J. Phys. I France 1 (1991) 159-166
Geometrical correlations and the origin of values at the maximum and intersects of in La 2-x (Sr) x CuORaphael Blumenfeld
Cavendish Laboratory, Madingley Road, University of Cambridge, CB3 0HE, G. B.
(Received 16 November 1990, accepted in final form 3 December 1990)
Many observations in doped La 2-x(Sr) xCuO 4, including the curve, are explained by a simple model of the holes distribution in the CuO planes. The excitation cloud surrounding a single hole is argued to spread on no more than about one plaquette. The Cooper pairs are proposed to be in a combination of at least two possible states, whose energy difference is smaller than the barrier between them. The experimental value of highest doping concentration x0=0.15 emerges as the limit when the pairs are most densely packed. Further doping above x0 is proposed to destroy superconductivity by generating normal islands, leading to a percolation-type structure. The model explains the observed disappearance of superconductivity above . It also accounts for observations of nonuniform x-dependent diamagnetic phase separation. Assuming low mobility of single hole excitations, this picture applies to x<x0 as well, in agreement with recent heat capacity measurements, and explains the observed onset of superconductivity at .
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