arXiv subjects

Andreas Hausoel

Publications and source records attributed to Andreas Hausoel.

10 recordsLinked to original sources

arXiv2026

Magnetic instabilities in three- and higher-dimensional Hubbard models with different lattice geometries: Impact of nonlocal spatial correlations

We analyze the impact of the lattice geometry on the thermodynamic transition to magnetically ordered phases in strongly interacting electron systems for various Bravais lattices in three, four and five dimensions, including both local and nonlocal correlation effects. In a first step we use the dynamical mean-field theory (DMFT), which takes into account purely local correlations, to calculate the magnetic susceptibilities of the Hubbard model on three- (3d-sc), four- (4d-sc), and five-dimensional (5d-sc) simple cubic/hypercubic, as well as on three-dimensional body- (bcc) and face-centered (fcc) cubic lattices, and determine the transition temperature to the corresponding magnetically ordered state. In a second step, we exploit the dynamical vertex approximation (D$Γ$A), a diagrammatic extension of DMFT, to include the effect of nonlocal correlations which are particularly important in the vicinity of the corresponding phase transition. For the bipartite 3d-sc, 4d-sc, 5d-sc and bcc lattices nonlocal fluctuations lead to a substantial reduction of the DMFT transition temperature consistent to the overall tendency of mean-field approaches to overestimate the stability of ordered phases. As expected, the magnitude of the difference between the DMFT, being exact in the limit of large connectivity/dimensions, and D$Γ$A transition temperatures decreases with increasing coordination number. On a more practical perspective, these results also provide a reasonable guidance to evaluate the expected overestimation of the DMFT ordering temperature for different material geometries. For the fcc lattice, on the other hand, the ordered phase observed in DMFT vanishes completely within D$Γ$A which is consistent with the existence of strong geometric frustration in this lattice.

cond-mat.str-el↗

arXiv2025

Superconducting phase diagram of finite-layer nickelates Nd$_{n+1}$Ni$_n$O$_{2n+2}$

Following the successful prediction of the superconducting phase diagram for infinite-layer nickelates, here we calculate the superconducting $T_{\mathrm{c}}$ vs. the number of layers $n$ for finite-layer nickelates using the dynamical vertex approximation. To this end, we start with density functional theory, and include local correlations non-perturbatively by dynamical mean-field theory for $n=2$ to 7. For all $n$, the Ni $d_{x^2-y^2}$ orbital crosses the Fermi level, but for $n>4$ there are additional $(π, π)$ pockets or tubes that slightly enhance the layer-averaged hole doping of the $d_{x^2-y^2}$ orbitals beyond the leading $1/n$ contribution stemming from the valence electron count. We finally calculate $T_{\mathrm{c}}$ for the single-orbital $d_{x^2-y^2}$ Hubbard model by dynamical vertex approximation.

cond-mat.str-el↗

arXiv2024

Closing in on possible scenarios for infinite-layer nickelates: comparison of dynamical mean-field theory with angular-resolved photoemission spectroscopy

Conflicting theoretical scenarios for infinite-layer nickelate superconductors have been hotly debated, particularly regarding whether {only} a single Ni-3$d_{x^2-y^2}$ band is relevant at low energies besides electron pockets or whether multi-orbital physics including Ni-3$d_{z^2}$ is instead essential. The first scenario has emerged from density-functional theory plus dynamical mean-field theory (DFT+DMFT) calculations. Comparing the previous DFT+DMFT spectra to recent angular-resolved photoemission spectroscopy (ARPES) experiments, we find excellent agreement for both the Fermi surface and the strongly renormalized quasi-particle bands, supporting the first scenario. Our key findings further suggest that the "waterfalls" observed in ARPES might emerge from the quasi-particle--to--Hubbard-band crossover, and that additional spectral weight close to the $A$-pocket {likely} originates from the Ni-3$d_{xy}$ orbital.

cond-mat.supr-con↗

arXiv2022

Aberration of the Green's function estimator in hybridization expansion continuous-time quantum Monte Carlo

We describe an aberration of the resampling estimator for the Green's function customarily used in hybridization expansion continuous-time quantum Monte Carlo. It occurs due to Pauli principle constraints in calculations of Anderson impurity models with baths consisting of a discrete energy spectrum. We identify the missing Feynman diagrams, characterize the affected models and discuss implications as well as solutions. This issue does not occur when using worm sampling or in the presence of continuous baths. However certain energy spectra can be inherently close to a discrete limit, and we explain why autocorrelation times can become very large in these cases.

cond-mat.str-el↗

arXiv2019

State- and superstate-sampling in hybridization-expansion continuous-time quantum Monte Carlo

