Microscopic investigation of superconductivity in transition metal chalcogenide materials
Haojie Guo
Donostia International Physics Center (DIPC), 20018 San Sebastián, Spain
Contact Email: haojie.guo@dipc.org
Owing to their relative chemical and structural simplicity compared to other many-body systems, transition metal chalcogenide (TMC) materials have emerged as a major platform in condensed matter physics for the investigation of many-body correlated quantum phenomena, including 2D magnetism, charge density wave (CDW), exciton states, quantum spin Hall states, heavy-fermion behavior, or superconductivity. In this talk, I will discuss our recent results on the microscopic characterization in real-space of the superconducting properties of different TMC materials by means of high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) and point-contact Andreev reflection spectroscopy at low temperatures (350 mK) and high-magnetic field (11 T) conditions. In particular, I will focus on deciphering the nature of superconductivity in two polymorphs (1T and 4Hb) of the isovalent chalcogenide alloy TaSSe. Our results reveal that 1T-TaSSe behaves as an isotropic, single-gap superconductor consistent with the BCS framework, and exhibiting a critical transition temperature of ≈ 1.5 K. Moreover, we demonstrate that the incoherent CDW mosaic phase in 1T-TaSSe does not play a major role in the onset of superconductivity in this material, which has previously been proposed as the spatial origin of the superconducting state. In contrast, the 4Hb-TaSSe, which interleaves alternating trigonal (1H) and octahedral (1T) polymorph layers, is a multigap superconductor, hosting two weakly coupled superconducting condensates with distinct properties, each living spatially on its respective layer. Remarkably, the Cooper pairing strength of these polymorphs shows opposing resilience to temperatures and magnetic fields. This unique behavior enables selective external control over individual condensates, establishing TMC polymorphs as platforms for engineering tunable multigap superconductors. Finally, and if time allows, I will describe how the superconducting properties of FeSe—an elemental and prototypical iron-based superconductor—present a dramatic dependence on thin-film layer thickness when grown epitaxially on graphene substrates via molecular beam epitaxy, a result that challenges the current understanding on the nature of superconductivity of this material.
