arXiv subjects

Frank Lechermann

Publications and source records attributed to Frank Lechermann.

At least 37 records · Page 2Linked to original sources

arXiv2023

Revisiting the metal-to-metal transition in $2H$-AgNiO$_2$

The layered delafossite compound AgNiO$_2$ with $2H$ stacking symmetry undergoes a structural metal-to-metal transition at $T_{\rm S}\sim 365$ K. It has been described in the past as a charge-ordering transition, where local $S=1$ spins are formed on part of the Ni sites. By means of first-principles many body calculations, we show that the transition is in fact a site-selective Mott transition on the Ni sublattice with only minor charge differentiation. Key to this finding is the uncovering of ligand-hole physics, rendering a Ni$^{2+}$ instead of a formal Ni$^{3+}$ oxidation state, in conjunction with strong local Coulomb repulsions.

cond-mat.str-el↗

arXiv2023

Electronic correlations and superconducting instability in La$_3$Ni$_2$O$_7$ under high pressure

Motivated by the report of superconductivity in bilayer La$_3$Ni$_2$O$_7$ at high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. Below 100\,K, a multi-orbital non-Fermi liquid state resulting from loss of Ni-ligand coherence within a flat-band dominated low-energy landscape is uncovered. The incoherent low-temperature Fermi surface displays strong mixing between Ni-$d_{z^2}$ and Ni-$d_{x^2-y^2}$ orbital character. In a model-Hamiltonian picture, spin fluctuations originating mostly from the Ni-$d_{z^2}$ orbital give rise to strong tendencies towards a superconducting instability with $B_{1g}$ or $B_{2g}$ order parameter. The dramatic enhancement of $T_{\rm c}$ in pressurized La$_3$Ni$_2$O$_7$ is due to stronger Ni-$d_{z^2}$ correlations compared to those in the infinite-layer nickelates.

cond-mat.str-el↗

arXiv2023

Quasiparticle approach to the transport in infinite-layer nickelates

The normal-state transport properties of superconducting infinite-layer nickelates are investigated within an interacting three-orbital model. It includes effective Ni-$d_{z^2}$, Ni-$d_{x^2-y^2}$ bands as well as the self-doping band degree of freedom. Thermopower, Hall coefficient and optical conductivity are modelled within a quasiparticle approximation to the electronic states. Qualitative agreement in comparison to experimentally available Hall data is achieved, with notably a temperature-dependent sign change of the Hall coefficient for larger hole doping $x$. The Seebeck coefficient changes from negative to positive in a non-trivial way with $x$, but generally shows only modest temperature dependence. The optical conductivity shows a pronounced Drude response and a prominent peak structure at higher frequencies due to interband transitions. While the quasiparticle picture is surely approximative to low-valence nickelates, it provides enlightening insights into the multiorbital nature of these challenging systems.

cond-mat.supr-con↗

arXiv2022

Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe3-xGeTe2

Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal that the competing magnetic fluctuations are driven by an orbital selective Mott transition, where only the plane-perpendicular a1g orbital of the Fe(3d) manifold remains itinerant. Our results highlight the multi-orbital character in Fe3-xGeTe2 that supports a rare coexistence of local and itinerant physics within this material.

cond-mat.str-el↗

arXiv2022

Superconducting Instabilities in Strongly-Correlated Infinite-Layer Nickelates

The discovery of superconductivity in infinite-layer nickelates has added a new family of materials to the fascinating growing class of unconventional superconductors. By incorporating the strongly correlated multi-orbital nature of the low-energy electronic degrees of freedom, we compute the leading superconducting instability from magnetic fluctuations relevant for infinite-layer nickelates. Specifically, by properly including the doping dependence of the Ni $d_{x^2-y^2}$ and $d_{z^2}$ orbitals as well as the self-doping band, we uncover a transition from $d$-wave pairing symmetry to nodal $s_\pm$ superconductivity, driven by strong fluctuations in the $d_{z^2}$-dominated orbital states. We discuss the properties of the resulting superconducting condensates in light of recent tunneling and penetration depth experiments probing the detailed superconducting gap structure of these materials.

