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D. Vollhardt

Publications and source records attributed to D. Vollhardt.

At least 19 recordsLinked to original sources
arXiv2021

Dynamical mean-field theory of the Anderson-Hubbard model with local and non-local disorder in tensor formulation

To explore correlated electrons in the presence of local and non-local disorder, the Blackman-Esterling-Berk method for averaging over off-diagonal disorder is implemented into dynamical mean-field theory using tensor notation. The impurity model combining disorder and correlations is solved using the recently developed fork tensor-product state solver, which allows one to calculate the single particle spectral functions on the real-frequency axis. In the absence of off-diagonal hopping, we establish exact bounds of the spectral function of the non-interacting Bethe lattice with coordination number $Z$. In the presence of interaction, the Mott insulating paramagnetic phase of the one-band Hubbard model is computed at zero temperature in alloys with site- and off-diagonal disorder. When the Hubbard $U$ parameter is increased, transitions from an alloy band-insulator through a correlated metal into a Mott insulating phase are found to take place.

cond-mat.str-el↗
arXiv2020

Scaling behavior of the momentum distribution of a quantum Coulomb system in a confining potential

We calculate the single-particle momentum distribution of a quantum many-particle system in the presence of the Coulomb interaction and a confining potential. The region of intermediate momenta, where the confining potential dominates, marks a crossover from a Gaussian distribution valid at low momenta to a power-law behavior valid at high momenta. We show that for all momenta the momentum distribution can be parametrized by a $q$-Gaussian distribution whose parameters are specified by the confining potential. Furthermore, we find that the functional form of the probability of transitions between the confined ground state and the $n^{th}$ excited state is invariant under scaling of the ratio $Q^2/ν_n$, where $Q$ is the transferred momentum and $ν_n$ is the corresponding excitation energy. Using the scaling variable $Q^2/ν_n$ the maxima of the transition probabilities can also be expressed in terms of a $q$-Gaussian.

cond-mat.other↗
arXiv2020

Ab initio typical medium theory of substitutional disorder

By merging single-site typical medium theory with density functional theory we introduce a self-consistent framework for electronic structure calculations of materials with substitutional disorder which takes into account Anderson localization. The scheme and details of the implementation are presented and applied to the hypothetical alloy Li$_{c}$Be$_{1-c}$, and the results are compared with those obtained with the coherent potential approximation. Furthermore we demonstrate that Anderson localization suppresses ferromagnetic order for a very low concentration of (i) carbon impurities substituting oxygen in MgO$_{1-c}$C$_{c}$, and (ii) manganese impurities substituting magnesium in Mg$_{1-c}$Mn$_c$O for the low-spin magnetic configuration.

cond-mat.dis-nn↗
arXiv2020

Lattice dynamics of palladium in the presence of electronic correlations

We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures $T \lesssim 20~K$ the phonon frequency in the vicinity of the Kohn anomaly, $ω_{T1}({\bf q}_{K})$, strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

cond-mat.str-el↗
arXiv2018

Predicting the conductance of strongly correlated molecules: the Kondo effect in perchlorotriphenylmethyl/Au junctions

Stable organic radicals integrated into molecular junctions represent a practical realization of the single-orbital Anderson impurity model. Motivated by recent experiments for perchlorotriphenylmethyl (PTM) molecules contacted to gold electrodes, we develop a method that combines density functional theory (DFT), quantum transport theory, numerical renormalization group (NRG) calculations and renormalized super-perturbation theory (rSPT) to compute both equilibrium and non-equilibrium properties of strongly correlated nanoscale systems at low temperatures effectively from first principles. We determine the possible atomic structures of the interfaces between the molecule and the electrodes, which allow us to estimate the Kondo temperature and the characteristic transport properties, which compare well with experiments. By using the non-equilibrium rSPT results we assess the range of validity of equilibrium DFT+NRG-based transmission calculations for the evaluation of the finite voltage conductance. The results demonstrate that our method can provide qualitative insights into the properties of molecular junctions when the molecule-metal contacts are amorphous or generally ill-defined, and that it can further give a fully quantitative description when the experimental contact structures are well characterized.

cond-mat.str-el↗
arXiv2018

Correlation strength, Lifshitz transition and the emergence of a two- to three-dimensional crossover in FeSe under pressure

