Orateur
Description
Molybdenum nitride (MoN) is a disordered superconductor, which garnered attention due to its high critical temperature ($\delta$-MoN: $T_c\sim 13\,\text{K}$), robustness in high magnetic fields and short coherence lengths, which allows the deposition of ultra-thin films below 5 nm, a property that is crucial for superconducting detector applications.
Here we report on another interesting property of MoN, namely its behavior under aging and thermal annealing. We access the low frequency complex conductivity $\hat{\sigma}(\omega)$ down to 90 GHz using THz spectroscopy and reveal a continuous suppression of the critical temperature $T_c\,$, energy gap $\Delta$, and spectral gap $\Omega$. In particular we find that the spectral gap is suppressed significantly faster than $T_c\,$, leading to an exotic state with a finite critical temperature but vanishing excitation gap, a clear signature of gapless superconductivity as described by the theory of Abrikosov and Gor'kov.
These findings show that the superconducting state in MoN is highly tunable giving rise to a broad range of accessible critical temperatures $T_c$ and excitation gaps $\Omega$, which is desirable in engineering superconducting photon detectors.