Observation of phase-coherent transport in d-wave junctions

The transport properties of microjunctions made of d-wave superconductors were studied by precisely measuring the conductance spectra of oxide junctions, YBa2Cu3O7-delta/La0.67Sr0.33MnO3 (YBCO/LSMO), of various sizes at a temperature of about 100 mK. When the junction size was larger than 30 mum, the conductance spectra showed sharp zero-bias peaks consistent with the Blonder-Tinkham-Klapwijk (BTK) formula for d-wave superconductors. When the size was less than 15 mum, however, the spectra were not consistent with the BTK formula; the sharp zero-bias peak was replaced by small peak-dip structures when the junction temperature was decreased, similar to features observed in Nb/Ag microjunctions. This peak-dip structure showed a periodic response to a relatively weak applied magnetic field (similar to0.2 T), suggesting an interference effect in these granular thin films. By increasing the magnetic field up to 7 T, the conductance near zero bias was reduced, possibly due to the decoherence effect. Our results show that the spectral features of small YBCO/LSMO junctions are well explained in terms of the phase dependent transport, indicating the compatibility between d-wave superconductivity and phase coherent transport.