By calculating thermodynamics properties of a Hubbard model on the anisotropic triangular lattice close to the isotropic point, we clearly observe a metal-insulator transition in the charge susceptibility and estimate critical interaction strength for such transition at different extends of frustrations. Determined phase diagram gives support to other numerical methods like variational Monte Carlo and path integral renormalization group and reveals that other methods like slave rotors, resonating valence bond theory, dynamical mean field theory and Brinkman-Rice approach gives substantially different value for a critical interaction strength. In the metallic side we observed Fermi liquid like behavior in charge susceptibility, entropy and specific heat and show that this phase crosses over to incoherent (bad metallic) phase at relatively low coherence temperature. Our results for the coherence temperature and quasi-particle mass renormalization in Fermi liquid phase agree well with experiments on organic charge transfer salts, which therefore give support to the description of these rather complicated molecular crystals with relatively simple microscopic model. Results on spin susceptibility, entropy and estimate of double occupancy show that in the bad metallic phase fluctuating local magnetic moment is already large and in this way give support to the picture of this phase proposed by dynamical mean field theory. In the insulating phase we demonstrate how frustration increases density of low lying spin excitations, which is, e.g., reflected in the maximum of the specific heat being at temperatures much below the energy of an exchange interaction.
COBISS.SI-ID: 26760743
We studied the electronic contribution of valence electrons to the thermal expansion. We fist derived general thermodynamic relations for description of thermal expansion and expressed it with elastic constants and dependence of entropy on structural parameters. We numerically simulated the electronic contribution via Hubbard model on the anisotropic triangular lattice and by using dependence of Hubbard parameters on the structural parameters. We showed that strong electronic correlations and spin frustration can enhance the electronic contribution to the thermal expansion by an order of magnitude. We also showed, that electronic contribution has non-monotonic temperature dependence and strong anisotropy, is negative in some regimes and has maximum at coherence temperature, while its maximum in insulating regime appears at similar temperatures as maximum in spin susceptibility and in specific heat. These properties qualitatively agree with experiment, while quantitative agreement and improvements like, e.g., including phononic contribution, remain future challenges.
COBISS.SI-ID: 28581671
In this work we showed how strong electronic correlations enhance thermoelectric power in bad metals. By using finite-temperature Lanczos method we simulated electrons described by Hubbard model on the triangular lattice, which is believed to be a good model for electrons in organic superconductors. We calculated thermoelectric power via Kelvin’s formula and showed that the thermoelectric power is enhanced to the values close to kB/e0, that it has nonmonotonic temperature dependece and maximum close to coherence temperature. These properties agree nicely with the experiment, while the description of orientational dependece and a Fermi liquid regime requires inclusion of lattice dimerization and the use of a Kubo formula.
COBISS.SI-ID: 28468519
We studied weakly coupled random S=1/2 Heisenberg chains with disorder that can be described with random values of super-exchange spin interactions. By using numerical simulations of chains and treating inter-chain couplings at the mean field level we showed that in agreement with experiment the disorder reduces the transition temperature into antiferromagnet as well as the ordered magnetic moment at zero temperature. We also showed that the local magnetic moments have a wide distribution of sizes, which has been experimentally observed with muon spin resonance.
COBISS.SI-ID: 28533543
By studying the Hubbard model in a limit of maximal spin polarization, which allows for exact solution, we showed that the mechanism behind the Mott metal-insulator transition is the holon-doublon binding. By extending the study towards nonpolarized systems and by using BCS-type approximation, random path approximation and numerical Lanczos method we show, that the transition is discontinuous and that the binding of a holon to a doublon is indicated in the density correlations.
COBISS.SI-ID: 29184551