Pressure effect and specific heat of RBa2Cu3Ox at distinct charge carrier concentrations: Possible influence of stripes

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dT(c)/dp, and in DeltaC(p).T-c, where DeltaC(p) is the jump in the specific heat at T-c: dT(c)/dp becomes zero when DeltaC(p) .T-c is maximal, whereas dT(c)/dp has a peak at lower oxygen contents where DeltaC(p).T-c vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole dopi ng by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic T-c(n(h)) behavior, the features coalesce for all sample s at n(h) approximate to 0.11 and n(h) approximate to 0.175, irrespective o f substitution and doping. Hence, this behavior seems to reflect an intrins ic property of the CuO2 planes. Analyzing our results we obtain different m echanisms in three doping regions: T-c changes in the optimally doped and o verdoped region are mainly caused by charge transfer. In the slightly under doped region an increasing contribution to dT(c)/dp is obtained when well o rdered CuO chain fragments serve as pinning centers for stripes. This behav ior is supported by our results on Zn doped NdBa2Cu3Ox, and is responsible for the well known dT(c)/dp peak observed in YBa2Cu3Ox, at x approximate to 6.7. Going to a hole content below n(h) approximate to 0.11 our results po int to a crossover from an underdoped superconductor to a doped antiferroma gnet, changing completely the physics of these materials.