Spontaneous and field enhanced sub-gaps in in-plane oriented (100)-Y sub(1-X)Ca sub(X)Ba sub(2)Cu sub(3)O sub(7-Y)/In tunnel junctions

摘要

Tunneling spectroscopy was performed on in-plane oriented (100)-Y sub(1-X)Ca sub(X)Ba sub(2)Cu sub(3)O sub(7-Y)/In tunnel junctions. For underdoped Y sub(1)Ba sub(2)Cu sub(3)O sub(7-Y) a zero-bias conductance peak (ZBCP) was observed which did not split down to 4.2K. For optimally doped YBCO a splitting of the ZBCP at zero magnetic field was observed below 6K and a large anisotropy of the splitting was observed as a function of field orientation. The splitting of the ZBCP is strong if the magnetic field is applied parallel to the (001) direction and weak if the field is parallel to the (010) or (100) direction. The splitting shows a large magnetic hysteresis if the magnetic field is parallel to the (001) direction. For Ca-doped YBCO no ZBCP was observed. Instead a sub-gap appeared below 10K, which widened if a magnetic field is applied. The data suggests an order parameter with a complex symmetry that depends on temperature, doping level and magnetic field.