Superconfucting cuprate ladder Sr(14-x)Ca(x)Cu(24)O(41): The need to go beyond both one and multiband ladder models*

Attempts to explain unconventional superconductivity (SC) in the cuprates have focused overwhelmingly on the layered materials. Significantly less effort has gone into into understanding the SC the ladder compounds (Sr,Ca)(14)Cu(24)O(41).  The assumption has been that single-band Hubbard model-based theories of 2-leg ladders suffice for these systems. We discuss the failures of both the simplest one- and multiband Hubbard models to explain SC in these systems. Experimentally, the appearance of SC under pressure is accompanied by a 1D-to-2D dimensional crossover [1] that has no theoretical explanation. NMR studies for the x=12 compound have found low-lying spin excitations [2,3]. Theoretically, we have recently shown from DMRG calculations for the 2-leg cuprate ladder containing both Cu and O that superconducting pair correlations decay extremely rapidly for realistic parameters [4].  We argue that taken together these observations indicate that neither the one- nor the multiband weakly doped Hubbard model is sufficient to understand the SC even in these simplest of superconducting cuprates.