epitaxial thin films grown on sapphire substrates by molecular-beam epitaxy.

Transition metal dichalcogenides (TMDs) provide a rich variety of emerging properties at monolayer limit originating from broken inversion symmetry and large spin-orbit coupling as typified by valley-polarized luminescence and transport in semiconducting TMDs such as MoS₂ and WSe₂, unconventional superconductivity in NbSe_2, enhancement of superconducting critical temperature in TaS₂, topological phase transition in WTe₂, and so on. The main fabrication methods of monolayer TMDs so far have been mechanical exfoliation and chemical-vapor deposition. The state-of-the-art molecular-beam epitaxy (MBE) should offer an alternative way to fabrication of TMD thin films and heterostructures, while so far conducting graphene substrates have been mainly used for spectroscopic studies. We have recently developed a fundamental route to layer-by-layer epitaxial growth of various TMD thin films on insulating sapphire substrates by MBE for transport studies [1], and demonstrated ambipolar transistor operation in WSe₂ epitaxial thin films. In this presentation, we report on superconducting transition with zero-resistance in NbSe₂ epitaxial thin films grown on sapphire substrates by MBE.
[1] M. Nakano et al., Nano Lett. 17, 5595 (2017).