Dimensionality-driven orthorhombic MoTe2 at room temperature

Abstract

Abstract:We use a combination of Raman spectroscopy and transport measurements to study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below ~250 K, we find that in moderately thin samples below ~12 nm, a single orthorhombic phase exists up to and beyond room temperature. This could be due to the effect of c-axis confinement, which lowers the energy of an out-of-plane hole band and stabilizes the orthorhombic structure. Our results suggest that Weyl nodes, predicated upon inversion symmetry breaking, may be observed in thin MoTe2 at room temperature. Main text:

Anisotropic Broadband Photoresponse of Layered Type‐II Weyl Semimetal MoTe2

“…The results indicate that the exfoliated thin MoTe 2 flakes used in our measurements are T d phase at room temperature after experiencing a cooling cycle down to 77 K and then warm up back to room temperature. Here we note the fact that 22 nm MoTe 2 flake is T d phase at room temperature is in contradiction to typical phase transition temperature below 250 K that is observed previously on bulk sample and the phase transition temperature turns out to be thickness dependent as the thickness thins down below 100 nm . Figure d illustrates a typical SPCM image of a MoTe 2 device together with its in situ scanning reflection microscopy.…”

Direct Observation at Room Temperature of the Orthorhombic Weyl Semimetal Phase in Thin Epitaxial MoTe2

“…This orthorhombic phase is typically reported in the literature below 250 or 150 K so questions are raised about the possible origin of its stability at RT as observed in our work. The reduced dimensionality analysis previously proposed to explain the indirect observation [38] of RT T d -MoTe 2 in 12 nm thick films exfoliated from bulk does not directly apply to our ultrathin (1-3 ML) films. Our DFT analysis for T d -MoTe 2 (Figure 7) shows that the enlarged lattice parameters of our epitaxial thin films result in a characteristic band structure where the uppermost valence band B along ΓΑ is fully occupied dispersing well below the Fermi level similar to what is observed for T d -WTe 2 .…”

“…[37] Recently, indirect evidence of RT γ-MoTe 2 has been reported in thin films exfoliated from bulk where low dimensionality is thought to play a role. [38] Direct observation of the γ-MoTe 2 phase at RT either on bulk or epitaxial material has not been reported.…”

Transport evidence of asymmetric spin–orbit coupling in few-layer superconducting 1Td-MoTe2

“…3 also confirm the few-layer MoTe 2 is in the 1 T d phase, that is, no phase transition is observed on lowering down the temperature. Therefore, the 1 T d phase could be the intrinsic feature of the CVD-grown few-layer MoTe 2 , which is presumably caused by the reduced thickness of MoTe 2 21 .…”