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Spectroscopic evidence for a type II Weyl semimetallic state in MoTe2

Abstract

In a type I Dirac or Weyl semimetal, the low-energy states are squeezed to a single point in momentum space when the chemical potential μ is tuned precisely to the Dirac/Weyl point1,2,3,4,5,6. Recently, a type II Weyl semimetal was predicted to exist, where the Weyl states connect hole and electron bands, separated by an indirect gap7,8,9,10. This leads to unusual energy states, where hole and electron pockets touch at the Weyl point. Here we present the discovery of a type II topological Weyl semimetal state in pure MoTe2, where two sets of Weyl points (, ) exist at the touching points of electron and hole pockets and are located at different binding energies above EF. Using angle-resolved photoemission spectroscopy, modelling, density functional theory and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations. We also find new surface ‘track states’ that form closed loops and are unique to type II Weyl semimetals. This material provides an exciting, new platform to study the properties of Weyl fermions.

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Figure 1: A simple model of the type II Weyl semimetal described by a two-band model given by equation (2) that exhibits four Weyl nodes.
Figure 2: Experimental Fermi surface and band structure of MoTe2.
Figure 3: Identification of WPs and Fermi arcs from experimental data.
Figure 4: Results of DFT calculations.

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Acknowledgements

The work at Ames Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division (ARPES measurements). Ames Laboratory is operated for the US Department of Energy by Iowa State University under contract No. DE-AC02-07CH11358. Data analysis, theory and modelling was supported by the Center for Emergent Materials, an NSF MRSEC, under grant DMR-1420451. T.M.M. acknowledges funding from NSF-DMR-1309461 and would like to thank the 2015 Princeton Summer School for Condensed Matter Physics for their hospitality. N.T. acknowledges partial support by a grant from the Simons Foundation (no. 343227). Work at ORNL (sample growth) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Scientific User Facilities Division (H.C.), and Materials Science and Engineering Division (J.Y.).

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N.T. and T.M.M. provided theoretical modelling and interpretation. J.Y. and Z.Z. grew the samples. M.O., M.-T.S. and R.A. performed DFT and Berry phase calculations. H.C. performed crystal structure determination. L.H., Y.W. and D.M. performed ARPES measurements and support. L.H. analysed ARPES data. The manuscript was drafted by L.H., T.M.M., N.T. and A.K. All authors discussed and commented on the manuscript.

Corresponding author

Correspondence to Adam Kaminski.

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The authors declare no competing financial interests.

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Huang, L., McCormick, T., Ochi, M. et al. Spectroscopic evidence for a type II Weyl semimetallic state in MoTe2. Nature Mater 15, 1155–1160 (2016). https://doi.org/10.1038/nmat4685

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