arXiv subjects

J. Kokalj

Publications and source records attributed to J. Kokalj.

15 recordsLinked to original sources

arXiv2020

Charge transport in the Hubbard model at high temperatures: triangular versus square lattice

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.

cond-mat.str-el↗

arXiv2017

Effective realization of random magnetic fields in compounds with large single-ion anisotropy

We show that spin $S=1$ system with large and random single--ion anisotropy can be at low energies mapped to a $S=1/2$ system with random magnetic fields. This is for example realized in \mbox{Ni(Cl$_{1-x}$Br$_{x}$)$_2$-4SC(NH$_2$)$_2$} compound (DTNX) and therefore it represents a long sought realization of random local (on-site) magnetic fields in antiferromagnetic systems. We support the mapping by numerical study of $S=1$ and effective $S=1/2$ anisotropic Heisenberg chains and find excellent agreement for static quantities and also for the spin conductivity. Such systems can therefore be used to study the effects of local random magnetic fields on transport properties.

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arXiv2015

Magnetic inhomogeneity on a triangular lattice: the magnetic-exchange versus the elastic energy and the role of disorder

Inhomogeneity in the ground state is an intriguing, emergent phenomenon in magnetism. Recently, it has been observed in the magnetostructural channel of the geometrically frustrated $α$-NaMnO$_2$, for the first time in the absence of active charge degrees of freedom. Here we report an in-depth numerical and local-probe experimental study of the isostructural sister compound CuMnO$_2$ that emphasizes and provides an explanation for the crucial differences between the two systems. The experimentally verified, much more homogeneous, ground state of the stoichiometric CuMnO$_2$ is attributed to the reduced magnetoelastic competition between the counteracting magnetic-exchange and elastic-energy contributions. The comparison of the two systems additionally highlights the role of disorder and allows an understanding of the puzzling phenomenon of phase separation in uniform antiferromagnets.

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arXiv2015

Antiferromagnetic order in weakly coupled random spin chains

The ordering of weakly coupled random antiferromagnetic $S=1/2$ chains, as relevant for recent experimentally investigated spin chain materials, is considered theoretically. The one-dimensional isotropic Heisenberg model with random exchange interactions is treated numerically on finite chains with the density-matrix renormalization-group approach as well as with the standard renormalization analysis, both within the mean-field approximation for interchain coupling $J_{\perp}$. Results for the ordering temperature $T_N$ and for the ordered moment $m_0$ are presented and are both reduced with the increasing disorder agreeing with experimental observations. The most pronounced effect of the random singlet concept appears to be a very large span of local ordered moments, becoming wider with decreasing $J_{\perp}$, consistent with $μ$SR experimental findings.

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arXiv2014

Enhancement of the thermal expansion of organic charge transfer salts by strong electronic correlations

Organic charge transfer salts exhibit thermal expansion anomalies similar to those found in other strongly correlated electron systems. The thermal expansion can be anisotropic and have a non-monotonic temperature dependence. We show how these anomalies can arise from electronic effects and be significantly enhanced, particularly at temperatures below 100 K, by strong electronic correlations. For the relevant Hubbard model the thermal expansion is related to the dependence of the entropy on the parameters ($t$, $t'$, and $U$) in the Hamiltonian or the temperature dependence of bond orders and double occupancy. The latter are calculated on finite lattices with the Finite Temperature Lanczos Method. Although many features seen in experimental data, in both the metallic and Mott insulating phase, are described qualitatively, the calculated magnitude of the thermal expansion is smaller than that observed experimentally.

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arXiv2014

Enhancement of the thermoelectric power by electronic correlations in bad metals: a study of the Kelvin formula

In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperature dependence and values that are orders of magnitude larger than for elemental metals. Kelvin proposed a particularly simple expression for the thermopower in terms of the temperature dependence of the chemical potential. We consider a Hubbard model on an anisotropic triangular lattice at half filling, a minimal effective Hamiltonian for several classes of organic charge transfer salts. The finite temperature Lanczos method is used to calculate the temperature dependence of the thermopower using the Kelvin formula. We find that electronic correlations significantly enhance the magnitude of the thermopower and lead to a non-monotonic temperature dependence. The latter reflects a crossover with increasing temperature from a Fermi liquid to a bad metal. Although, the Kelvin formula gives a semi-quantitative description of some experimental results it cannot describe the directional dependence of the sign of the thermopower in some materials.

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arXiv2013

Local spin relaxation within the random Heisenberg chain

Finite-temperature local spin dynamics within the random spin-1/2 antiferromagnetic Heisenberg chain is studied numerically. The aim is to explain measured NMR spin-lattice relaxation times in BaCu_{2}(Si_{0.5}Ge_{0.5})_{2}O_{7}, which is the realization of a random spin chain. In agreement with experiments we find that the distribution of relaxation times within the model shows a very large span similar to the stretched-exponential form. The distribution is strongly reduced with increasing T but stays finite also in the high-T limit. Our results reveal the crucial role of the anisotropy (interaction), since the behavior is essentially contrast with the ones for XX model (equivalent to noninteracting fermions), where we do not find any significant T dependence of the distribution.

