J. Phys. I France
Volume 1, Numéro 2, February 1991
Page(s) 159 - 166
DOI: 10.1051/jp1:1991122

J. Phys. I France 1 (1991) 159-166

Geometrical correlations and the origin of $\mathit{x}$ values at the maximum and intersects of ${T_{\mathsf{c}}({x})}$ in La 2-x (Sr) x CuO $\mathsf{_{4}}$

Raphael 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 $T_{\rm c}(x)$ 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 $T_{\rm c}$ 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 $x\approx 0.3$. 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 $x\approx 0.075$.

© Les Editions de Physique 1991

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