DIMENSIONAL CROSSOVERS IN THE DOPED LADDER SYSTEM - SPILL GAP, SUPERCONDUCTIVITY AND INTERLADDER COHERENT BAND MOTION

Based on the perturbative renormalization group (PRG) approach, we hav e studied dimensional crossovers in Hubbard ladders coupled via weak i nterladder one-particle hopping, t(perpendicular to). found that the o ne-particle crossover is strongly suppressed through growth of the int raladder scattering processes which lead the isolated Hubbard ladder s ystem toward the spin gap metal (SGM) phase. Consequently when t(perpe ndicular to) sets in, there exists, for any finite intraladder Hubbard repulsion, U > 0, the region where the two-particle crossover dominat es the one-particle crossover and consequently the d-wave superconduct ing transition, which is regarded as a bipolaron condensation, occurs. By solving the scaling equations for the interladder one-particle and two-particle hopping amplitudes, we give phase diagrams of the system with respect to U, t(perpendicular to 0) (initial value of t(perpendi cular to)) and the temperature, T. We compared the above dimensional c rossovers with those in a weakly coupled chain system: clarifying the difference between them.