An Ab-Initio Study of the Effect of Impurities on Electron-Phonon Energy Transfer in Niobium*

A precise understanding of electron-phonon energy transfer is crucial to the optimization of superconducting radio frequency (SRF) cavities, photocathodes, and other material systems in which electrons are out of thermal equilibrium with phonons. The role played by interstitial impurities is a point of particular interest; SRF scientists have long believed that impurities affect electron-phonon energy transfer in niobium cavities, but it is difficult to distinguish this effect from other impurity-induced effects. An inability to precisely control impurity concentrations and directly measure electron temperature further complicates the problem.

In order to shed light on this rich and important topic, we take an ab-initio approach, using recently developed packages for our in-house density functional theory software JDFTx to explore the dependence of electron-phonon energy transfer on electron and phonon temperatures, and on the concentrations of common impurities such as H, C, N, and O. We perform our calculations using a Wannier representation of the electronic state, which makes possible the dense k-space sampling necessary to study processes at cryogenic temperatures in the large unit cells necessary to study impurities.