Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density

Abstract

We investigate how a combination of a nonmagnetic-impurity scattering rate $\gamma$ and finite subgap states parametrized by Dynes $\Gamma$ affects various physical quantities relevant to to superconducting devices: kinetic inductance $L_k$, complex conductivity $\sigma$, surface resistance $R_s$, quality factor $Q$, and depairing current density $J_d$. All the calculations are based on the Eilenberger formalism of the BCS theory. We assume the device materials are extreme type-II $s$-wave superconductors. It is well known that the optimum impurity concentration ($\gamma/\Delta_0 \sim 1$) minimizes $R_s$. Here, $\Delta_0$ is the pair potential for the idealized ($\Gamma\to 0$) superconductor for the temperature $T\to 0$. We find the optimum $\Gamma$ can also reduce $R_s$ by one order of magnitude for a clean superconductor ($\gamma/\Delta_0 < 1$) and a few tens $\%$ for a dirty superconductor ($\gamma/\Delta_0 > 1$). Also, we find a nearly-ideal ($\Gamma/\Delta_0 \ll 1$) clean-limit superconductor exhibits a frequency-independent $R_s$ for a broad range of frequency $\omega$, which can significantly improve $Q$ of a very compact cavity with a few tens of GHz frequency. As $\Gamma$ or $\gamma$ increases, the plateau disappears, and $R_s$ obeys the $\omega^2$ dependence. The subgap-state-induced residual surface resistance $R_{\rm res}$ is also studied, which can be detected by an SRF-grade high-$Q$ 3D resonator. We calculate $L_k(\gamma, \Gamma,T)$ and $J_d(\gamma, \Gamma,T)$, which are monotonic increasing and decreasing functions of $(\gamma, \Gamma,T)$, respectively. Measurements of $(\gamma, \Gamma)$ of device materials can give helpful information on engineering $(\gamma, \Gamma)$ via materials processing, by which it would be possible to improve $Q$, engineer $L_k$, and ameliorate $J_d$.