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The transition metal dichalcogenide TiSe2 has received significant research attention over the past four decades. Different studies have presented ways to suppress the 200 K charge-density-wave transition, vary lowerature resistivity by several orders of magnitude, and stabilize magnetism or superconductivity. Here we give the results of a synthesis technique whereby samples were grown in a high-pressure environment with up to 180 bar of argon gas. Above 100 K, properties are nearly unchanged from previous reports, but a distinct hysteretic resistance region begins around 80 K, accompanied by insulating lowerature behavior. An accompanying decrease in carrier concentration is seen in Hall effect measurements, and photoemission data show a removal of an electron pocket from the Fermi surface in an insulating sample. We conclude that high inert gas pressure synthesis accesses an underlying nonmetallic ground state in a material long speculated to be an excitonic insulator.

The noncentrosymmetric Weyl semimetal candidate MoTe 2 was investigated through neutron-diffraction and transport measurements at pressures up to 1.5 GPa and at temperatures down to 40 mK. Centrosymmetric and noncentrosymmetric structural phases were found to coexist in the superconducting state. Density functional theory (DFT) calculations reveal that the strength of the electron-phonon coupling is similar for both crystal structures. Furthermore, it was found that by controlling nonhydrostatic components of stress, it is possible to mechanically control the ground-state crystal structure. This allows for the tuning of crystal symmetry in the superconducting phase from centrosymmetric to noncentrosymmetric. DFT calculations support this strain control of crystal structure. This mechanical control of crystal symmetry gives a route to tuning the band topology of MoTe 2 and possibly the topology of the superconducting state., Competing Interests: C.H. and N.B. synthesized materials. C.H., W.R. II, Y.W., J.L., and H.C. performed neutron-scattering measurements. Bulk property measurements were performed by C.H., I-L.L., T.M., C.E., P.N., J.L., P.P., J.P. and N.B. L.H. and T.Y. performed DFT calculations. All authors contributed to the manuscript. The authors declare no competing financial interests.