Observation of Superconducting Collective Modes from Competing Pairing Instabilities in Single-Layer NbSe2

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 the phase competition to be scrutinized. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, the existence of a soft collective mode in single-layer NbSe2, observed as a characteristic resonance excitation in high-resolution tunneling spectra is reported. This resonance is observed along with higher harmonics, its frequency ohm/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, this resonance is interpreted as a collective Leggett mode that represents the fluctuation toward a proximate f-wave triplet state, due to subleading attraction in the triplet channel. These findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path toward unconventional superconductivity in simple, scalable, and transferable 2D superconductors.

Automated summary

This study reports the existence of a soft collective mode in single-layer NbSe2, observed as a characteristic resonance excitation in high-resolution tunneling spectra. The frequency and amplitude of this resonance change with temperature and magnetic field, establishing a direct link to the superconducting state.