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Investigations of specific heat and T c under pressure are presented for YBa 2 Cu 3 O x (Y123), NdBa 2 Cu 3 O x (Nd123) and Y 1−y Ca y Ba 2 Cu 3 O x (YCa123) for various oxygen contents. In Y123, at the maximum of the specific-heat jump ΔC/T c the pressure dependence of the transition temperature dT c /dp shows a zero crossing. Inversely, at the maximum of dT c /dp the jump in the specific heat becomes minimal. With Ca doping these joint features shift to lower oxygen contents x, whereas Nd123 causes a shift into the opposite direction. Considering the hole doping by Ca and the larger chain fragmentation in Nd123, we rescaled our results with respect to the hole content n h in the CuO 2 planes. In this plot features coalesce in two fixed hole concentrations, n h ≈0.11 and n h 0.17, irrespective of the substitution and doping. The maximum in ΔC/T c may be related to the appearance of a quantum critical point due to the disappearance of a pseudogap for hole contents above n h ≈0.17. The features at n h ≈0.11 point to a drastic change in system properties at this hole content that may be connected to various anomalies found in other properties reported in the literature.

The linear compressibilities and the thermal expansion of an untwined, nearly optimal doped YBa{sub 2}Cu{sub 3}O{sub 6.94} single crystal were measured along the three crystallographic axes with a high-resolution dilatometer mounted in a high-pressure cell. The measurements were performed in a temperature range from 50 K to 320 K under hydrostatic gas pressure of maximal 0.65 GPa. At a temperature of 300 K the measured bulk modulus is reduced and shows a very strong enhancement under increasing pressure due to pressure-induced oxygen ordering. This ordering process can also be related to a glass transition seen in the thermal expansion above 225 K. At lower temperatures, when oxygen ordering is frozen, the bulk modulus is increased and shows a significantly reduced enhancement under pressure. This pressure dependence, however, is still anomalous high, probably caused by bonds of extremely different compressibilities within the unit cell. The widely spread literature data of the bulk modulus can be explained by this high pressure dependence.

Westudied T c of Y Ba 2 Cu 3 O x (Y123), Y 0.89 Ca 0.11 Ba 2 Cu 3 O x (YCa123) and NdBa 2 Cu 3 O x (Nd123) single crystals with various oxygen contents x. Compared to T c (x) of Y123 the T c (x) curves of YCa123 are shifted to lower oxygen contents and the maximum transition temperature T c,max decreases with increasing Ca content whereas in Nd123 T c (x) is shifted to higher oxygen contents and T c,max is increased. According to the universal parabolic T c (n h ) behavior the differences in T c (x) of Y123, YCa123 and Nd123 can be ascribed to different hole concentrations n h in the CuO 2 planes caused by doping via changes in chemistry or structure. In order to study the influence of structural changes on T c we examined the hydrostatic pressure effect dT c /dp (p ≤0.6GPa). In the underdoped region, at n h ≈0.11, the examined compounds show a peak in dT c /dp which is very pronounced for systems with well ordered CuO chains. As this peak occurs at the same n h in all investigated systems it is not caused by oxygen ordering, but its origins might be found in a strong influence of lattice deformations on the electronic structure.

In YBa 2 Cu 3 O x (x < 7.0), above 225 K a oxygen ordering in the CuO chains is visible as a glass-like transition in thermal-expansion measurements. The large contribution of this process to the thermal expansion and the T c changes observed under application of external hydrostatic pressure point to a strong dependence of the oxygen ordering and its activation energy E a on the unit-cell volume V. If, however, V is changed chemically by replacing Y with other rare earth ions, R, the values of E a reported in the literature show only a slight dependence on V. We have measured the thermal expansion of a YBa 2 Cu 3 O 6.94 single crystal under hydrostatic pressure up to 0.5 GPa. The activation energy at ambient pressure was determined to be 1.0 ± 0.1 eV with a pressure dependence dE a /dp = 78 meV/GPa. The volume dependence dE a /dV = -48 meV/A 3 obtained from these pressure experiments is clearly higher than dE a /dV = -5.4 meV/A 3 from R-ion substitution. Thus we conclude that a non-uniform compression of interatomic distances within the unit cell has to be considered to explain the difference between dE a /dV(p) and dE a /dV(R). The contribution, Δα V , of the ordering process to the thermal-expansion coefficient, α V , increases with pressure, indicating that more and more oxygen atoms take part in the process. From an extrapolation to negative pressure via R-ion substitution we expect a vanishing (R=Nd) or inverted (R=La) pressure effect on oxygen ordering.