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, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.Comment: 6 pages, 5 figures Proccedings of the 'Stripes 2000' Conference, Rome (2000

Reentrant vortex lattice transformation in four-fold symmetric superconductors

The physics behind the rhombic$\to$square$\to$rhombic flux line lattice transformation in increasing fields is clarified on the basis of Eilenberger theory. We demonstrate that this reentrance observed in LuNi$_2$B$_2$C is due to intrinsic competition between superconducting gap and Fermi surface anisotropies. The calculations reproduce not only it but also predict yet not found lock-in transition to a square lattice with different orientation in higher field. In view of physical origin given, this sequence of transitions is rather generic to occur in four-fold symmetric superconductors.Comment: 5 pages, 4 figures,submitted to Phys. Rev. Let