arXiv subjects

Alen Horvat

Publications and source records attributed to Alen Horvat.

5 recordsLinked to original sources

arXiv2019

Non-Fermi-liquid fixed point in multi-orbital Kondo impurity model relevant for Hund's metals

Due to the separation between the spin and the orbital screening scales, the normal state of Hund's metals at ambient temperature can be loosely characterized as a partially coherent state with fluctuating spins and quenched orbital moments. With the aim to characterize this situation more precisely, we investigate the Kondo-Kanamori impurity model that describes the low-energy local physics of three-orbital Hund's metals occupied by two or four electrons. Within this model one can diminish the mixed spin-orbital terms and thereby enhance the separation between the two screening scales, allowing a more precise investigation of the intermediate state. Using the numerical renormalization group we calculate the impurity entropy as well as the temperature and frequency dependence of the spin and the orbital susceptibilities. We uncover a non-Fermi-liquid two-channel overscreened SU(3) fixed point that controls the behavior in the intermediate regime. We discuss its fingerprints in the frequency dependence of local orbital susceptibility and the shape of the spectral function.

cond-mat.str-el↗

arXiv2017

Spin-orbit coupling in three-orbital Kanamori impurity model and its relevance for transition-metal oxides

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 $λ$ we identify three regimes: usual Hund's impurity for $|λ|<λ_c$, van-Vleck non-magnetic impurity for $λ> λ_c$, and a $J=2$ impurity for $λ< -λ_c$. 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, $λ_c =T_K^\mathrm{orb}$ 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.

cond-mat.str-el↗

arXiv2016

Kondo effect at low electron density and high particle-hole asymmetry in 1D, 2D, and 3D

Using the perturbative scaling and the NRG, we study the characteristic energy scales in the Kondo impurity problem as a function of the exchange coupling constant $J$ and the conduction electron density. We discuss the relation between the impurity binding energy $ΔE$ and the Kondo temperature $T_K$. We find that the two are proportional only for large values of $J$, whereas in the weak-coupling limit the energy gain is quadratic in $J$, while the Kondo temperature is exponentially small. The exact relation between the two quantities depends on the detailed form of the density of states of the band. In the limit of low electron density the Kondo screening is affected by the strong particle-hole asymmetry due to the presence of the band-edge van Hove singularities. We consider the cases of 1D, 2D, and 3D tight-binding lattices with inverse-square-root, step function, and square-root onsets of the density of states that are characteristic of the respective dimensionalities. We always find two different regimes depending on whether $T_K$ is higher or lower than $μ$, the chemical potential measured from the bottom of the band. For 2D and 3D, we find a sigmoidal cross-over between the large-$J$ and small-$J$ asymptotics in $ΔE$, and a clear separation between $ΔE$ and $T_K$ for $T_K < μ$. For 1D, there is in addition a sizable intermediate-$J$ regime where the Kondo temperature is quadratic in $J$ due to the diverging density of states at the band edge. Furthermore, we find that in 1D the particle-hole asymmetry leads to a large decrease of $T_K$ compared to the standard result obtained by approximating the density of states to be constant (flat-band approximation), while in 3D the opposite is the case; this is due to the non-trivial interplay of the exchange and potential scattering renormalization in the presence of particle-hole asymmetry.

cond-mat.str-el↗

arXiv2016

Low-energy physics of three-orbital impurity model with Kanamori interaction

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.

cond-mat.str-el↗

arXiv2015

Theoretical prediction of antiferromagnetism in layered perovskite Sr$_2$TcO$_4$

We theoretically investigate magnetic properties of Sr$_2$TcO$_4$, a 4d transition-metal layered perovskite of the K$_2$NiF$_4$-type with half-filled t$_{2g}$ states. The effect of local Coulomb repulsion between the t$_{2g}$ orbitals is included within the density-functional theory (DFT)+U and DFT+dynamical mean-field theory (DMFT) methods. The DFT+DMFT predicts paramagnetic Sr$_2$TcO$_4$ to be close to the Mott insulator-to-metal transition, similarly to the cubic compound SrTcO$_3$. The inter-site exchange interactions computed within the DFT+DMFT framework point to a strong antiferromagnetic coupling between the neighboring Tc sites within the layer. We then evaluate the Néel temperature $T_N$ within a classical Monte Carlo approach including dipolar interactions, which stabilize the magnetic order in the frustrated K$_2$NiF$_4$ lattice structure. Our approach is applied to a set of layered and cubic perovskites. The obtained $T_N$ are in fair agreement with experiment. Within the same approach we predict $T_N$ of Sr$_2$TcO$_4$ to be in the 500-600K range.

cond-mat.str-el↗