| In the research of this thesis, the critical current density Jc of YBa2Cu3Ox bicrystalline rings with various grain boundary angles theta (4° ≤ theta ≤ 31°) and oxygen concentrations x (6.4 ≤ x ≤ 6.9) were studied using AC-susceptibility measurements under hydrostatic pressure. In high pressure measurements on optimally doped YBCO, the Jc (across grain boundary) was enhanced at the rate dlnJc/dP ≈ 20%/GPa. On the other hand, for the underdoped rings, dln Jc/dP ≈ 40 to 60%/GPa was measured. The pressure derivative dlnJc/dP does not exhibit a monotonic dependence on the misorientation angle. We also observed the effects of pressure-induced oxygen ordering (oxygen rearrangement) on both Jc and Tc for optimal and under-doped YBCO rings after releasing the pressure. This gives evidence for oxygen vacancies in the grain boundary region. Moreover, from a simple calculation of the tunneling current, it appears that the Jc enhancement is caused by carrier transfer from the CuO-chains to the CuO2-planes generated by the oxygen rearrangements under pressure.;The pressure derivative of the superconducting Tc of single and polycrystalline samples of MgB2 was determined for the first time under hydrostatic pressure conditions. This pressure derivative is found to be dTc/dP ≃ -1.14(3) K/GPa and -1.11(2) K/GPa in single crystal and polycrystalline samples, respectively. These values are consistent with BCS theory with electron-phonon coupling. |