We report a detailed theoretical study of the electronic structure, spectral properties, and lattice parameters of bulk FeSe under pressure using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory method (DFT+DMFT). In particular, we perform a structural optimization and compute the evolution of the lattice parameters (volume, $c/a$ ratio, and the internal $z$ position of Se) and the electronic structure of the tetragonal (space group $P4/nmm$) paramagnetic FeSe. Our results for the lattice parameters are in good quantitative agreement with experiment. The $c/a$ ratio is slightly overestimated by about $3$~\%, presumably due to the absence of the van der Waals interactions between the FeSe layers in our calculations. The lattice parameters determined within DFT are off the experimental values by a remarkable $\sim$$6$-$15$~\%, implying a crucial importance of electron correlations. Upon compression to $10$~GPa, the $c/a$ ratio and the lattice volume show a decrease by $2$ and $10$~\%, respectively, while the Se $z$ coordinate weakly increases by $\sim$$2$~\%. Most importantly, our results reveal a topological change of the Fermi surface (Lifshitz transition) which is accompanied by a two- to three-dimensional crossover. Our results indicate a small reduction of the quasiparticle mass renormalization $m^*/m$ by about $5$~\% for the $e$ and less than $1$~\% for the $t_2$ states, as compared to ambient pressure. The behavior of the momentum-resolved magnetic susceptibility $χ({\bf q})$ shows no topological changes of magnetic correlations under pressure, but demonstrates a reduction of the degree of the in-plane $(π,π)$ stripe-type nesting. Our results for the electronic structure and lattice parameters of FeSe are in good qualitative agreement with recent experiments on its isoelectronic counterpart FeSe$_{1-x}$S$_x$.

cond-mat.str-el↗
arXiv2017

LDA+DMFT approach to ordering phenomena and the structural stability of correlated materials

Materials with correlated electrons often respond very strongly to external or internal influences, leading to instabilities and states of matter with broken symmetry. This behavior can be studied theoretically either by evaluating the linear response characteristics, or by simulating the ordered phases of the materials under investigation. We developed the necessary tools within the dynamical mean-field theory (DMFT) to search for electronic instabilities in materials close to spin-state crossovers and to analyze the properties of the corresponding ordered states. This investigation, motivated by the physics of LaCoO$_3$, led to a discovery of condensation of spinful excitons in the two-orbital Hubbard model with a surprisingly rich phase diagram. The results are reviewed in the first part of the article. Electronic correlations can also be the driving force behind structural transformations of materials. To be able to investigate correlation-induced phase instabilities we developed and implemented a formalism for the computation of total energies and forces within a fully charge self-consistent combination of density functional theory and DMFT. Applications of this scheme to the study of structural instabilities of selected correlated electron materials such as Fe and FeSe are reviewed in the second part of the paper.

cond-mat.str-el↗
arXiv2017

Scaling behavior of the Compton profile of alkali metals

The contribution of the valence electrons to the Compton profiles of the alkali metals is calculated using density functional theory. We show that the Compton profiles can be modeled by a $q-$Gaussian distribution, which is characterized by an anisotropic, element dependent parameter $q$. Thereby we derive an unexpected scaling behavior of the Compton profiles of all alkali metals.

cond-mat.mtrl-sci↗
arXiv2017

Effect of electron correlations on the electronic structure and phase stability of FeSe upon lattice expansion

We present results of a detailed theoretical study of the electronic, magnetic, and structural properties of the chalcogenide parent system FeSe using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we predict a remarkable change of the electronic structure of FeSe which is accompanied by a complete reconstruction of the Fermi surface topology (Lifshitz transition) upon a moderate expansion of the lattice volume. The phase transition results in a change of the in-plane magnetic nesting wave vector from $(π,π)$ to $(π,0)$ and is associated with a transition from itinerant to orbital-selective localized magnetic moments. We attribute this behavior to a correlation-induced shift of the van Hove singularity of the Fe $t_{2}$ bands at the M-point across the Fermi level. Our results reveal a strong orbital-selective renormalization of the effective mass $m^*/m$ of the Fe $3d$ electrons upon expansion. The largest effect occurs in the Fe $xy$ orbital, which gives rise to a non-Fermi-liquid-like behavior above the transition. The behavior of the momentum-resolved magnetic susceptibility $χ({\bf q})$ demonstrates that magnetic correlations are also characterized by a pronounced orbital selectivity, suggesting a spin-fluctuation origin of the nematic phase of paramagnetic FeSe. We conjecture that the anomalous behavior of FeSe upon expansion is associated with the proximity of the Fe $t_{2}$ van Hove singularity to the Fermi level and the sensitive dependence of its position on external conditions.

cond-mat.str-el↗
arXiv2016

Typical-medium, multiple-scattering theory for disordered systems with Anderson localization

The typical medium dynamical cluster approximation (TMDCA) is reformulated in the language of multiple scattering theory to make possible first principles calculations of the electronic structure of substitutionally disordered alloys including the effect of Anderson localization. The TMDCA allows for a systematic inclusion of non-local multi-site correlations and at same time provides an order parameter, the typical density of states, for the Anderson localization transition. The relation between the dynamical cluster approximation and the multiple scattering theory is analyzed, and is illustrated for a tight-binding model.

cond-mat.dis-nn↗
arXiv2016

Multitude of phases in correlated lattice fermion systems with spin-dependent disorder

The magnetic phases induced by the interplay between disorder acting only on particles with a given spin projection ("spin-dependent disorder") and a local repulsive interaction is explored. To this end the magnetic ground state phase diagram of the Hubbard model at half-filling is computed within dynamical mean-field theory combined with the geometric average over disorder, which is able to describe Anderson localization. Five distinct phases are identified: a ferromagnetically polarized metal, two types of insulators, and two types of spin-selective localized phases. The latter four phases possess different long-range order of the spins. The predicted phase diagram may be tested experimentally using cold fermions in optical lattices subject to spin-dependent random potentials.

