Conductance anomalies in constriction-type superconducting Josephson junctions induced by hydrogen impurities

To microscopically investigate the influence of hydrogen on transport properties in superconductors, we studied the current–voltage characteristics of hydrogen-absorbed niobium (Nb) and hydrogen-adsorbed lead (Pb) superconductor–normal metal–superconductor (SNS) Josephson junctions. The junctions were fabricated by stretching Nb and Pb wires using a mechanically controllable break junction technique. In hydrogen-absorbed Nb SNS Josephson junctions, where hydrogen was absorbed into the Nb lattice while a fraction of H2 molecules remained adsorbed on the junction surface by low-temperature loading at ∼4.2 K, the differential conductance (dI/dV) spectra exhibit peak-shaped anomalies at V ∼ 0.8 mV and above 1 mV, together with multiple spike-like anomalies having an almost uniform spacing of about 0.1 mV within the superconducting gap. In the NbH0.1 SNS Josephson junction, where hydrogen is preloaded above room temperature and the surface is free of H2 molecules, the dI/dV spectra exhibit several peak-shaped anomalies at V ∼ 0.8 mV and above 1 mV, whose temperature dependence is consistent with higher-order multiple Andreev reflection (MAR) processes. In contrast, hydrogen-covered Pb SNS Josephson junction showed only multiple spike-like anomalies. These results suggest that proton tunneling assists hole transport in the normal region of the SNS junction, giving rise to peak-shaped anomalies at higher order MAR voltages, whereas the spike-like anomalies originate from excitations of hydrogen molecules adsorbed on the junction surface.