Superconducting Energy Scales and Anomalous Dissipative Conductivity in Thin Films of Molybdenum Nitride

TL;DR

This study investigates the superconducting properties of molybdenum nitride thin films, revealing that their energy scales align with BCS theory while exhibiting an unusual dissipative conductivity likely due to disorder effects.

Contribution

It provides detailed terahertz spectroscopy analysis of MoN thin films, highlighting the coexistence of conventional superconducting energy scales with anomalous dissipation linked to disorder.

Findings

Superconducting energy scales match BCS theory predictions.

Anomalously large dissipative conductivity persists deep into the superconducting state.

Disorder influences the superconductor-insulator transition regime.

Abstract

We report investigations of molybdenum nitride (MoN) thin films with different thickness and disorder and with superconducting transition temperature 9.89 K $\ge{T_c}\ge$ 2.78 K. Using terahertz frequency-domain spectroscopy we explore the normal and superconducting charge carrier dynamics for frequencies covering the range from 3 to 38 cm$^{-1}$ (0.1 to 1.1 THz). The superconducting energy scales, i.e. the critical temperature $T_c$, the pairing energy $\Delta$, and the superfluid stiffness $J$, and the superfluid density $n_s$ can be well described within the Bardeen-Cooper-Schrieffer theory for conventional superconductors. At the same time, we find an anomalously large dissipative conductivity, which cannot be explained by thermally excited quasiparticles, but rather by a temperature-dependent normal-conducting fraction, persisting deep into the superconducting state. Our results on this disordered system constrain the regime, where discernible effects stemming from the disorder-induced superconductor-insulator transition possibly become relevant, to MoN films with a transition temperature lower than at least 2.78 K.