arXiv subjects

Wolfram Hergert

Publications and source records attributed to Wolfram Hergert.

12 recordsLinked to original sources

arXiv2022

Electron magnetic moment of transient chiral phonons in KTaO$_3$

High intensity THz lasers allow for the coherent excitation of individual phonon modes. The ultrafast control of emergent magnetism by means of phonons opens up new tuning mechanisms for functional materials. While theoretically predicted phonon magnetic moments are tiny, recent experiments hint towards a significant magnetization in various materials. To explain these phenomena, we derive a coupling mechanism between the phonon angular momentum and the electron spin. This coupling introduces the transient level-splitting of spin-up and spin-down channels and a resulting magnetization. We estimate this magnetization on the example of the lowest infrared active mode in the perovskite KTaO$_3$. Our results show an electronic magnetic moment of $\approx 10^{-1}$ $μ_B$ per unit cell, depending on the doping level and electron temperature.

cond-mat.mtrl-sci↗

arXiv2021

Influence of surface band bending on a narrow band gap semiconductor: Tunneling atomic force studies of graphite with Bernal and rhombohedral stacking orders

Tunneling atomic force microscopy (TUNA) was used at ambient conditions to measure the current-voltage ($I$-$V$) characteristics at clean surfaces of highly oriented graphite samples with Bernal and rhombohedral stacking orders. The characteristic curves measured on Bernal-stacked graphite surfaces can be understood with an ordinary self-consistent semiconductor modeling and quantum mechanical tunneling current derivations. We show that the absence of a voltage region without measurable current in the $I$-$V$ spectra is not a proof of the lack of an energy band gap. It can be induced by a surface band bending due to a finite contact potential between tip and sample surface. Taking this into account in the model, we succeed to obtain a quantitative agreement between simulated and measured tunnel spectra for band gaps $(12 \ldots 37)$\,meV, in agreement to those extracted from the exponential temperature decrease of the longitudinal resistance measured in graphite samples with Bernal stacking order. In contrast, the surface of relatively thick graphite samples with rhombohedral stacking reveals the existence of a maximum in the first derivative $dI/dV$, a behavior compatible with the existence of a flat band. The characteristics of this maximum are comparable to those obtained at low temperatures with similar techniques.

cond-mat.mtrl-sci↗

arXiv2019

First-principles investigations of the magnetic phase diagram of Gd$_{1-x}$Ca$_{x}$MnO$_{3}$

We studied for the first time the magnetic phase diagram of the rare-earth manganites series Gd$_{1-x}$Ca$_{x}$MnO$_{3}$ (GCMO) over the full concentration range based on density functional theory. GCMO has been shown to form solid solutions. We take into account this disordered character by adapting special quasi random structures at different concentration steps. The magnetic phase diagram is mainly described by means of the magnetic exchange interactions between the Mn sites and Monte Carlo simulations were performed to estimate the corresponding transition temperatures. They agree very well with recent experiments. The hole doped region $x<0.5$ shows a strong ferromagnetic ground state, which competes with A-type antiferromagnetism at higher Ca concentrations $x>0.6$.

cond-mat.mtrl-sci↗

arXiv2019

Substrate-Controlled Magnetism: Fe nanowires on vicinal Cu surfaces

Here we present a novel approach to control magnetic interactions in atomic-scale nanowires. Our ab initio calculations demonstrate the possibility to tune magnetic properties of Fe nanowires formed on vicinal Cu surfaces. Both intrawire and interwire magnetic exchange parameters are extracted from DFT calculations. This study suggests that the effective interwire magnetic exchange parameters exhibit Ruderman--Kittel--Kasuya--Yosida-like (RKKY) oscillations as a function of Fe interwire separation. The choice of vicinal Cu surface offers possibilities for controlling the magnetic coupling. Furthermore, an anisotropic Heisenberg model was used in Monte Carlo simulations to examine the stability of these magnetic configurations at finite temperature. The predicted critical temperatures of the Fe nanowires on Cu(422) and Cu(533) surfaces are well-above room temperature.

cond-mat.mtrl-sci↗

arXiv2018

Tuning the probability of defect formation via substrate strains in Sr$_2$FeMoO$_6$ films

Since oxide materials like Sr$_2$FeMoO$_6$ are usually applied as thin films, we studied the effect of biaxial strain, resulting from the substrate, on the electronic and magnetic properties and, in particular, on the formation energy of point defects. From our first-principles calculations, we determined that the probability of forming point defects - like vacancies or substitutions - in Sr$_2$FeMoO$_6$ could be adjusted by choosing a proper substrate. For example, the amount of anti-site disorder can be reduced with compressive strain in order to obtain purer Sr$_2$FeMoO$_6$ as needed for spintronic applications, while the formation of oxygen vacancies is more likely for tensile strain, which improves the functionality of Sr$_2$FeMoO$_6$ as a basis material of solid oxide fuel cells. In addition, we were also be able to include the oxygen partial pressure in our study by using its thermodynamic connection with the chemical potential. Strontium vacancies become for example more likely than oxygen vacancies at a pressure of 1$\,$bar. Hence, this degree of freedom might offer in general another potential method for defect engineering in oxides besides, e.g., experimental growth conditions like temperature or gas pressure.

