Zero Bias Conductance Peak in HTS above Critical Temperature

It has recently been proposed that if phase incoherent preformed pairs exist above critical temperature (Tc), Andreev reflection should be observable above Tc too. A distinctive feature of Andreev reflection is a zero bias conductance peak (ZBCP) in the conductance/voltage (G/V) characteristics. We have performed an extensive search for the ZBCP in HTS/normal-metal planar junctions including samples which have been annealed in Argon in an attempt to induce underdoping and enhancement of the pseudogap temperature. We have observed a maximum in the G/V characteristics above Tc in a number of samples and contact configurations. In some samples this behavior persists up to room temperature, but has a very broad voltage scale, much greater than the gap voltage. In YBa 2 Cu 3 O x/ Au and YBa 2 Cu 3 O x/ Ag contacts the form and amplitude of the ZBCP is close to the theoretical prediction, but the origin of this effect could also be in sample heating. One particularly curious result comes from an Au/LSMO/YBCO sample that shows a clear sub-gap ZBCP over the temperature range 15–266 K.

Cooper Pair-Like Systems at High Temperature and their Role on Fluctuations Near the Critical Temperature

A logarithmic behavior is hidden in the linear temperature regime of the electrical resistivity R(T) of some YBCO sample below 2T c where "pairs" break apart, fluctuations occur and "a gap is opening". An anomalous effect also occurs near 200 K in the normal state Hall coefficient. In a simulation of oxygen diffusion in planar 123 YBCO, an anomalous behavior is found in the oxygen-vacancy motion near such a temperature. We claim that the behavior of the specific heat above and near the critical temperature should be reexamined in order to show the influence and implications of fluctuations and dimensionality on the nature of the phase transition and on the true onset temperature.

Thermal Neutron Irradiation and Large Pinning Enhancement in Superconducting Materials

ABSTRACTUp to 25 times enhancement of magnetic critical current density has been observed at 70 K and 2500 O for 3% 6Li-doped sintered YBCO sample irradiated with thermal neutrons at a dose of 1 × 1018 n/cm2. High energy reaction product of 6Li and thermal neutron, the 2.73 MeV 3H and 2.05 MeV 4He produced uniform damage throughout the bulk sample. The strong field dependence of the Jc enhancement points to the weak center nature of the radiation defects. The upper critical field of this weak center decreases with temperature, and is about 800, 300, 150 Oe at 30, 50, 70 K respectively.