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We study the electronic thermal conductivity $\kappa_\textrm{el}$ and the thermal diffusion constant $D_\textrm{Q,el}$ in the square lattice Hubbard model using the finite-temperature Lanczos method. We exploit the Nernst-Einstein relation for thermal transport and interpret the strong non-monotonous temperature dependence of $\kappa_\textrm{el}$ in terms of that of $D_\textrm{Q,el}$ and the electronic specific heat $c_\textrm{el}$. We present also the results for the Heisenberg model on a square lattice and ladder geometries. We study the effects of doping and consider the doped case also with the dynamical mean-field theory. We show that $\kappa_\textrm{el}$ is below the corresponding Mott-Ioffe-Regel value in almost all calculated regimes, while the mean free path is typically above or close to lattice spacing. We discuss the opposite effect of quasi-particle renormalization on charge and heat diffusion constants. We calculate the Lorenz ratio and show that it differs from the Sommerfeld value. We discuss our results in relation to experiments on cuprates. Additionally, we calculate the thermal conductivity of overdoped cuprates within the anisotropic marginal Fermi liquid phenomenological approach., Comment: 8+5 figures

We investigate the nonequilibrium electronic structure and characteristic timescales in a candidate excitonic insulator, ${\mathrm{Ta}}_{2}\mathrm{Ni}{\mathrm{Se}}_{5}$, using time- and angle-resolved photoemission spectroscopy with a temporal resolution of 50 fs. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a timescale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time associated with the rise of the photoemission intensity above the Fermi energy decreases with increasing excitation strength, while the relaxation time of the electron population towards equilibrium shows an opposite behavior. We argue that these experimental observations can be consistently explained by an excitonic origin of the band gap in the material. The excitonic picture is supported by microscopic calculations based on the nonequilibrium Green's function formalism for an interacting two-band system. We interpret the speedup of the rise time with fluence in terms of an enhanced scattering probability between photoexcited electrons and excitons, leading to an initially faster decay of the order parameter. We show that the inclusion of electron-phonon coupling at a semiclassical level changes only the quantitative aspects of the proposed dynamics, while the qualitative features remain the same. The experimental observations and microscopic calculations allow us to develop a simple and intuitive phenomenological model that captures the main dynamics after photoexcitation in ${\mathrm{Ta}}_{2}\mathrm{Ni}{\mathrm{Se}}_{5}$.

We study the spin diffusion and spin conductivity in the square lattice Hubbard model by using the finite-temperature Lanczos method. We show that the spin diffusion behaves differently from the charge diffusion and has a nonmonotonic $T$ dependence. This is due to a progressive liberation of charges that contribute to spin transport and enhance it beyond that active at low temperature due to the Heisenberg exchange. We further show that going away from half filling and zero magnetization increases the spin diffusion, but that the increase is insufficient to reconcile the difference between the model calculations and the recent measurements on cold atoms.

High-temperature bad-metal transport has been recently studied both theoretically and in experiments as one of the key signatures of strong electronic correlations. Here we use the dynamical mean field theory (DMFT) and its cluster extensions, as well as the finite-temperature Lanczos method (FTLM) to explore the influence of lattice frustration on the thermodynamic and transport properties of the Hubbard model at high temperatures. We consider the triangular and the square lattice at half-filling and at 15\% hole-doping. We find that for $T \gtrsim 1.5t$ the self-energy becomes practically local, while the finite-size effects become small at lattice-size $4 \times 4$ for both lattice types and doping levels. The vertex corrections to optical conductivity, which are significant on the square lattice even at high temperatures, contribute less on the triangular lattice. We find approximately linear temperature dependence of dc resistivity in doped Mott insulator for both types of lattices., 11 pages

