Quasiparticle Injector-Detector Experiments on Superconductors and Proximity Effect Bilayers

Multi-junction NIS tunneling experiments allow for the formation and measurement of highly non-equilibrium steady state quasiparticle distributions in superconductors. By injecting quasiparticles from one junction and detecting them using a second, one can examine the transport of quasiparticles at millikelvin temperatures.

Recently, we have used this approach to demonstrate how the relaxation of quasiparticles can be engineered by the proximity effect [1]. In bilayers consisting of a very thin layer of normal metal Cu under an Al wire, we report a significant increase in the effective inelastic relaxation rate for quasiparticles while maintaining robust superconductivity. This effect is distinct from quasiparticle trapping, and verifies earlier quasiclassical predictions on modified quasiparticle relaxation in proximity bilayers [2], and could be easily applied to tests using superconducting qubits.

Additionally, we report several measurements in pure superconductors where the chemical potential model appears to fail in unexpected ways. For instance, we have observed systematic discrepancies in the symmetry between the chemical potential induced during the injection of electron-like and hole-like quasiparticles under different types of transport measurements. Similarly, when voltage biasing two junctions simultaneously, we observe competing transport asymmetries not consistent with charge imbalance. These may potentially be due to nonlocal or thermal transport effects which become significant at low temperatures and mesoscopic length scales.

[1] K. M. Ryan and V. Chandrasekhar, Enhanced Quasiparticle Relaxation in a Superconductor via the Proximity Effect, arXiv:2409.05233.

[2] A. A. Golubov and E. P. Houwman, Quasiparticle relaxation rates in a spatially inhomogeneous superconductor, Physica C: Superconductivity 205, 147 (1993).