cond-mat.str-el↗
arXiv2015

Spin-selective localization of correlated lattice fermions

The interplay between local, repulsive interactions and disorder acting only on one spin orientation of lattice fermions ("spin-dependent disorder") is investigated. The nonmagnetic disorder vs. interaction phase diagram is computed using Dynamical Mean-Field Theory in combination with the geometric average over disorder. The latter determines the typical local density of states and is therefore sensitive to Anderson localization. The effect of spin-dependent disorder is found to be very different from that of conventional disorder. In particular, it destabilizes the metallic solution and leads to a novel spin-selective, localized phase at weak interactions and strong disorder.

cond-mat.dis-nn↗
arXiv2015

Metal-Insulator Transition and Lattice Instability of Paramagnetic V2O3

We determine the electronic structure and phase stability of paramagnetic V$_2$O$_3$ at the Mott-Hubbard metal-insulator phase transition, by employing a combination of an ab initio method for calculating band structures with dynamical mean-field theory. The structural transformation associated with the metal-insulator transition is found to occur upon a slight expansion of the lattice volume by $\sim 1.5$ %, in agreement with experiment. Our results show that the structural change precedes the metal-insulator transition, implying a complex interplay between electronic and lattice degrees of freedom at the transition. Electronic correlations and full charge self-consistency are found to be crucial for a correct description of the properties of V$_2$O$_3$.

cond-mat.str-el↗
arXiv2014

Correlation-driven topological Fermi surface transition in FeSe

The electronic structure and phase stability of paramagnetic FeSe is computed by using a combination of ab initio methods for calculating band structure and dynamical mean-field theory. Our results reveal a topological change (Lifshitz transition) of the Fermi surface upon a moderate expansion of the lattice. The Lifshitz transition is accompanied with a sharp increase of the local moments and results in an entire reconstruction of magnetic correlations from the in-plane magnetic wave vector $(π,π)$ to $(π,0)$. We attribute this behavior to a correlation-induced shift of the Van Hove singularity originating from the $d_{xy}$ and $d_{xz}/d_{yz}$ bands at the M-point across the Fermi level. We propose that superconductivity is strongly influenced, or even induced, by a Van Hove singularity.

cond-mat.str-el↗
arXiv2014

Effect of correlations and doping on the spin susceptibility of iron pnictides: the case of KFe2As2

The temperature dependence of the paramagnetic susceptibility of the iron pnictide superconductor KFe2As2 and its connection with the spectral properties of that material is investigated by a combination of density functional theory (DFT) in the local density approximation and dynamical mean-field theory (DMFT). Unlike other iron pnictide parent compounds where the typical oxidation state of iron is 2, the formal valence of Fe in KFe2As2 is 2.5, corresponding to an effective doping with 0.5 hole per iron atom compared to, for example, BaFe2As2. This shifts the chemical potential and thereby reduces the distance between the peaks in the spectral functions of KFe2As2 and the Fermi energy as compared to BaFe2As2. The shift, which is clearly seen on the level of DFT as well as in DMFT, is further enhanced by the strong electronic correlations in KFe2As2. In BaFe2As2 the presence of these peaks results [Phys. Rev. B 86, 125124 (2012)] in a temperature increase of the susceptibility up to a maximum at ~1000 K. While the temperature increase was observed experimentally the decrease at even higher temperatures is outside the range of experimental observability. We predict that in KFe2As2 the situation is different. Namely, the reduction of the distance between the peaks and the Fermi level due to doping is expected to shift the maximum in the susceptibility to much lower temperatures, such that the decrease of the susceptibility should become visible in experiment.

cond-mat.str-el↗
arXiv2014

Electronic correlations determine the phase stability of iron up to the melting temperature

We present theoretical results on the high-temperature phase stability and phonon spectra of paramagnetic bcc iron which explicitly take into account many-body effects. Several peculiarities, including a pronounced softening of the [110] transverse (T$_1$) mode and a dynamical instability of the bcc lattice in harmonic approximation are identified. We relate these features to the $α$-to-$γ$ and $γ$-to-$δ$ phase transformations in iron. The high-temperature bcc phase is found to be highly anharmonic and appears to be stabilized by the lattice entropy.

cond-mat.str-el↗
arXiv2013

First-principles Calculation of Atomic Forces and Structural Distortions in Strongly Correlated Materials

We introduce a novel computational approach for the investigation of complex correlated electron materials which makes it possible to evaluate interatomic forces and thereby determine atomic displacements and structural transformations induced by electronic correlations. It combines \textit{ab initio} band structure and dynamical mean-field theory and is implemented with the linear-response formalism regarding atomic displacements. We apply this new technique to explore structural transitions of prototypical correlated systems such as elemental hydrogen, SrVO$_3$, and KCuF$_3$.

cond-mat.str-el↗