Anomalous Flux Quantization in the Spin-Imbalanced Attractive Hubbard Ring

We investigate the one-dimensional Hubbard ring with attractive interaction in the presence of imbalanced spin populations by using the exact diagonalization method. The singlet pairing correlation function is found to show spatial oscillations with power-law decay as expected in the Fulde-Ferrell-Larkin-Ovchinnikov state of a Tomonaga-Luttinger liquid. In the strong coupling regime, the system shows an anomalous flux quantization of period h=4e, half of the superconducting flux quantum of h=2e, as recently predicted by mean-field analysis, together with various flux quanta smaller than h=4e. Notably, the observed flux quanta are determined by the difference between the system size NL and electron number N_e as h=(N_L-N_e)e.Comment: 6 pages, 8 figure

Superconductivity in the CuO double chain of Pr_2Ba_4Cu_7O_15-delta on the basis of Tomonaga-Luttinger liquid theory

Recently, Matsukawa et al. have discovered a new superconductor Pr_2Ba_4Cu_7O_15-delta in which metallic CuO double chains are responsible for the superconductivity. To investigate the superconductivity, we employ the d-p double chain model where the tight-binding parameters are determined so as to fit the LDA band structure. On the basis of the Tomonaga-Luttinger liquid theory, we obtain the phase diagram including the superconducting phase in the weak coupling limit. We also calculate the Luttinger liquid parameter K_rho as a function of the electron density $n$ by using the Hartree-Fock approximation. With increasing $n$ from quarter filling, K_rho increases, and then exceeds 1/2 when the superconducting correlation becomes most dominant. K_rho has a maximum at an optimal density between quarter- and half-filling. These results are consistent with the experimental observation.Comment: 4 pages, 4 figure