Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films

TL;DR

This study investigates the terahertz optical conductivity of ALD-grown NbN thin films, revealing signatures of Dynes superconductivity and deviations from BCS theory, with implications for understanding disordered superconductors.

Contribution

It demonstrates how Dynes electrodynamics explains THz response in disordered NbN films, highlighting the impact of pair-breaking effects on superconducting properties.

Findings

Deviations from BCS in 20nm NbN film's optical conductivity.

Observation of a step-like absorption feature at half the spectral gap.

Dynes model accurately describes the THz response with a small pair-breaking rate.

Abstract

The frequency-dependent complex optical conductivity reflects key properties of superconductors, such as the energy gap in the density of states (DOS) and the superfluid density. For disordered superconductors, the optical conductivity often can be described within Bardeen-Cooper-Schrieffer (BCS) theory, while in corresponding tunneling experiments, deviations in the observed DOS typically require modelling by the phenomenological Dynes formula. The implications of such Dynes DOS for optics were rarely discussed so far. Here we probe the terahertz conductivity of superconducting NbN thin films with thicknesses ranging from 4.5 to 20nm, which were grown by atomic layer deposition (ALD). Our frequency range from 0.3 to 2.1 THz covers energies below and above the spectral gap. For 20nm thick NbN, we find in the optical conductivity distinct deviations from the BCS model, including a step-like characteristic in the absorption at half the zero-temperature spectral gap. These observations can be fully captured by Dynes electrodynamics with a small and temperature-independent pair-breaking rate. For the other films, we also observe signs of Dynes electrodynamics, and we discuss the evolution of the energy gap, the superfluid density, and the pair-breaking rate as function of film thickness.