Observation of topologically protected states at crystalline phase boundaries in single-layer WSe2

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), 1T′-WSe2. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1T′-WSe2 and a semiconducting domain of 1H-WSe2 in contiguous single layers. The QSHI nature of single-layer 1T′-WSe2 is 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 statesThis research was supported by the VdW Heterostructure program (KCWF16) (STM spectroscopy and QPI mapping) funded by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contract No. DE-AC02-05CH11231. Support was also provided by National Science Foundation award EFMA1542741 (surface treatment and topographic characterization). The work at the ALS (sample growth and ARPES measurements) is supported by the Office of Basic Energy Sciences, US DOE under Contract No. DE-AC02-05CH11231. The work at the Stanford Institute for Materials and Energy Sciences and Stanford University (ARPES measurements) was supported by the Office of Basic Energy Sciences, US DOE under contract No. DE-AC02-76SF00515. S. T. acknowledges the support by CPSF-CAS Joint Foundation for Excellent Postdoctoral Fellows. H. R. acknowledges fellowship support from NRF, Korea through Max Planck Korea/POSTECH Research Initiatives No. 2016K1A4A4A01922028 and No. 2011-0031558. A.P. and O.V.Y. acknowledge support by the ERC Starting grant “TopoMat” (Grant No. 306504) (theoretical formalism development). Q.W. acknowledges support from NCCR Marvel (hybrid functional calculations). First-principles calculations were performed at the Swiss National Supercomputing Centre (CSCS) under project s832 and the facilities of Scientific IT and Application Support Center of EPFL. The work at Nanjing University (Y.Z.) is supported by the Fundamental Research Funds for the Central Universities N°. 020414380037 (surface structure analysis). The SIMES work is supported by DOE BES, Division of Materials Sciences. M.M.U. acknowledges support by Spanish MINECO under grant no. MAT2017-88377-C2-1-R (data analysis