We exactly solve a two-particle
UV model to explore the behavior of pairs in a face-centered cubic (FCC) lattice within an extended Hubbard model. The conditions for pair formation, pair mass, pair size, and the Bose-Einstein condensation (BEC) temperature are examined. Our results show that strongly bound, superlight and small pairs can be generated in the FCC lattice, which are much lighter than pairs in other 3D lattices. We estimate that such pairs can Bose condense at high temperatures even if the lattice constant is large. Therefore, in the search for materials with high superconducting transition temperatures, three-dimensional materials with an underlying FCC structure should be investigated due to the possibility of superlight small pairs.
The majority of fulleride superconductors with unusually high transition-temperature to kinetic-energy ratios have a face-centred-cubic (FCC) structure. We demonstrate that, within extended Hubbard models with strong Coulomb repulsion, paired fermions in FCC lattices have qualitatively different properties than pairs in other three-dimensional cubic lattices. Our results show that strongly bound, light, and small pairs can be generated in FCC lattices across a wide range of the parameter space. We estimate that such pairs can Bose condense at high temperatures even if the lattice constant is large (as in the fullerides).
We study charge and heat transport in the square lattice Hubbard model at strong coupling using the finite-temperature Lanczos method. We construct the diffusion matrix and estimate the effect of thermoelectric terms on diffusive and hydrodynamic time evolution. The thermoelectric terms prevent the interpretation of the diffusion in terms of a single time scale. We discuss our results in relation to cold-atom experiments and measurements of heat conductivity based on the measurements of heat diffusion.