Perovskite rhodates are characterized by intermediate strengths of both electronic correlation as well as spin-orbit coupling (SOC) and usually behave as moderately correlated metals. A recent publication [Phys. Rev. B 95, 245121 (2017)] on epitaxial ${\mathrm{SrRhO}}_{3}$ thin films reported a bad-metallic behavior and suggested the occurrence of antiferromagnetism below 100 K. We have further studied this ${\mathrm{SrRhO}}_{3}$ thin film by hard x-ray photoemission spectroscopy and found a very small density of states (DOS) at the Fermi level (${E}_{F}$), which is consistent with the previously reported bad-metallic behavior. However, the absence of DOS persists up to room temperature, which contradicts the explanation of antiferromagnetic transition at $\ensuremath{\sim}100$ K. We also employed electronic structure calculations within the framework of density functional theory and dynamical mean-field theory. In contrast to the photoemission results, our calculations indicate metallic behaviors of both bulk ${\mathrm{SrRhO}}_{3}$ and the ${\mathrm{SrRhO}}_{3}$ thin film, and a stronger correlation effect was observed in the thin film than that in the bulk. The calculated uniform magnetic susceptibility is substantially larger in the thin film than that in the bulk. We also investigated the role of SOC and found only a moderate modulation of the band structure. Hence SOC is not expected to significantly affect the electronic correlation in ${\mathrm{SrRhO}}_{3}$. Extrinsic effects of finite-thickness effects beyond our calculations and localization effects may play important roles to induce the negligible spectral weight at ${E}_{F}$.

Monoclinic semimetallic ${\mathrm{Mo}}_{8}{\mathrm{O}}_{23}$ belongs to a multifunctional series of compounds showing multiple ordering phenomena that have not achieved much attention until now. Previous x-ray studies of this compound have revealed an incommensurate ordering transition at ${T}_{\mathrm{IC}}\ensuremath{\sim}350$ K, followed by a structural transition to commensurate order at ${T}_{\mathrm{IC}\ensuremath{-}\mathrm{C}}=285$ K. In addition, an enigmatic resistance maximum is observed at ${T}_{\mathrm{el}}\ensuremath{\sim}150$ K, whose origin has so far proved elusive. Aiming to disentangle these multiple orders we use the polarized transient optical spectroscopy supplemented by Raman spectroscopy to study the electronic relaxation dynamics and lattice vibrational modes in ${\mathrm{Mo}}_{8}{\mathrm{O}}_{23}$ single crystals. Remarkably, both the coherent vibrational mode response and single-particle response display extrema of damping/relaxation times close to ${T}_{\mathrm{el}}$ with the concurrent appearance of new coherent vibrational modes and a characteristic polarization asymmetry which saturates below ${T}_{\mathrm{el}}$. The single-particle relaxation data analysis shows the appearance of a temperature-independent gap in the electronic excitation spectrum below ${T}_{\mathrm{IC}}$ and additional temperature-dependent gap opening near ${T}_{\mathrm{el}}$. Concurrently, a low-frequency vibrational mode shows anomalous softening around ${T}_{\mathrm{m}}\ensuremath{\sim}200$ K, far below ${T}_{\mathrm{IC}\ensuremath{-}\mathrm{C}}$ and ${T}_{\mathrm{IC}}$. The observations are interpreted in terms of the appearance of a hidden gapped state below ${T}_{\mathrm{el}}$ that has so far eluded detection by structural analyses.

We investigate the influence of spin-orbit coupling $\ensuremath{\lambda}$ in strongly-correlated multiorbital systems that we describe by a three-orbital Hubbard-Kanamori model on a Bethe lattice. We solve the problem at all integer fillings $N$ with the dynamical mean-field theory using the continuous-time hybridization expansion Monte Carlo solver. We investigate how the quasiparticle renormalization $Z$ varies with the strength of spin-orbit coupling. The behavior can be understood for all fillings except $N=2$ in terms of the atomic Hamiltonian (the atomic charge gap) and the polarization in the $j$ basis due to spin-orbit induced changes of orbital degeneracies and the associated kinetic energy. At $N=2, \ensuremath{\lambda}$ increases $Z$ at small $U$ but suppresses it at large $U$, thus eliminating the characteristic Hund's metal tail in $Z(U)$. We also compare the effects of the spin-orbit coupling to the effects of a tetragonal crystal field. Although this crystal field also lifts the orbital degeneracy, its effects are different, which can be understood in terms of the different form of the interaction Hamiltonian expressed in the respective diagonal single-particle basis.