cond-mat.supr-con↗

arXiv2022

Emergent flat-band physics in $d^{9-δ}$ multilayer nickelates

Recent experiments have shown that the reduced multilayer rare-earth (RE) nickel oxides of form RE$_{p+1}$Ni$_p$O$_{2p+2}$ may belong to the novel family of superconducting lanthanide nickelates. Here, the correlated electronic structure of Pr$_{4}$Ni$_3$O$_{8}$ and Nd$_{6}$Ni$_5$O$_{12}$ is studied by means of an advanced realistic many-body framework. It is revealed that the low-energy physics of both systems is dominated by an interplay of Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$ degrees of freedom. Whilst the Ni-$d_{x^2-y^2}$ orbitals are always highly correlated near an (orbital-selective) Mott-insulating regime, the Ni-$d_{z^2}$ orbitals give rise to intriguing non-dispersive features. At low temperature, the Pr compound still displays QP-like Ni-$d_{x^2-y^2}$-derived states at the Fermi level, but the interacting fermiology of the Nd compound is outshined by an emergent Ni-$d_{z^2}$-controlling flat band. These findings translate well to the previous characterization of doped infinite-layer nickelates, and hence further make the case for a mechanism of unconventional superconductivity which is distinct from the one in high-$T_{\rm c}$ cuprates.

cond-mat.str-el↗

arXiv2022

Dynamical Mean Field Studies of Infinite Layer Nickelates: Physics Results and Methodological Implications

This article summarizes recent work on the many-body (beyond density functional theory) electronic structure of layered rare-earth nickelates, both in the context of the materials themselves and in comparison to the high-temperature superconducting (high-$T_c$) layered copper-oxide compounds. It aims to outline the current state of our understanding of layered nickelates and to show how the analysis of these fascinating materials can shed light on fundamental questions in modern electronic structure theory. A prime focus is determining how the interacting physics defined over a wide energy range can be estimated and "downfolded" into a low energy theory that would describe the relevant degrees of freedom on the $\sim 0.5$ eV scale and that could be solved to determine superconducting and spin and charge density wave phase boundaries, temperature-dependent resistivities, and dynamical susceptibilities.

cond-mat.str-el↗

arXiv2021

Assessing the correlated electronic structure of lanthanum nickelates

The series of nickel-oxide compounds LaNiO$_3$ (formal Ni$(d^7)$), La$_2$NiO$_4$ (formal Ni$(d^8)$) and LaNiO$_2$ (formal Ni$(d^9)$) is investigated by first-principles many-body, using a combination of density functional theory, self-interaction correction and dynamical mean-field theory. The characteristics of these different nickelates, in good agreement with available experimental data, is revealed by employing a compound-independent choice for the local Coulomb interactions. The dichotomy within the low-energy dominant Ni-$e_g$ sector of $\{d_{z^2},d_{x^2-y^2}\}$ kind is rising with growing Ni$(3d)$ filling across the series. An intermediate-coupling scheme for spin-polarized calculations is introduced, which leads to very weak Ni ordered moments for the infinite-layer compound LaNiO$_2$ in contrast to the robust-moment system La$_2$NiO$_4$.

cond-mat.str-el↗

arXiv2021

From basic properties to the Mott design of correlated delafossites

The natural-heterostructure concept realized in delafossites highlights these layered oxides. While metallic, band- or Mott-insulating character may be associated with individual layers, inter-layer coupling still plays a decisive role. We review the correlated electronic structure of PdCoO$_2$, PdCrO$_2$ and AgCrO$_2$, showing that layer-entangled electronic states can deviate from standard classifications of interacting systems. This finding opens up possibilities for materials design in a subtle Mott-critical regime. Manipulated Hidden-Mott physics, correlation-induced semimetallicity, or Dirac/flat-band dispersions in a Mott background are emerging features. Together with achievements in the experimental preparation, this inaugurates an exciting research field in the arena of correlated materials.

cond-mat.str-el↗

arXiv2021

Doping-dependent character and possible magnetic ordering of NdNiO$_2$

The novel nickelate superconductors of infinite-layer type feature challenging electronic pecularities in the normal-state phase diagram with doping. Distinct many-body behavior and different dispersion regimes of the entangled $\{$Ni-$d_{z^2}$, Ni-$d_{x^2-y^2}$$\}$ orbital sector give rise to highly rich physics, which is here studied for the case of the NdNiO$_2$ system. An analysis based on advanced realistic dynamical mean-field theory unveils that the superconducting hole-doped region is the meeting place of a (self-)doped Mott insulator from the underdoped side, and a bad Hund metal from the overdoped side. Fermi-level crossing of the Ni-$d_{z^2}$ flat-band ties both regimes together to form a singular arena for unconventional superconductivity. We furthermore shed light on the intriguing problem of elusive magnetism in infinite-layer nickelates. Antiferromagnetic (AFM) order with small Ni moments is shown to be a vital competitor at low temperature. At stoichiometry, C-AFM order with ferromagnetic spin-alignment along the $c$-axis benefits from a conceivable coexistence with Kondo(-lattice) screening.

