Repulsive vs. attractive Hubbard model: transport properties and spin-lattice relaxation rate

Abstract

We contrast the transport properties (dc resistivity, Seebeck coefficient), optical conductivity, spectral functions, dynamical magnetic susceptibility, and the NMR $1/T_1$ spin-lattice relaxation rate of the repulsive and attractive infinite-dimensional Hubbard models in the paramagnetic phase for a generic band filling. The calculations are performed in a wide temperature interval using the dynamical mean-field theory with the numerical renormalization group as the impurity solver. The attractive case exhibits significantly more complex temperature dependences which can be explained by the behavior of the half-filled Hubbard model in external magnetic field with constant magnetization, to which the attractive Hubbard model maps through the partial particle-hole transformation. The resistivity is non-monotonous for strongly attractive case: it peaks significantly above the MIR value at a temperature $T_\mathrm{max}$ where the quasiparticle band disappears. For both signs of $U$ we find particle-hole asymmetry in the self-energy at low energies, but with the opposite kind of excitations having longer lifetime. This leads to a strong suppression of the slope of the Seebeck coefficient in the attractive case, rather than an enhancement as in the repulsive case. The spin-lattice relaxation rate in the strongly attractive case has a non-monotonic temperature dependence, thereby revealing the pairing fluctuations.