arXiv subjects

Frank Lechermann

Publications and source records attributed to Frank Lechermann.

At least 73 records · Page 4Linked to original sources

arXiv2015

Large-amplitude spin oscillations triggered by nonequilibrium strongly correlated $t_{2g}$ electrons

Laser-induced ultrafast (fs) magnetization experiments in antiferromagnets have recently attracted large attention, paving the road for inherently fast spin dynamics in the THz regime without invoking stray fields. The technical importance is emphasized by the rising new research field of antiferromagnetic (AFM) spintronics, where superexchange-dominated strongly correlated compounds provide an interesting materials playground. An intriguing question is whether the Coulomb interaction may be a key to control AFM order on ultrafast time scales. Therefore, we study (de)magnetization processes in a time-dependent multiorbital Hubbard model, focusing on $t_{2g}$ electrons in a wider doping range. Depending on filling, we reveal large-amplitude spin oscillations via interaction quenches from the antiferromagnetic or paramagnetic state. Nonequilibrium ultrafast spin-orientation effects in prominent correlated transition-metal oxides are therefrom predicted.

cond-mat.str-el↗

arXiv2014

Electron correlation and magnetism at the LaAlO$_3$/SrTiO$_3$ interface: A DFT+DMFT investigation

We shed light on the interplay between structure and many-body effects relevant for itinerant ferromagnetism in LaAlO$_3$/SrTiO$_3$ heterostructures. The realistic correlated electronic structure is studied by means of the (spin-polarized) charge self-consistent combination of density functional theory (DFT) with dynamical mean-field theory (DMFT) beyond the realm of static correlation effects. Though many-body behavior is also active in the defect-free interface, a ferromagnetic instability occurs only with oxygen vacancies. A minimal Ti two-orbital $e_g$-$t_{2g}$ description for the correlated subspace is derived. Magnetic order affected by quantum fluctuations builds up from effective double exchange between nearly-localized $e_g$ and mobile $xy$ electrons.

cond-mat.mtrl-sci↗

arXiv2014

From Hubbard bands to spin-polaron excitations in the doped Mott material Na$_x$CoO$_2$

We investigate the excitation spectrum of strongly correlated sodium cobaltate within a realistic many-body description beyond dynamical mean-field theory (DMFT). At lower doping around $x$=0.3, rather close to Mott-critical half-filling, the single-particle spectral function of Na$_x$CoO$_2$ displays an upper Hubbard band which is captured within DMFT. Momentum-dependent self-energy effects beyond DMFT become dominant at higher doping. Around a doping level of $x\sim 0.67$, the incoherent excitations give way to finite-energy spin-polaron excitations in close agreement with optics experiments. These excitations are a direct consequence of the formation of bound states between quasiparticles and paramagnons in the proximity to in-plane ferromagnetic ordering.

cond-mat.str-el↗

arXiv2014

How chromium doping affects the correlated electronic structure of V2O3

The archetypical strongly correlated Mott-phenomena compound V2O3 is known to show a paramagnetic metal-insulator transition driven by doping with chromium atoms and/or (negative) pressure. Via charge self-consistent density-functional theory+dynamical mean-field theory calculations we demonstrate that these two routes cannot be understood as equivalent. To this end, the explicit description of Cr-doped V2O3 by means of supercell calculations and the virtual crystal approximation is performed. Already the sole introduction of chromium's additional electron to the system is shown to modify the overall correlated electronic structure substantially. Correlation-induced charge transfers between Cr and the remaining V ions occur and the transition-metal orbital polarization is increased by the electron doping, in close agreement with experimental findings.

cond-mat.str-el↗

arXiv2014

Theoretical prediction of a strongly correlated Dirac metal

Recently, the most intensely studied objects in the electronic theory of solids have been strongly correlated systems and graphene. However, the fact that the Dirac bands in graphene are made up of $sp^{2}$-electrons, which are subject to neither strong Hubbard repulsion $U$ nor strong Hund's rule coupling $J$ creates certain limitations in terms of novel, interaction-induced physics that could be derived from Dirac points. Here we propose GaCu$_{3}$(OH)$_{6}$Cl$_{2}$ (Ga-substituted herbertsmithite) as a correlated Dirac-Kagome metal combining Dirac electrons, strong interactions and frustrated magnetism. Using density functional theory (DFT), we calculate its crystallographic and electronic properties, and observe that it has symmetry-protected Dirac points at the Fermi level. Its many-body physics is excitingly rich, with possible charge, magnetic and superconducting instabilities. Through a combination of various many-body methods we study possible symmetry-lowering phase transitions such as Mott-Hubbard, charge or magnetic ordering, and unconventional superconductivity, which in this compound assumes an $f$-wave symmetry.

