Layer-selective Cooper pairing in an alternately stacked transition metal dichalcogenide

TL;DR

This paper demonstrates that the layered superconductor 4Hb-TaSSe hosts two distinct, weakly coupled superconducting gaps in alternating layers, enabling layer-specific control and offering new avenues for tunable multigap superconductor devices.

Contribution

It reveals the existence of layer-selective multigap superconductivity in 4Hb-TaSSe, combining experimental spectroscopy and theoretical modeling to understand interlayer coupling effects.

Findings

Identification of two distinct superconducting gaps in alternating layers.

Observation of different resilience of gaps to temperature and magnetic fields.

Theoretical explanation of high critical field due to interlayer hybridization.

Abstract

Multigap superconductivity emerges when superconducting gaps form on distinct Fermi surfaces. Arising from locally overlapping atomic orbitals, multiple superconducting bands introduce a new internal degree of freedom in the material that, however, escapes external control due to their coexistence in real space in the known multigap superconductors. Here, we show that the layered superconductor 4Hb-TaSSe - composed of alternating trigonal (H) and octahedral (T) polymorph layers - is a multigap superconductor, featuring two weakly coupled superconducting condensates with distinct properties, spatially separated in alternating layers. Using high-resolution quasiparticle tunneling and Andreev reflection spectroscopy in the two polymorph layers, we identify two superconducting gaps that vary in size and internal structure. The intrinsic Cooper pairing in each polymorph is corroborated by the temperatures and magnetic fields at which the gaps open up, which differ in each polymorph layer and show opposing resilience to these parameters. This behavior enables selective external actuation upon the condensates. Our theoretical model based on ab-initio calculations reproduces key features of the observed superconducting gaps in the presence of finite interlayer hybridization and explains the unusually high critical field observed in the T-layer. Our results establish TMD polymorphs as platforms for engineering tunable multigap superconductors, offering new opportunities in layered superconducting device architectures.