Influence of the structural modulations and the Chain-ladder interaction in the $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$ compounds.

8 octobre 2004International audienceWe studied the effects of the incommensurate structural modulations on the ladder subsystem of the $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$ family of compounds using ab-initio explicitly-correlated calculations. From these calculations we derived $t-J$ model as a function of the fourth crystallographic coordinate $\tau$ describing the incommensurate modulations. It was found that in the highly calcium-doped system, the on-site orbital energies are strongly modulated along the ladder legs. On the contrary the two sites of the ladder rungs are iso-energetic and the holes are thus expected to be delocalized on the rungs. Chain-ladder interactions were also evaluated and found to be very negligible. The ladder superconductivity model for these systems is discussed in the light of the present results