Photoinduced topological phase transition in monolayer 1T$^\prime$-MoS$_2$

TL;DR

This work studies nonequilibrium topological phases in a monolayer $1T^{\prime}$-MoS$_2$ under high-frequency circularly polarized light. Using a low-energy $k\cdot p$ model and a van Vleck Floquet–Magnus expansion, it derives an effective static Hamiltonian with light-induced masses $M_x$ and $M_z$ that depend on drive strength $\xi$, helicity $\eta$, and the perpendicular-field parameter $\alpha$. The authors compute quasienergy spectra, Berry curvatures, and spin- and valley-resolved Chern numbers, revealing a sequence of driven phase transitions among QSH, QVH/BI, S-QHI, and P-QHI as $\xi$ is varied. The results demonstrate tunable, spin- and valley-selective topological control in $1T^{\prime}$-MoS$_2$, highlighting its potential as a platform for optically programmable topological valleytronics in transition-metal dichalcogenides.

Abstract

We investigate the nonequilibrium topological phases of monolayer 1T$^\prime$--MoS$_2$ under high-frequency circularly polarized driving using a low-energy $k\!\cdot\!p$ Hamiltonian combined with a van Vleck expansion. The off-resonant field generates spin- and valley-dependent mass corrections that reshape the Berry curvature profile and shift the conditions for band inversion. By evaluating the quasienergy bands, Berry curvatures, Hall conductivities, and spin- valley-resolved Chern numbers, we identify a sequence of light-controlled topological transitions marked by well-defined gap closings. Depending on the Floquet coupling strength and the electric-field parameter, the system evolves between the equilibrium quantum spin Hall (QSH) state and a set of driven phases including spin-polarized quantum Hall insulator (S-QHI), quantum valley Hall (QVH or BI) and photo-induced quantum Hall insulator (P-QHI) regimes. The results establish 1T$^\prime$--MoS$_2$ as a tunable platform where circular driving selectively manipulates spin and valley degrees of freedom, enabling controlled access to non-equilibrium topological phases in transition-metal dichalcogenides.

Figures (4)

• Figure 1: (a) Side view and (b) top view of the crystal structure of monolayer 1T$^{\prime}$-MoS$_2$. Molybdenum (Mo) and sulfur (S) atoms are represented by green and dark-gray spheres, respectively. The dashed lines highlight the Mo–Mo dimerization pattern, and the blue parallelogram marks the primitive unit cell used in our calculations.(c) Spin- and valley-resolved band structure of monolayer 1T$^{\prime}$-MoS$_2$ near the Fermi level, showing the effects of SOC and valley asymmetry. The red and blue curves correspond to the spin-up and spin-down states at the $\kappa = +$ valley, while the yellow and black curves represent the same at the $\kappa = -$ valley. A small gap opening at $k_y = 0$ reflects SOC-induced band splitting. (d) Corresponding DOS illustrating a small minimum near the Fermi energy and pronounced van Hove singularities on both sides, consistent with the anisotropic band structure of the 1T$'$ phase.
• Figure 2: Floquet-engineered band structures and corresponding Berry curvatures of monolayer 1T$'$--MoS$_2$ for different spin and valley indices under various light--matter coupling strengths ($\xi$) and electric field ($\alpha$). Panels (a--f) display the quasienergy spectra (solid lines) and Berry curvature distributions (dashed lines) for spin-up (red) and spin-down (blue) states at valleys $\kappa = \pm 1$. Panels (a,b) correspond to $\xi \simeq 0.02$ and (c,d) to $\xi \simeq 0.064$, while (e,f) show results for $\xi \simeq 0.105$.
• Figure 3: Spin- and valley-resolved anomalous Hall conductivity $\sigma_{xy}$ of monolayer 1T$^{\prime}$–MoS$_2$ as a function of the Fermi energy for different Floquet coupling strengths $\xi=0$ (black solid), $\xi=0.02$ (blue dashed), $\xi=0.064$ (red dotted), and $\xi=0.105$ (green solid) for $\alpha<1$. Panels (a) and (b) correspond to the spin-up and spin-down Hall conductivities, respectively, while panels (c) and (d) show the valley-resolved responses for $\kappa=+1$ and $\kappa=-1$.
• Figure 4: Spin and valley Chern numbers of monolayer 1T$'$--MoS$_2$ as a function of the Floquet coupling strength ($\xi$) for different electric field amounts. Panels (a,b) show the spin Chern numbers $C_{\uparrow}$ and $C_{\downarrow}$, while panels (c,d) present the valley Chern numbers $C_{+}$ and $C_{-}$ for $\alpha < 1$ and $\alpha > 1$, respectively. The insets illustrate the variation of the light-induced band gap with $\xi$ for each spin or valley configuration. s,