cond-mat.str-el↗

arXiv2013

Extended dynamic Mott-transition in the two-band Hubbard model out of equilibrium

We reformulate the time-dependent Gutzwiller approximation by M. Schiró and M. Fabrizio [Phys. Rev. Lett. 105, 076401 (2010)] in the framework of slave-boson mean-field theory, which is used to investigate the dynamical Mott transition of the generic two-band Hubbard model at half filling upon an interaction quench. Interorbital fluctuations lead to notable changes with respect to the single-band case. The singular dynamical transition is replaced by a broad regime of long-lived fluctuations between metallic and insulating states, accompanied by intriguing precursor behavior. A mapping to a spin model proves helpful to analyze the different regions in terms of the evolution of an Ising-like order parameter. Contrary to the static case, singlet occupations remain vital in the Mott-insulating regime with finite Hund's exchange.

cond-mat.str-el↗

arXiv2013

Getting back to Na_xCoO_2: spectral and thermoelectric properties

Sodium cobaltate Na_xCoO_2 as dopable strongly correlated layered material with a triangular sublattice still poses a challenging problem in condensed matter. The intriguing interplay between lattice, charge, spin and orbital degrees of freedom leads to a complex phase diagram bounded by a nominal Mott (x=0) regime and a band-insulating (x=1) phase. By means of the charge self-consistent density functional theory (DFT) plus dynamical mean-field theory (DMFT) scheme, built on a pseudopotential framework combined with a continuous-time quantum Monte-Carlo solver, we here study the one-particle spectral function A(k,ω) as well as the thermopower S(T). The computations may account for the suppression of the e_g' pockets in A(k,ω) at lower doping in line with photoemission experiments. Enhancement of the thermopower is verified within the present elaborate multi-orbital method to treat correlated materials. In addition, the two-particle dynamic spin susceptibility χ_s(ω,q) is investigated based on a simplified tight-binding approach, yet by including vertex contributions in the DMFT linear response. Besides the identification of paramagnon branches at higher doping, a prominent high-energy antiferromagnetic mode close to x=0.67 is therewith identified in χ_s(ω,q), which can be linked to extended hopping terms on the CoO_2 sublattice.

cond-mat.str-el↗

arXiv2013

Formation of orbital-selective electron states in LaTiO$_3$/SrTiO$_3$ superlattices

The interface electronic structure of correlated LaTiO$_3$/SrTiO$_3$ superlattices is investigated by means of the charge self-consistent combination of the local density approximation (LDA) to density functional theory (DFT) with dynamical mean-field theory (DMFT). Utilizing a pseudopotential technique together with a continuous-time quantum Monte-Carlo approach, the resulting complex multiorbital electronic states are addressed in a coherent fashion beyond static mean-field. General structural relaxations are taken into account on the LDA level and cooperate with the driving forces from strong electronic correlations. This alliance leads to an Ti($3d_{xy}$) dominated low-energy quasiparticle peak and a lower Hubbard band in line with photoemission studies. Furthermore correlation effects close to the band-insulating bulk SrTiO$_3$ limit as well as the Mott-insulating bulk LaTiO$_3$ limit are studied via realistic single-layer embeddings.

cond-mat.str-el↗

arXiv2012

Multi-orbital physics in Fermi liquids prone to magnetic order

The interplay of spin-orbit-coupling and strong electronic correlations is studied for the single-layer and the bilayer compound of the strontium ruthenate Ruddlesden-Popper series by a combination of first-principles band-structure theory with mean-field rotationally invariant slave bosons. At equilibrium strongly renormalized (spin-orbit-split) quasiparticle bands are traced and a thorough description of the low-energy regime for the nearly ferromagnetic bilayer system in accordance with experimental data is presented. The metamagnetic response of Sr$_3$Ru$_2$O$_7$ in finite magnetic field $H$ is verified and a detailed analysis of the underlying correlated electronic structure provided. Intriguing multi-orbital physics on both local and itinerant level, such as e.g. competing paramagnetic and diamagnetic contributions, is observed with important differences depending on the magnetic-field angle $θ$ with the crystallographic c axis.

