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Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1
T′-WSe2. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1 T′-WSe2and a semiconducting domain of 1 H-WSe2in contiguous single layers. The QSHI nature of single-layer 1 T′-WSe2is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states.
Superconductivity in monolayer niobium diselenide (NbSe2) on bilayer graphene is studied by electrical transport. Monolayer NbSe2is grown on bilayer graphene by molecular beam epitaxy and capped with a selenium film to avoid degradation in air. The selenium capped samples have
TC = 1.9 K. In situmeasurements down to 4 K in ultrahigh vacuum show that the effect of the selenium layer on the transport is negligible. The superconducting transition and upper critical fields in air exposed and selenium capped samples are compared.
Schematic of monolayer NbSe2/bilayer graphene with selenium capping layer and electrical contacts.