Recently, a joint research team in China studied the two-dimensional (2D) transition metal dichalcogenide (TMD) MoTe₂ and found the abnormal surface STM image below 70 K does not originate from the displacement of the Te atoms but the strong electron-lattice coupling.

MoTe₂ is a typical representative of the TMD MX₂ family. It can be grown in 2H, 1T’ and T_d phases at different temperatures and has superconductivity and non-trivial topological properties.

Recent studies on the low temperature magnetoresistance "turn on" phenomenon of Td-phase MoTe₂ have found an anisotropic negative thermal expansion effect, and the surface STM images have very significant differences at 70 K and 7 K.

This indicates that MoTe₂ may undergo a temperature-induced electronic phase transition at low temperature, meanwhile, a similar conclusion had been obtained based on the abnormal change of carrier concentration.

Prof. YANG Xiaoping and Prof. LU Qingyou at High Magnetic Field Laboratory, Chinese Academy of Sciences under Hefei Institutes of Physical Sciences conducted research on the temperature-dependent surface STM image, electronic structure, lattice dynamics and topological properties of the 2D transition metal dichalcogenide (TMD) MoTe₂.

The temperature-induced structure variation and its effect on physical properties is pivotal in material preparation for devices application. In order to understand the microscopic origin of the observed anisotropic negative thermal expansion and the anomalous surface STM images below 70 K, Prof. YANG Xiaoping studied theoretically the temperature-dependent electronic structure, lattice dynamics and topological properties of monolayer Td-MoTe₂.

Remarkably, the study indicates that the nonequivalent Te atoms caused by MX₆ octahedral distortion have qualitatively different contributions to both phonon spectrum and electronic structure. The in-plane longitudinal acoustic mode and Te₍₂₎ atoms play an important role in the uniaxial negative thermal expansion and the temperature-dependent electronic phase transition, respectively.

Interestingly, under the scalar relativistic approximation, a band renormalization occured, accompanied by a Dirac phase transition from type II to type I, upon cooling below 70 K. Introducing spin-orbit coupling induced a temperature dependent trivial semimetal–semiconductor transition.

The results of the study may explain the experimental phenomenon very well that the abnormal surface STM image below 70 K does not originate from the displacement of the Te atoms but the band renormalization owing to strong electron-lattice coupling.

This work could be a direct theoretical support for the low-temperature research, experimental preparation and device development of the 2D TMD materials MX₂. Also, the intrinsic physical mechanism revealed in this work can be applied to other 2D TMD materials with the local MX₆ octahedron structure distortion.

This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China (NSFC), Hefei Science Center CAS, and the Maintenance and Renovation Project for CAS Major Scientific and Technological Infrastructure. A portion of this work was supported by the High Magnetic Field Laboratory of Anhui Province and Anhui Laboratory of Advanced Photon Science and Technology.

GGA band structure with spin-orbit coupling at 7 K(a) and 70 K (b). The size of the symbol represents the contributionof the corresponding orbital. The partial charge density is calculatedfor the occupied states below valence band maximum at 7 K (c) orthe Fermi level at 70 K (d) by 0.1 eV. The blue rectangle presents the2D unit cell. (Image by YANG Xiaoping)