cond-mat.str-el↗

arXiv2012

Approaching finite-temperature phase diagrams of strongly correlated materials: a case study for V2O3

Examining phase stabilities and phase equilibria in strongly correlated materials asks for a next level in the many-body extensions to the local-density approximation (LDA) beyond mainly spectroscopic assessments. Here we put the charge-self-consistent LDA+dynamical mean-field theory (DMFT) methodology based on projected local orbitals for the LDA+DMFT interface and a tailored pseudopotential framework into action in order to address such thermodynamics of realistic strongly correlated systems. Namely a case study for the electronic phase diagram of the well-known prototype Mott-phenomena system V$_2$O$_3$ at higher temperatures is presented. We are able to describe the first-order metal-to-insulator transitions with negative pressure and temperature from the self-consistent computation of the correlated total energy in line with experimental findings.

cond-mat.mtrl-sci↗

arXiv2012

Impact of the Dzyaloshinskii-Moriya interaction in strongly correlated itinerant systems

Spin-only approaches to anisotropic effects in strongly interacting materials are often insufficient for systems close to the Mott regime. Within a model context, here the consequences of the low-symmetry relevant Dzyaloshinskii-Moriya (DM) interaction are studied for strongly correlated, but overall itinerant, systems. Namely, we investigate the Hubbard bilayer model supplemented by a DM term at half filling and in the hole-doped regime. As an add-on, further results for the two- impurity Anderson model with DM interaction are also provided. The model Hamiltonians are treated by means of the rotational invariant slave boson technique at saddle point within a (cellular) cluster approach. Already small values of the anisotropic interaction prove to have a strong influence on the phases and correlation functions with increasing U. An intriguing metallic spin-flop phase is found in the doped bilayer model and a reduction of the RKKY exchange in the two-impurity model.

cond-mat.str-el↗

arXiv2011

Competing orders in Na$_x$CoO$_2$ from strong correlations on a two-particle level

Based on dynamical mean-field theory with a continuous-time quantum Monte-Carlo impurity solver, static as well as dynamic spin and charge susceptibilites for the phase diagram of the sodium cobaltate system Na$_x$CoO$_2$ are discussed. The approach includes important vertex contributions to the q-dependent two-particle response functions by means of a local approximation to the irreducible vertex function in the particle-hole channel. A single-band Hubbard model suffices to reveal several charge- and spin-instability tendencies in accordance with experiment, including the stabilization of an effective kagome sublattice close to x=0.67, without invoking the doping-dependent Na-potential landscape. The in-plane antiferromagnetic-to-ferromagnetic crossover is additionally verified by means of the computed Korringa ratio. Moreover an intricate high-energy mode in the transverse spin susceptiblity is revealed, pointing towards a strong energy dependence of the effective intersite exchange.

cond-mat.str-el↗

arXiv2011

Strong correlations enhanced by charge-ordering in highly doped cobaltates

We present an explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit, which relies on the key effect of charge ordering. Blocking a significant fraction of the lattice sites deeply modifies the electronic structure in a way that is shown to be quantitatively consistent with photoemission experiments. It also makes the system highly sensitive to interactions (especially to intersite ones), hence accounting for the strong correlations effects observed in this regime, such as the high effective mass and quasiparticle scattering rate. These conclusions are supported by a theoretical study of an extended Hubbard model with a realistic band structure on an effective kagomè lattice.

cond-mat.str-el↗

arXiv2011

Orthogonal Polynomial Representation of Imaginary-Time Green's Functions

We study the expansion of single-particle and two-particle imaginary-time Matsubara Green's functions of quantum impurity models in the basis of Legendre orthogonal polynomials. We discuss various applications within the dynamical mean-field theory (DMFT) framework. The method provides a more compact representation of the Green's functions than standard Matsubara frequencies and therefore significantly reduces the memory-storage size of these quantities. Moreover, it can be used as an efficient noise filter for various physical quantities within the continuous-time quantum Monte Carlo impurity solvers recently developed for DMFT and its extensions. In particular, we show how to use it for the computation of energies in the context of realistic DMFT calculations in combination with the local density approximation to the density functional theory (LDA+DMFT) and for the calculation of lattice susceptibilities from the local irreducible vertex function.

