High bias anomaly in YBa_2Cu_3O_(7-x)/LaMnO_(3+δ)/YBa_2Cu_3O_(7-x) superconductor/ferromagnetic insulator/superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator

摘要

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa_2Cu_3O_(7-x)/LaMnO_(3+δ)/YBa_2Cu_3O_(7-x)) tunnel junctions. At a large bias voltage V ～ 1 V we observe a steplike onset of excess current that occurs below the superconducting transition temperature T < T_c and is easily suppressed by a magnetic field. The phenomenon is attributed to a different type of the superconducting proximity effect of nonequilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly nonequilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.