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1 Materials Science Poland

1 2004

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Remarks on the phase diagram of high-temperature superconductors: pressure dependence

Stankowski J., Krupski M., Micnas R.

Materials Science Poland

2004

Vol. 22, No. 3

175--185

The Tc(x) dependence of high-temperature superconductors shows that superconductivity occurs above a critical concentration x1. For concentrations exceeding x1, Tc increases until an optimal concentration xopt is reached and drops with further increase of x. Above xopt, i.e., in the overdoped region, there exists a single Tc which can be taken as the temperature of the Cooper pair formation and, simultaneously, of their condensation (BEC) to the superconducting state. For carrier concentrations below the optimal value, x < xopt, there are two characteristic temperatures, > Tc. At T*, phase incoherent local pairs (LP's) are formed and only at Tc the system undergoes the superconducting phase transition. The existence of these two characteristic temperatures, T* and Tc, reflects various phenomena related to strong electron correlations. We review the pressure effects in the cuprate family YBCO and propose their explanation within the Hubbard model and crossover from BCS cooperative pairing to Bose-Einstein of preformed pairs. The scaling of the pressure effects above and below xopt is analyzed in terms of two parameters: the transfer integral t and on-site energy U. With increasing pressure, t increases and the density of states at the Fermi level decreases. Above xopt, Tc ?1/2zt and dTc/dp < 0. Below xopt, however, the derivative dTc/dp > 0, since Tc ?t2/U.