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arXiv2013

Thermodynamics of a bad metal-Mott insulator transition in the presence of frustration

We study a range of thermodynamic properties (charge susceptibility, specific heat, entropy and spin susceptibility) of the Hubbard model on the anisotropic triangular lattice at half filling by means of the numerical finite-temperature Lanczos method. We observe clear signatures of a metal-Mott insulator transition in the charge susceptibility, and show that the metallic phase is characterized by a small charge susceptibility, large entropy, low coherence temperature, large renormalized quasiparticle mass, and large spin susceptibility. We show that the local magnetic moment in the metallic phase is large and comparable to the local moment in the insulating phase. These are all characteristics of bad metallic behavior. In addition, we show how frustration increases the density of low-lying spin excitations in the Mott insulating phase and decreases longer range spin correlations.

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arXiv2011

Consistent description of the metallic phase of overdoped cuprate superconductors as an anisotropic marginal Fermi liquid

We consider a model self-energy consisting of an isotropic Fermi liquid term and a Marginal Fermi liquid term which is anisotropic over the Fermi surface, vanishing in the same directions as the superconducting gap and the pseudogap. This model self-energy gives a consistent description of experimental results from Angle-Dependent Magneto-Resistance (ADMR), specific heat, de Haas-van Alphen, and measurements of the quasi-particle dispersion near the Fermi surface from photoemission. In particular, we reconcile the strongly doping dependent anomalous scattering rate observed in ADMR with the almost doping independent specific heat.

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arXiv2010

Dynamical central peak and spinon deconfinement in frustrated spin chains

Studying the dynamical spin structure factor in frustrated spin chains with spontaneously dimerized ground state we show that besides the gapped spin-wave excitations there appears at finite temperatures also a sharp central peak. The latter can be attributed to deconfined spinons, accounted well within the variational approach. The central peak remains well pronounced within the local spin dynamics and may be relevant for experiments on materials with 1D frustrated spin chains.

cond-mat.str-el↗

arXiv2010

Reply to the Comments by B. Farid and A.M. Tsvelik, arXiv:0909.2886v1, and B. Farid, arXiv:0909.2887v1

In the recent preprints the authors claim that conclusions on the violation of the Luttinger sum rule (LSR) as put forward in published papers by the present authors are false or at least not conclusive enough. It concerns the validity of the LSR within the Mott-Hubbard insulator as manifested in the 1D model of spinless fermions, and the Hubbard model on a triangular lattice. We show below that arguments against the presented analysis are rather easily rejected while still the general question of the validity and breakdown of the LSR remains to be answered.

cond-mat.str-el↗

arXiv2009

Finite-temperature dynamics with the density-matrix renormalization group method

We present a novel numerical method for the evaluation of dynamical response functions at finite temperatures in one-dimensional strongly correlated systems. The approach is based on the density-matrix renormalization group method, combined with the finite-temperature Lanczos diagonalization. The feasibility of the method is tested on the example of dynamical spin correlations in the anisotropic Heisenberg chain, in particular it yields nontrivial results for the critical behavior in the isotropic case.

cond-mat.str-el↗

arXiv2008

Breakdown of the Luttinger sum rule within the Mott-Hubbard insulator

The validity of the Luttinger sum rule is investigated within the prototype tight-binding model of interacting fermions in one dimension, i.e., the t-V model including the next-nearest neighbor hopping t' in order to break the particle-hole symmetry. Scaling analysis of finite-system results at half-filling reveals evident breakdown of the sum rule in the regime of large gap at V >> t, while the sum rule appears to recover together with vanishing of the Mott-Hubbard gap.

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arXiv2008

Violation of the Luttinger sum rule within the Hubbard model on a triangular lattice

The frequency-moment expansion method is developed to analyze the validity of the Luttinger sum rule within the Mott-Hubbard insulator, as represented by the generalized Hubbard model at half filling and large $U$. For the particular case of the Hubbard model with nearest-neighbor hopping on a triangular lattice lacking the particle-hole symmetry results reveal substantial violation of the sum rule.

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arXiv2006

Luttinger sum rule for finite systems of correlated electrons

The validity of the Luttinger sum rule is considered for finite systems of interacting electrons, where the Fermi volume is determined by location of zeroes of Green's function. It is shown that the sum rule in the paramagnetic state is evidently violated within the planar t-J model at low doping while for the related Hubbard model, even in the presence of next-nearest-neighbor hopping, no clearcut exception is found.

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