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

• Published in: Advanced materials (Weinheim) Vol. 34; no. 41; pp. e2206078 - n/a
• Main Authors: Wan, Wen, Dreher, Paul, Muñoz‐Segovia, Daniel, Harsh, Rishav, Guo, Haojie, Martínez‐Galera, Antonio J., Guinea, Francisco, de Juan, Fernando, Ugeda, Miguel M.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.10.2022
• Subjects: 2D materials; Atomic energy levels; collective modes; Correlation; electronic structure; Higher harmonics; Layered materials; molecular beam epitaxy; Order parameters; Resonance; Superconductivity; Transition metal compounds; transition metal dichalcogenides; Unconventional superconductivity; Unconventional superconductors
• DOI: 10.1002/adma.202206078
• ISSN: 0935-9648

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 Ω/2Δ is anticorrelated with the local superconducting gap Δ, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2), 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. The experimental observation of Leggett collective modes in the superconducting state of a 2D transition metal dichalcogenide (TMD) is reported. Leggett modes have been rarely seen in nature and represent the first type of collective modes observed in 2D TMD superconductors, which highlights the rather unconventional superconducting properties of this family of materials in the single‐layer limit.