We present an implementation of the rotationally invariant slave boson technique as an impurity solver for density functional theory plus dynamical mean field theory (DFT+DMFT). Our approach provides explicit relations between quantities in the local correlated subspace treated with DMFT and the Bloch basis used to solve the DFT equations. In particular, we present an expression for the mass enhancement of the quasiparticle states in reciprocal space. We apply the method to the study of the electronic correlations in Sr2RuO4 under anisotropic strain. We find that the spin-orbit coupling plays a crucial role in the mass enhancement differentiation between the quasi-one-dimensional α and β bands, and on its momentum dependence over the Fermi surface. The mass enhancement, however, is only weakly affected by either uniaxial or biaxial strain, even across the Lifshitz transition induced by the strain. Fil: Facio, Jorge Ismael. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Leibniz-institut Für Festkörper- Und Werkstoffforschung Dresden; Alemania Fil: Mravlje, Jernej. Jozef Stefan Institute; Eslovenia Fil: Pourovskii, Leonid. Collège de France; Francia. Université Paris-Saclay; Francia Fil: Cornaglia de la Cruz, Pablo Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Vildosola, Veronica Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina

We investigate the effects of the spin-orbit coupling (SOC) in a three-orbital impurity model with Kanamori interaction using the numerical renormalization group method. We focus on the impurity occupancy $N_d=2$ relevant to the dynamical mean-field theory studies of Hund's metals. Depending on the strength of SOC $\lambda$ we identify three regimes: usual Hund's impurity for $|\lambda| \lambda_c$, and a $J=2$ impurity for $\lambda < -\lambda_c$. They all correspond to a Fermi liquid but with very different quasiparticle phase shifts and different physical properties. The crossover between these regimes is controlled by an emergent scale, the orbital Kondo temperature, $\lambda_c =T_K^\mathrm{orb}$ that drops with increasing interaction strength. This implies that oxides with strong electronic correlations are more prone to the effects of the spin-orbit coupling., Comment: 5 pages, 3 figures

We discuss the low-energy physics of the three-orbital Anderson impurity model with the Coulomb interaction term of the Kanamori form which has orbital SO(3) and spin SU(2) symmetry and describes systems with partially occupied $t_{2g}$ shells. We focus on the case with two electrons in the impurity that is relevant to Hund's metals. Using the Schrieffer-Wolff transformation we derive an effective Kondo model with couplings between the bulk and impurity electrons expressed in terms of spin, orbital, and orbital quadrupole operators. The bare spin-spin Kondo interaction is much smaller than the orbit-orbit and spin-orbital couplings or is even ferromagnetic. Furthermore, the perturbative scaling equations indicate faster renormalization of the couplings related to orbital degrees of freedom compared to spin degrees of freedom. Both mechanisms lead to a slow screening of the local spin moment. The model thus behaves similarly to the related quantum impurity problem with a larger SU(3) orbital symmetry (Dworin-Narath interaction) where this was first observed. We find that the two problems actually describe the same low-energy physics since the SU(3) symmetry is dynamically established through the renormalization of the splittings of coupling constants to zero. The perturbative renormalization group results are corroborated with the numerical-renormalization group (NRG) calculations. The dependence of spin Kondo temperatures and orbital Kondo temperatures as a function of interaction parameters, the hybridization, and the impurity occupancy is calculated and discussed., Comment: 12 pages

A comparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials ${\text{CaRuO}}_{3}$ and ${\text{SrRuO}}_{3}$ is presented. Phase diagrams are determined for both materials as a function of Hubbard repulsion $U$ and Hund's rule coupling $J$. Metallic and insulating phases are found, as are ferromagnetic and paramagnetic states. The locations of the relevant phase boundaries are determined. Based on the computed phase diagrams, Mott dominated and Hund's dominated regimes of strong correlation are distinguished. Comparison of calculated properties to experiments indicates that the actual materials are in the Hund's coupling dominated region of the phase diagram so can be characterized as Hund's metals, in common with other members of the ruthenate family. Comparison of the phase diagrams for the two materials reveals the role played by rotational and tilt $({\text{GdFeO}}_{3}$-type) distortions of the ideal perovskite structure. The presence of magnetism in ${\text{SrRuO}}_{3}$ and its absence in ${\text{CaRuO}}_{3}$ despite the larger mass and larger tilt/rotational distortion amplitude of ${\text{CaRuO}}_{3}$ can be understood in terms of density of states effects in the presence of strong Hund's coupling. Comparison of the calculated low-$T$ properties of ${\text{CaRuO}}_{3}$ to those of ${\text{SrRuO}}_{3}$ provides insight into the effects of magnetic order on the properties of a Hund's metal. The study provides a simultaneous description of magnetism and correlations and explicates the roles played by band theory and Hubbard and Hund's interactions.

We investigate the electronic structure of a perovskite-type Pauli paramagnet ${\mathrm{SrMoO}}_{3}$ (${{t}_{2g}}^{2}$) thin film using hard x-ray photoemission spectroscopy and compare the results to realistic calculations that combine density functional theory within the local-density approximation (LDA) with dynamical mean-field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially filled ${t}_{2g}$ bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. This interpretation is supported by DMFT model calculations including Hund's rule coupling, which show a renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around $\ensuremath{-}2.5$ eV that is not present in the LDA+DMFT results, pointing to a source of correlations that is not present in our calculations that include only on-site interactions. We interpret this weight as a plasmon satellite, which is supported by the measured core-level spectra that all show satellites at this energy.

We investigate the frequency and temperature dependence of the low-energy electron dynamics in a Landau Fermi liquid with a local self-energy. We show that the frequency and temperature dependencies of the optical conductivity obey universal scaling forms, for which explicit analytical expressions are obtained. For the optical conductivity and the associated memory function, we obtain a number of surprising features that differ qualitatively from the Drude model and are universal characteristics of a Fermi liquid. Different physical regimes of scaling are identified, with marked non-Drude features in the regime where hbar omega ~ kB T. These analytical results for the optical conductivity are compared to numerical calculations for the doped Hubbard model within dynamical mean-field theory. For the "universal" low-energy electrodynamics, we obtain perfect agreement between numerical calculations and analytical scaling laws. Both results show that the optical conductivity displays a non-Drude "foot", which could be easily mistaken as a signature of breakdown of the Fermi liquid, while it actually is a striking signature of its applicability. The aforementioned scaling laws provide a quantitative tool for the experimental identification and analysis of the Fermi-liquid state using optical spectroscopy, and a powerful method for the identification of alternative states of matter, when applicable., Published version

Recent angular-resolved photoemission experiments on LaNiO$_3$ reported a renormalization of the Fermi velocity of $e_g$ quasiparticles, a kink in their dispersion at $ -0.2$ eV and a large broadening and weakened dispersion of the occupied $t_{2g}$ states. We show here that all these features result from electronic correlations and are quantitatively reproduced by calculations combining density-functional theory and dynamical mean-field theory. The importance and general relevance of correlation effects in filled bands coupled by inter-orbital interactions to a partially-filled band are pointed out., Comment: 5 pages, 5 figures

We investigate the competition between pair entanglement of two spin qubits in double quantum dots attached to leads with various topologies and the separate entanglement of each spin with nearby electrodes. Universal behavior of entanglement is demonstrated in dependence on the mutual interactions between the spin qubits, the coupling to their environment, temperature and magnetic field. As a consequence of quantum phase transition an abrupt switch between fully entangled and unentangled states takes place when the dots are coupled in parallel., Comment: 3 figures

Zero temperature linear response conductance of molecules with Coulomb interaction and with various types of phonon modes is analysed together with local occupation, local moment, charge fluctuations and fluctuations of molecular deformation. Deformation fluctuations are quantitatively related to charge fluctuations which exhibit similarity also to static charge susceptibility., Comment: 4 pages, color figures