High bias anomaly in YBa2Cu3O7/LaMnO_{3+delta}/YBa2Cu3O7 Superconductor/Ferromagnetic Insulator/Superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator

TL;DR

This study reveals a long-range spin-triplet superconducting proximity effect through a ferromagnetic insulator, evidenced by excess current and photon emission in superconductor/ferromagnetic insulator/superconductor junctions, challenging conventional understanding.

Contribution

It provides experimental evidence for a long-range, unconventional spin-triplet superconducting proximity effect mediated by non-equilibrium electrons in a ferromagnetic insulator.

Findings

Observation of a step-like excess current onset below T_c

Detection of photon emission at large bias voltage

Proximity effect extends over 20 nm in the ferromagnetic insulator

Abstract

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+\delta}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like 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 novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly non-equilibrium 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.