Due to the intrinsic complexity of the quantum many-body problem, quantum Monte Carlo algorithms and their corresponding Monte Carlo configurations can be defined in various ways. Configurations corresponding to few Feynman diagrams often lead to severe sign problems. On the other hand, computing the configuration weight becomes numerically expensive in the opposite limit in which many diagrams are grouped together. Here we show that for continuous-time quantum Monte Carlo in the hybridization expansion the efficiency can be substantially improved by dividing the local impurity trace into fragments, which are then sampled individually. For this technique, which also turns out to preserve the fermionic sign, a modified update strategy is introduced in order to ensure ergodicity. Our (super)state sampling is particularly beneficial to calculations with many $d$-orbitals and general local interactions, such as full Coulomb interaction. For illustration, we reconsider the simple albeit well-known case of a degenerate three-orbital model at low temperatures. This allows us to quantify the coherence properties of the "spin-freezing" crossover, even close to the Mott transition.

cond-mat.str-el↗

arXiv2018

w2dynamics: Local one- and two-particle quantities from dynamical mean field theory

We describe the hybridization-expansion continuous-time quantum Monte Carlo code package "w2dynamics", developed in Wien and Würzburg. We discuss the main features of this multi-orbital quantum impurity solver for the Anderson impurity model, dynamical mean field theory as well as its coupling to density functional theory. The w2dynamics package allows for calculating one- and two-particle quantities; it includes worm and further novel sampling schemes. Details about its download, installation, functioning and the relevant parameters are provided.

cond-mat.str-el↗

arXiv2017

DFT+DMFT study on soft moment magnetism and covalent bonding in SrRu$_2$O$_6$

Dynamical mean-field theory is used to study the three orbital model of the $d^3$ compound SrRu$_2$O$_6$ both with and without explicitly including the O-$p$ states. Depending on the size of the Hund's coupling $J$, at low to intermediate temperatures we find solutions corresponding to Mott or correlated covalent insulator. The latter can explain the experimentally observed absence of Curie susceptibility in the paramagnetic phase. At high temperatures a single phase with smoothly varying properties is observed. SrRu$_2$O$_6$ provides an ideal system to study the competition between the local moment physics and covalent bonding since both effects are maximized, while spin-orbit interaction is found to play a minor role.

cond-mat.str-el↗

arXiv2017

Strongly correlated double Dirac fermions

Double Dirac fermions have recently been identified as possible quasiparticles hosted by three-dimensional crystals with particular non-symmorphic point group symmetries. Applying a combined approach of ab-initio methods and dynamical mean field theory, we investigate how interactions and double Dirac band topology conspire to form the electronic quantum state of Bi$_2$CuO$_4$. We derive a downfolded eight-band model of the pristine material at low energies around the Fermi level. By tuning the model parameters from the free band structure to the realistic strongly correlated regime, we find a persistence of the double Dirac dispersion until its constituting time reveral symmetry is broken due to the onset of magnetic ordering at the Mott transition. We analyze pressure as a promising route to realize a double-Dirac metal in Bi$_2$CuO$_4$.

cond-mat.str-el↗

arXiv2016

Worm Improved Estimators in Continuous-time Quantum Monte Carlo

We derive the improved estimators for general interactions and employ these for the continuous-time quantum Monte Carlo method. Using a worm algorithm we show how measuring higher-ordered correlators leads to an improved high-frequency behavior in irreducible quantities such as the one-particle self-energy or the irreducible two-particle vertex for non-density-density interactions. A good knowledge of the asymptotics of the two-particle vertex is essential for calculating non-local electronic correlations using diagrammatic extensions to the dynamical mean field theory as well as for calculating susceptibilities. We test our algorithm against analytic results for the multi-orbital atomic-limit and the Falicov-Kimball model.

cond-mat.str-el↗

arXiv2015

Continuous-time Quantum Monte Carlo using Worm Sampling

We present a worm sampling method for calculating one- and two-particle Green's functions using continuous-time quantum Monte Carlo simulations in the hybridization expansion (CT-HYB). Instead of measuring Green's functions by removing hybridization lines from partition function configurations, as in conventional CT-HYB, the worm algorithm directly samples the Green's function. We show that worm sampling is necessary to obtain general two-particle Green's functions which are not of density-density type and that it improves the sampling efficiency when approaching the atomic limit. Such two-particle Green's functions are needed to compute off-diagonal elements of susceptibilities and occur in diagrammatic extensions of the dynamical mean field theory and efficient estimators for the single-particle self-energy.

cond-mat.str-el↗