Low quasiparticle coherence temperature in the one band-Hubbard model: A slave-boson approach

We use the Kotliar-Ruckenstein slave-boson formalism to study the temperature dependence of paramagnetic phases of the one-band Hubbard model for a variety of band structures. We calculate the Fermi liquid quasiparticle spectral weight $Z$ and identify the temperature at which it decreases significantly to a crossover to a bad metal region. Near the Mott metal-insulator transition, this coherence temperature $T_\textrm{coh}$ is much lower than the Fermi temperature of the uncorrelated Fermi gas, as is observed in a broad range of strongly correlated electron materials. After a proper rescaling of temperature and interaction, we find a universal behavior that is independent of the band structure of the system. We obtain the temperature-interaction phase diagram as a function of doping, and we compare the temperature dependence of the double occupancy, entropy, and charge compressibility with previous results obtained with Dynamical Mean-Field Theory. We analyse the stability of the method by calculating the charge compressibility.Comment: 12 figures, 11 page

Haldane phase in one-dimensional topological Kondo insulators

We investigate the groundstate properties of a recently proposed model for a topological Kondo insulator in one dimension (i.e., the $p$-wave Kondo-Heisenberg lattice model) by means of the Density Matrix Renormalization Group method. The non-standard Kondo interaction in this model is different from the usual (i.e., local) Kondo interaction in that the localized spins couple to the "$p$-wave" spin density of conduction electrons, inducing a topologically non-trivial insulating groundstate. Based on the analysis of the charge- and spin-excitation gaps, the string order parameter, and the spin profile in the groundstate, we show that, at half-filling and low energies, the system is in the Haldane phase and hosts topologically protected spin-1/2 end-states. Beyond its intrinsic interest as a useful "toy-model" to understand the effects of strong correlations on topological insulators, we show that the $p$-wave Kondo-Heisenberg model can be implemented in $p-$band optical lattices loaded with ultra-cold Fermi gases.Comment: 8 pages, 4 figures, 1 appendi

Orbital-selective bad metals due to Hund’s rule and orbital anisotropy: A finite-temperature slave-spin treatment of the two-band Hubbard model

We study the finiteerature properties of the half-filled two-band Hubbard model in the presence of Hund's rule coupling and orbital anisotropy. We use the mean-field treatment of the Z2 slave-spin theory with a finiteerature extension of the zeroerature gauge variable previously developed by Hassan and de' Medici [Phys. Rev. B 81, 035106 (2010)10.1103/PhysRevB.81.035106]. We consider the instability of the Fermi-liquid phases and how it is enhanced by the Hund's rule. We identify paramagnetic solutions that have zero quasiparticle weight with a bad metal, and the first-order transition temperature between the bad metal and the Fermi-liquid phase as a coherence temperature that signals the crossover to the bad metallic state. When orbital anisotropy is present, we found an intermediate transition to an orbital-selective bad metal (OSBM), where the narrow band becomes a bad metal while the wide band remains a renormalized Fermi liquid. The temperatures Tcoh and TOSBM at which the system transitions to the bad metal phases can be orders of magnitude less than the Fermi temperature associated with the noninteracting band. The parameter dependence of the temperature at which the OSBM is destroyed can be understood in terms of a ferromagnetic Kondo-Hubbard lattice model. In general, Hund's rule coupling enhances the bad metallic phases, reduces interorbital charge fluctuations, and increases spin fluctuations. The qualitative difference found in the ground state whether the Hund's rule is present or not, related to the degeneracy of the low-energy manifold, is also maintained for finite temperatures.Fil: Mezio, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. University of Queensland; AustraliaFil: McKenzie, Ross H.. University of Queensland; Australi

Excitaciones elementales y ruptura de simetrías en antiferromagnetos frustrados

A lo largo de esta tesis hemos investigado distintos modelos de antiferromagnetos (AF) bidimensionales frustrados utilizando la teoría de campo medio de bosones de Schwinger. Esta teoría provee una formulación invariante SU(2), lo que permite satisfacer tanto el teorema de Lieb-Mattis como el teorema de Mermin-Wagner, mientras que también puede tratar fases ordenadas a través de una condensación de bosones. Otra característica importante es que los elementos constituyentes de esta teoría son espinones bosónicos de espín-1/2, lo cual permite investigar posibles escenarios de fraccionalización de las excitaciones de espín, algo que se cree que puede llegar a ocurrir en AF 2D frustrados. Además, puede ser formulada en el contexto de una teoría large-N, permitiendo entonces realizar cálculos de correcciones 1/N sobre las soluciones de campo medio.Fil: Mezio, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentin