Effects of pressure and silver impurities on superconducting properties of the HTSC YBa2Cu4O8

The effect of hydrostatic pressure on the temperature of the superconducting transition of YBa2(Cu(1-x)Ag(x))4O(8 +/- delta) (x = 0; 0,05; 0,1; 0,15) has been studied. We have found that the pressure effect in this system is remarkably different from that in the YBa2(Cu(1-x)Ag(x))3O(7-delta) system. The observed features are discussed in the context of the Van Hove singularity of the electron energy spectra of oxide cuprates. The consistency of the superconducting features of the 124 and 125 systems under pressure and some models of high-Tc superconductivity are discussed. The type of nonlinearity of the temperature dependence of resistivity in the normal-state 124 system and the effect of Cu - Ag substitution are studied, and the phase diagram is plotted. We have found an increase of the crossover temperature on copper substitution and discussed its agreement with the Nagaosa-Lee theory. (Author)