Zero bias conductance peak in HTS above the critical temperature

It has recently been proposed that if phase incoherent preformed pairs exist above critical temperature (T-c), Andreev reflection should be observable above T-c 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 YBa2Co3Ox/Au and YBa2Cu3Ox/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.