneto-Resistance of Single Proton-Implanted Lithium-Doped ZnO Microwires

The magneto-transport properties of single proton-implanted ZnO and of Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7\%) doped ZnO microwires showed a non monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implantation and Li doping. The magnetic field dependence of the resistance might be explained by the formation of a magnetic/non magnetic heterostructure in the wire after proton implantation.

cond-mat.mtrl-sci↗
arXiv2013

Exchange interaction and its tuning in magnetic binary chalcogenides

Using a first-principles Green's function approach we study magnetic properties of the magnetic binary chalcogenides Bi2Te3, Bi2Se3, and Sb2Te3. The magnetic coupling between transition-metal impurities is long-range, extends beyond a quintuple layer, and decreases with increasing number of d electrons per 3d atom. We find two main mechanisms for the magnetic interaction in these materials: the indirect exchange interaction mediated by free carriers and the indirect interaction between magnetic moments via chalcogen atoms. The calculated Curie temperatures of these systems are in good agreement with available experimental data. Our results provide deep insight into magnetic interactions in magnetic binary chalcogenides and open a way to design new materials for promising applications.

cond-mat.mtrl-sci↗
arXiv2013

Defect-Induced Magnetism in Solids

In the last years the number of nominally non-magnetic solids showing magnetic order induced by some kind of defects has increased continuously. From the single element material graphite to several covalently bonded non-magnetic compounds, the influence of defects like vacancies and/or non-magnetic ad-atoms on triggering magnetic order has attracted the interest of experimentalists and theoreticians. We review and discuss the main theoretical approach as well as recently obtained experimental evidence based on different experimental methods that supports the existence of defect-induced magnetism (DIM) in non-magnetic as well as in magnetic materials.

cond-mat.mtrl-sci↗
arXiv2009

Structural phase transitions and fundamental band gaps of Mg(x)Zn(1-x)O alloys from first principles

The structural phase transitions and the fundamental band gaps of Mg(x)Zn(1-x)O alloys are investigated by detailed first-principles calculations in the entire range of Mg concentrations x, applying a multiple-scattering theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are treated within the coherent potential approximation (CPA). The calculations for various crystal phases have given rise to a phase diagram in good agreement with experiments and other theoretical approaches. The phase transition from the wurtzite to the rock-salt structure is predicted at the Mg concentration of x = 0.33, which is close to the experimental value of 0.33 - 0.40. The size of the fundamental band gap, typically underestimated by the local density approximation, is considerably improved by the self-interaction correction. The increase of the gap upon alloying ZnO with Mg corroborates experimental trends. Our findings are relevant for applications in optical, electrical, and in particular in magnetoelectric devices.

cond-mat.mtrl-sci↗
arXiv2008

Onset of Magnetic Order in Strongly-Correlated Systems from ab initio Electronic Structure Calculations: Application to Transition Metal Oxides

We describe an ab initio theory of finite temperature magnetism in strongly-correlated electron systems. The formalism is based on spin density functional theory, with a self-interaction corrected local spin density approximation (SIC-LSDA). The self-interaction correction is implemented locally, within the KKR multiple-scattering method. Thermally induced magnetic fluctuations are treated using a mean-field `disordered local moment' (DLM) approach and at no stage is there a fitting to an effective Heisenberg model. We apply the theory to the 3d transition metal oxides, where our calculations reproduce the experimental ordering tendencies, as well as the qualitative trend in ordering temperatures. We find a large insulating gap in the paramagnetic state which hardly changes with the onset of magnetic order.

cond-mat.str-el↗
arXiv2005

Self-interaction correction in multiple scattering theory

We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LSD method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of physical quantities averaged over ensembles of such potentials using the coherent potential approximation (CPA). This facilitates applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well as intermediate valences. As a demonstration of the method, we study the well-known $α$-$γ$ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

cond-mat.str-el↗
arXiv2005

Ab initio study of mirages and magnetic interactions in quantum corrals

The state of the art ab initio calculations of quantum mirages,the spin-polarization of surface-state electrons and the exchange interaction between magnetic adatoms in Cu and Co corrals on Cu(111) are presented. We find that the spin-polarization of the surface-state electrons caused by magnetic adatoms can be projected to a remote location and can be strongly enhanced in corrals compared to an open surface.Our studies give a clear evidence that quantum corrals could permit to tailor the exchange interaction between magnetic adatoms at large separations. The spin-polarization of surface-state electrons at the empty focus in the Co corral used in the experimental setup of Manoharan et al., (Nature 403, 512 (2000)) is revealed.

cond-mat.mtrl-sci↗
arXiv2004

Ab initio approach for atomic relaxations in supported magnetic clusters

We present a newly developed scheme for atomic relaxations of magnetic supported clusters. Our approach is based on the full potential Korringa-Kohn-Rostoker Green's function method and the second moment tight-binding approximation for many-body potentials. We demonstrate that only a few iterations in ab initio calculations are necessary to find an equilibrium structure of supported clusters. As an example, we present our results for small Co clusters on Cu(001). Changes in electronic and magnetic states of clusters due to atomic relaxations are revealed.

cond-mat.mtrl-sci↗
arXiv2004

Long-range electronic interactions between adatoms on transition metal surfaces

Ab initio calculations of surface-state mediated interactions between Cu adatoms on transition metal surfaces are presented. We concentrate on Co/Cu(111) and Co(0001) substrates and compare results with our calculations for Cu(111). Our studies show that surface states of Co/Cu(111) and Co(0001) are spin-polarized. We reveal that long-range interactions between adatoms are mainly determined by sp-majority states. In contrast to Cu(111) and Co/Cu(111), the interaction between adatoms on Co(0001) is strongly suppressed at large adsorbate separations.

cond-mat.mtrl-sci↗
arXiv2003

Resonant inelastic x-ray scattering (RIXS) spectra of magnesium diboride

Using the tight-binding linear muffin-tin orbitals method, the soft x-ray fluorescence K-emission spectra of boron in MgB_2, excited close to the absorption edge, are estimated. In the calculations the angle of incidence between the direction of the incoming photon and the hexagonal axis of the specimen is 60 degrees and 75 degrees. Comparison with experiment is possible in the former case where good agreement is found. Furthermore, another resonant feature below the Fermi energy is predicted for the larger angle. This feature can be related to the excitations to the antibonding B pi-band in the neighbourhood of the L-H line in the Brillouin zone.

cond-mat.supr-con↗
arXiv2003

Prediction of a new class of half-metallic antiferromagnets

We report on vacancy induced half-metallicity in the prototype Mott-insulating substances MnO and NiO. By embedding a cation-vacancy into a magnetic system a new road opens up to the construction of half-metallic antiferromagnets. For Ni$_{0.97}$O we find a half-metallic antiferromagnet a system hitherto only proposed for complicated crystal structures.

cond-mat.str-el↗