Non-Adiabatic Phase Transition from 2H to 1T in Transition Metal Dichalcogenides: A Real-Time Time-Dependent Density Functional Theory Study

Transition metal dichalcogenides (TMDCs) exhibit various structural phases, including the 2H phase with trigonal prismatic (C_3h) symmetry and the 1T phase with octahedral (O_h) symmetry. The electronic configuration of the transition metal, particularly in Group VI elements such as Mo and W, significantly influences the thermodynamic stability of these phases. Group VI metals possess a d2 electron configuration, stabilizing the 2H phase through the occupation of the d_z2 orbital in C_3h symmetry. In contrast, the 1T phase remains metastable with partially filled t_2g states in O_h symmetry. Understanding and controlling the phase transitions between these states is critical for various applications because of the different physical properties of the 2H and 1T phases.

Traditionally, density functional theory (DFT) with the nudged elastic band (NEB) method has been used to calculate the energy barriers for phase transition. However, these phase transitions often involve non-adiabatic dynamics that extend beyond the scope of conventional DFT. This study employed real-time time-dependent density functional theory (RT-TDDFT) with Ehrenfest dynamics to simulate the non-adiabatic phase transition from the 2H to 1T phase. By introducing an external vector potential and wave-packet dynamics, we provide a more accurate description of the phase transition mechanism. These insights provide a foundation for experimental manipulation of TMDCs' electronic and structural properties, potentially advancing their use in future applications.