cond-mat.str-el↗

arXiv2020

Multiorbital processes rule the Nd$_{1-x}$Sr$_x$NiO$_2$ normal state

The predominant Ni-multiorbital nature of infinite-layer neodynium nickelate at stoichiometry and with doping is revealed. We investigate the correlated electronic structure of NdNiO$_2$ at lower temperatures and show that first-principles many-body theory may account for Kondo(-lattice) features. Yet those are not only based on localized Ni-$d_{x^2-y^2}$ and a Nd-dominated self-doping band, but heavily builds on the participation of Ni-$d_{z^2}$ in a Hund-assisted manner. In a tailored three-orbital study, the half-filled regime of the former inplane Ni orbital remains surprisingly robust even for substantial hole doping $δ$. Reconstructions of the interacting Fermi surface designate the superconducting region within the experimental phase diagram. They furthermore provide clues to recent Hall measurements as well as to the astounding weakly-insulating behavior at larger experimental $δ$. Finally, a strong asymmetry between electron and hole doping, with a revival of Ni single-orbital features in the former case, is predicted. Superconductivity in Nd$_{1-x}$Sr$_x$NiO$_2$ is unlike the one in cuprates of distinct multiorbital kind, building up on nearly localized Ni-$d_{x^2-y^2}$ and itinerant Ni-$d_{z^2}$.

cond-mat.str-el↗

arXiv2020

Spatial inhomogeneity and the metal-insulator transition in Ca$_3$(Ru$_{1-x}$Ti$_x$)$_2$O$_7$

Turning a pristine Mott insulator into a correlated metal by chemical doping is a common procedure in strongly correlated materials physics, e.g. underlying the phenomenology of high-$T_c$ cuprates. The ruthenate bilayer compound Ca$_3$Ru$_2$O$_7$ is a prominent example of a reversed case, namely a correlated metal at stoichiometry that realizes a transition into an insulating state via Ti doping. We here investigate this puzzling metal-insulator transition (MIT) by first-principles many-body theory and elucidate a challenging interplay between electronic correlations and symmetry breakings on the Ru sublattice. While average effects on the Ca$_3$Ru$_2$O$_7$ crystal structure are still relevant, key to the MIT is the cooperation of electronic correlations with the spatial inhomogeneity in the defect regime. Together they give rise to the emergence of site-selective Mott criticality and competing orbital-ordering tendencies.

cond-mat.str-el↗

arXiv2020

Unconventional Hund Metal in a Weak Itinerant Ferromagnet

The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi liquid theory. While magnetic fluctuations play an important role in the materials' unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund's exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets.

cond-mat.str-el↗

arXiv2020

Theoretical design of highly correlated electron states in delafossite heterostructures

Delafossites represent natural heterostructures which can host rather different electronic characteristics in their constituting layers. The design of novel heterostructure architectures highlighting the competition between such varying layer properties is promising from the viewpoint of basic research as well as for future technological applications. By means of the combination of density functional theory and dynamical mean-field theory, we here unveil the formation of highly correlated electron states in delafossite heterostructures build from metallic PdCrO$_2$ and insulating AgCrO$_2$. Due to the sophisticated coupling between layers of strong and of weak internal electron-electron interaction, correlation-induced semimetals at ambient temperature and doped Mott-insulators at lower temperature are predicted. The unique electronic structure of delafossite heterostructures opens a door to research on novel challenging quantum matter.

cond-mat.mtrl-sci↗

arXiv2020

Late transition-metal oxides with infinite-layer structure: Nickelates versus cuprates