cond-mat.str-el↗

arXiv2011

LDA+Slave-Boson approach to the correlated electronic structure of the metamagnetic bilayer ruthenate Sr$_3$Ru$_2$O$_7$

The combination of the local-density approximation (LDA) with the rotationally invariant slave-boson theory (RISB) is used to investigate the realistic correlated electronic structure of Sr$_3$Ru$_2$O$_7$. From Wannier-downfolding the low-energy band structure to a three-band model for the Ru($t_{2g}$) states, the interacting problem is solved including intra- and inter-orbital Hubbard terms as well as spin-flip and pair-hopping interactions. Therewith it is possible to obtain valuable insight into the orbital occupations, relevant local spin multiplets and the fermiology with increasing correlation strength. Besides generic correlation-induced band-narrowing and -shifting, an intriguing quasiparticle structure close to the Fermi level is found in the neigborhood of the notorious $γ_2$ pocket in the Brillouin zone. Along the $Γ$$-$$X$ direction in {\bf k}-space, that structure appears very sensitive to electronic self-energy effects. The subtle sensitivity, connected also its manifest multi-orbital character, may put this very low-energy structure in context with the puzzling metamagnetic properties of the compound.

cond-mat.str-el↗

arXiv2010

Considerable non-local electronic correlations in strongly doped Na$_x$CoO$_2$

The puzzling electronic correlation effects in the sodium cobaltate system are studied by means of the combination of density functional theory and the rotationally invariant slave boson (RISB) method in a cellular-cluster approach. Realistic non-local correlations are hence described in the short-range regime for finite Coulomb interactions on the underlying frustrated triangular lattice. A local Hubbard U is sufficient to model the gross in-plane magnetic behavior with doping x, namely antiferromagnetic correlations at intermediate doping and the onset of ferromagnetic order above x>3/4 with a mixed phase for 0.62<x<3/4. Important insight is thereby provided by the occupations of local cluster multiplets retrieved from the RISB framework. The extended modeling of the x$\ge$2/3 doping regime with an additional inter-site Coulomb repulsion V on an experimentally verified effective kagomé lattice allows to account for relevant charge-ordering physics. Therewith a fluctuating charge-density-wave state with small quasiparticle weight and a maximum in-plane magnetic susceptibility may be identified at x$\sim$3/4, just where the magnetic ordering sets in.

cond-mat.str-el↗

arXiv2010

Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3

Albeit structurally and electronically very similar, at low temperature the quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator transition via the appearance of a charge-density-wave state, while BaVSe3 apparently remains metallic down to zero temperature. This different behavior upon cooling is studied by means of density functional theory and its combination with the dynamical mean-field theory and the rotationally-invariant slave-boson method. We reveal several subtle differences between these chalcogenides that provide indications for the deviant behavior of BaVSe3 at low temperature. In this regard, a smaller Hubbard U in line with an increased relevance of the Hund's exchange J plays a vital role.

cond-mat.str-el↗

arXiv2010

Realistic modeling of the electronic structure and the effect of correlations for Sn/Si(111) and Sn/Ge(111) surfaces

The correlated electronic structure of the submonolayer surface systems Sn/Si(111) and Sn/Ge(111) is investigated by density-functional theory (DFT) and its combination with explicit many-body methods. Namely, the dynamical mean-field theory and the slave-boson mean-field theory are utilized for the study of the intriguing interplay between structure, bonding and electronic correlation. In this respect, explicit low-energy one- and four($sp^2$-like)-band models are derived using maximally-localized Wannier(-like) functions. In view of the possible low-dimensional magnetism in the Sn submonolayers we compare different types of magnetic orders and indeed find a 120$^\circ$ antiferromagnetic ordering to be stable in the ground state. With single-site methods and cellular-cluster extensions the influence of a finite Hubbard $U$ on the surface states in a planar and a reconstructed structural geometry is furthermore elaborated.

cond-mat.mtrl-sci↗