Finite-temperature transport in finite-size Hubbard rings in the strong-coupling limit

We study the current, the curvature of levels, and the finite temperature c harge stiffness, D(T,L), in the strongly correlated limit U much greater th an t for Hubbard rings of L sites, with U the on-site Coulomb repulsion and t the hopping integral. Our study is done for finite-size systems and any band filling. Up to order t, eve derive our results following two independe nt approaches, namely, using the ablution provided by the Bethe ansatz and the Solution provided by an algebraic method, where the electronic operator s are represented in a slave-fermion picture. We find that, in the U=infini ty case, the finite-temperature charge stiffness is finite for electronic d ensities n smaller than 1. These results are essentially those of spinless fermions in a lattice of size L, apart from small corrections coming from a statistical flux, due to the spin degrees:of freedom. Up to order t, the M ott-Hubbard gap is Delta(MH)= U-4t, and we find that D(T) is finite for n < 1, but is zero at half filling. This result comes from the effective flux felt by the holon excitations, which, due to the presence of doubly occupie d sites, is renormalized to Phi(eff) = phi(N-h-N-d)/(N-d+N-h), and which is zero at-half filling, with N-d and N-h being the number of doubly occupied and empty lattice sites, respectively. Further, for half filling, the curr ent transported by any eigenstate of the system is zero and, therefore, D(T ) is also zero.