Direct evidence for two-gap superconductivity in hydrogen-intercalated titanium diselenide

Abstract

Transition-metal dichalcogenides (TMDs) offer an extremely rich material platform in the exploration of unconventional superconductivity. The unconventional aspects include exotic coupling mechanisms such as the Ising pairing, a complex interplay with other electronic orders such as charge-density waves (CDWs), symmetry-breaking and topological effects, and non-trivial gap structures such as multi-gap and possible nodal phases. Among TMDs, titanium diselenide (1$T$-TiSe${}_2$) is one of the most studied and debated cases. Hints to an anomalous structure of its superconducting order parameter have emerged over the years, possibly linked to its texturing in real and reciprocal space due to the presence of a 2×2×2 CDW phase, or to a pressure-driven multi-band Fermi surface. However, direct evidence for an unconventional structure of the superconducting gap in this material is still lacking. In this work, the combination of the temperature dependence of the upper critical field with point-contact Andreev reflection and scanning tunneling spectroscopy measurements all consistently indicate the existence of two distinct superconducting gaps in the recently-discovered H-intercalated TiSe${}_2$ superconductor. These results provide unambiguous evidence for a non-trivial superconducting phase in TiSe${}_2$ and stimulate further research aimed at clarifying its connection with the orbital character and its interplay with the underlying CDW order.