The correlated electronic structure of the infinite-layer compounds NdNiO$_2$ and SrCuO$_2$ at stoichiometry and with finite hole doping is compared. Key differences are elucidated from an advanced first-principles many-body perspective. Contrary to the charge-transfer insulating cuprate, the self-doped nickelate remains non-insulating even for large interaction strength, though the Ni-$d_{x^2-y^2}$ spectral weight is also gapped in that limit. Hybridization between Ni$(3d)$ and Nd$(5d)$ is crucial for the appearance of the self-doping band. Upon realistic hole doping, Sr$_{1-y}$CuO$_2$ shows the expected mixed oxygen-Cu-$d_{x^2-y^2}$ (Zhang-Rice) states at low-energy. In the case of Nd$_{1-x}$Sr$_x$NiO$_2$, the self-doping band is shifted to higher energies and a doping-dependent $d_{z^2}$-versus-$d_{x^2-y^2}$ competition on Ni is revealed. The absence of prominent Zhang-Rice physics in infinite-layer nickelates might be relevant to understand the notable difference in the superconducting $T_{\rm c}$'s.

cond-mat.str-el↗

arXiv2019

Interplay of charge-transfer and Mott-Hubbard physics approached by an efficient combination of self-interaction correction and dynamical mean-field theory

Late transition-metal oxides with small charge-transfer energy $Δ$ raise issues for state-of-the-art correlated electronic structure schemes such as the combination of density functional theory (DFT) with dynamical mean-field theory (DMFT). The accentuated role of the oxygen valence orbitals in these compounds asks for an enhanced description of ligand-based correlations. Utilizing the rocksalt-like NiO as an example, we present an advancement of charge self-consistent DFT+DMFT by including self-interaction correction (SIC) applied to oxygen. This introduces explicit onsite O correlations as well as an improved treatment of intersite $p-d$ correlations. Due to the efficient SIC incorporation in a pseudopotential form, the DFT+sicDMFT framework is an advanced but still versatile method to address the interplay of charge-transfer and Mott-Hubbard physics. We revisit the spectral features of stoichiometric NiO and reveal the qualitative sufficiency of local DMFT self-energies in describing spectral peak structures usually associated with explicit nonlocal processes. For Li$_x$Ni$_{1-x}$O, prominent in-gap states are verified by the present theoretical study.

cond-mat.str-el↗

arXiv2019

A key role of correlation effects in the Lifshitz transition in Sr$_2$RuO$_4$

Uniaxial pressure applied along an Ru-Ru bond direction induces an elliptical distortion of the largest Fermi surface of Sr$_2$RuO$_4$, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity. In the present paper, we report new measurements, employing new uniaxial stress apparatus and new measurements of the low-temperature elastic moduli, of the strain at which this Lifshitz transition occurs: a longitudinal strain $\varepsilon_{xx}$ of $(-0.44\pm0.06)\cdot10^{-2}$, which corresponds to a B$_{1g}$ strain $\varepsilon_{xx} - \varepsilon_{yy}$ of $(-0.66\pm0.09)\cdot10^{-2}$. This is considerably smaller than the strain corresponding to a Lifshitz transition in density functional theory calculations, even if the spin-orbit coupling is taken into account. Using dynamical mean-field theory we show that electronic correlations reduce the critical strain. It turns out that the orbital anisotropy of the local Coulomb interaction on the Ru site is furthermore important to bring this critical strain close to the experimental number, and thus well into the experimentally accessible range of strains.

cond-mat.str-el↗

arXiv2018

Doping a Bad Metal: Origin of Suppression of Metal-Insulator Transition in Non-Stoichiometric VO$_2$

Rutile ($R$) phase VO$_2$ is a quintessential example of a strongly correlated bad-metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V dimerized monoclinic phase below T$_{MIT} \sim 340K$. It has been experimentally shown that one can control this transition by doping VO$_2$. In particular, doping with oxygen vacancies ($V_O$) has been shown to completely suppress this MIT {\em without} any structural transition. We explain this suppression by elucidating the influence of oxygen-vacancies on the electronic-structure of the metallic $R$ phase VO$_2$, explicitly treating strong electron-electron correlations using dynamical mean-field theory (DMFT) as well as diffusion Monte Carlo (DMC) flavor of quantum Monte Carlo (QMC) techniques. We show that $V_O$'s tend to change the V-3$d$ filling away from its nominal half-filled value, with the $e_{g}^π$ orbitals competing with the otherwise dominant $a_{1g}$ orbital. Loss of this near orbital polarization of the $a_{1g}$ orbital is associated with a weakening of electron correlations, especially along the V-V dimerization direction. This removes a charge-density wave (CDW) instability along this direction above a critical doping concentration, which further suppresses the metal-insulator transition. Our study also suggests that the MIT is predominantly driven by a correlation-induced CDW instability along the V-V dimerization direction.

cond-mat.mtrl-sci↗