Enhancement of superconducting transition temperature in Nb/Pd bilayers upon rapid thermal hydrogenation

We studied the hydrogenation effect on the superconducting properties of Nb/Pd bilayers using a modified rapid thermal annealing approach at ~4 bar of pressure and up to 400 °C temperature. Below 300 °C annealing temperatures, the superconducting transition temperature (T_c) is enhanced from 8.77 to 9.06 K and is correlated with the compression of the unit cell. At higher annealing temperatures, a weak lattice expansion occurs, and the Tc is gradually suppressed. Furthermore, the rapid thermal hydrogenation results differently depending on the substrate of Nb/Pd bilayers. For the c-cut Al₂O₃ substrate, the T_c reduction started at 250 °C, while for the r-cut Al₂O₃, this occurred at 350 °C. We associated these features with the elastic behavior of Nb film upon hydrogenation. A model is proposed based on the increase of Tc caused by the compressive stress produced by the rapid nucleation and growth of hydrides. Our discoveries provide insights into how superconductivity can be manipulated by rapid thermal hydrogenation.