cond-mat.mtrl-sci↗

arXiv2018

Variation of magnetic properties of Sr$_2$FeMoO$_6$ due to oxygen vacancies

Oxygen vacancies can be of utmost importance for improving or deteriorating physical properties of oxide materials. Here, we studied from first-principles the electronic and magnetic properties of oxygen vacancies in the double perovskite Sr$_2$FeMoO$_6$ (SFMO). We show that oxygen vacancies can increase the Curie temperature in SFMO, although the total magnetic moment is reduced at the same time. We found also that the experimentally observed valence change of the Fe ions from $3+$ to $2+$ in the x-ray magnetic circular dichroism (XMCD) measurements is better explained by oxygen vacancies than by the assumed mixed valence state. The agreement of the calculated x-ray absorption spectra and XMCD results with experimental data is considerably improved by inclusion of oxygen vacancies.

cond-mat.mtrl-sci↗

arXiv2016

Quantitative description of short-range order and its influence on the electronic structure in Ag-Pd alloys

We investigate the effect of short-range order (SRO) on the electronic structure in alloys from the theoretical point of view using density of states (DOS) data. In particular, the interaction between the atoms at different lattice sites is affected by chemical disorder, which in turn is reflected in the fine structure of the DOS and, hence, in the outcome of spectroscopic measurements. We aim at quantifying the degree of potential SRO with a proper parameter. The theoretical modeling is done with the Korringa-Kohn-Rostoker Green's function method. Therein, the extended multi-sublattice non-local coherent potential approximation is used to include SRO. As a model system, we use the binary solid solution Ag$_c$Pd$_{1-c}$ at three representative concentrations $c=0.25$, $0.5$ and $0.75$. The degree of SRO is varied from local ordering to local segregation through an intermediate completely uncorrelated state. We observe some pronounced features, which change over the whole energy range of the valence bands as a function of SRO in the alloy. These spectral variations should be traceable in modern photoemission experiments.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic properties of defect-free and oxygen-deficient cubic SrCoO$_{3-δ}$

We investigated theoretically electronic and magnetic properties of the perovskite material SrCoO$_{3-δ}$ with $δ\leq 0.15$ using a projector-augmented plane-wave method and a Green's function method. This material is known from various experiments to be ferromagnetic with a Curie temperature of 260$\,$K to 305$\,$K and a magnetic moment of 1.5${\,μ_\text{B}}$ to 3.0${\,μ_\text{B}}$. Applying the magnetic force theorem as it is formulated within Green's function method, we calculated for SrCoO$_{3-δ}$ the magnetic exchange parameters and estimated the Curie temperature. Including correlation effects by an effective $U$ parameter within the GGA$+U$ approach and verifying this by hybrid functional calculations, we obtained the Curie temperatures in dependence of the oxygen deficiency close to the experimental values.

cond-mat.mtrl-sci↗

arXiv2015

The effect of hydrostatic pressure and uniaxial strain on the electronic structure of Pb$_{\text{1-x}}$Sn$_{\text{x}}$Te

The electronic structure of Pb$_{1-x}$Sn$_{x}$Te is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations $x$, Pb$_{1-x}$Sn$_{x}$Te is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap can be tuned by the alloy composition. Thus, there is range of potential usage of Pb$_{1-x}$Sn$_{x}$Te for spintronic applications.

physics.comp-ph↗

arXiv2015

Numerical solution of the relativistic single-site scattering problem for the Coulomb and the Mathieu potential

For a reliable fully-relativistic Korringa-Kohn-Rostoker Green function method, an accurate solution of the underlying single-site scattering problem is necessary. We present an extensive discussion on numerical solutions of the related differential equations by means of standard methods for a direct solution and by means of integral equations. Our implementation is tested and exemplarily demonstrated for a spherically symmetric treatment of a Coulomb potential and for a Mathieu potential to cover the full-potential implementation. For the Coulomb potential we include an analytic discussion of the asymptotic behaviour of irregular scattering solutions close to the origin ($r\ll1$).

cond-mat.mtrl-sci↗

arXiv2011

Room temperature p-induced surface ferromagnetism

We prove a spontaneous magnetization of the oxygen-terminated ZnO (0001) surface by utilizing a multi-code, SIESTA and KKR, first-principles approach, involving both LSDA+U and selfinteraction corrections (SIC) to treat electron correlation effects. Critical temperatures are estimated from Monte Carlo simulations, showing that at and above 300 K the surface is thermodynamically stable and ferromagnetic. The observed half-metallicity and long-range magnetic order originate from the presence of p-holes in the valence band of the oxide. The mechanism is universal in ionic oxides and points to a new route for the design of ferromagnetic low dimensional systems.

cond-mat.mtrl-sci↗

arXiv2009

Exchange coupling in transition metal monoxides: Electronic structure calculations

An ab initio study of magnetic exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Neel temperatures T_N and spin wave dispersions are calculated. The Neel temperatures are obtained using mean field approximation, random phase approximation and Monte Carlo simulations. The pressure dependence of T_N is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.

cond-mat.other↗