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. (in press)
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. (in press)
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.