Observation of superconducting collective modes from competing pairing instabilities in single-layer NbSe$_2$

TL;DR

This study reports the detection of a collective Leggett mode in single-layer NbSe₂, revealing competing pairing instabilities and highlighting the role of electronic correlations in 2D superconductors.

Contribution

The paper provides experimental evidence of a Leggett mode in single-layer NbSe₂, linking it to fluctuations towards a proximate f-wave triplet state, a novel insight into 2D superconductivity.

Findings

Observation of a soft collective mode in tunneling spectra.

Resonance frequency anticorrelated with local gap.

Mode amplitude diminishes with temperature and magnetic field.

Abstract

In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow to scrutinize the phase competition. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, we report the existence of a soft collective mode in single-layer NbSe$_2$, observed as a characteristic resonance excitation in high resolution tunneling spectra. This resonance is observed along with higher harmonics, its frequency $\Omega/2\Delta$ is anticorrelated with the local superconducting gap $\Delta$, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (T$_C$ and H$_{C2}$), which sets an unambiguous link to the superconducting state. Aided by a microscopic model that captures the main experimental observations, we interpret this resonance as a collective Leggett mode that represents the fluctuation towards a proximate f-wave triplet state, due to subleading attraction in the triplet channel. Our findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path towards unconventional superconductivity in simple, scalable and transferable 2D superconductors.