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Floquet-engineered Valley Topology with Anisotropic Response in 1T'-WSe$_2$ and Janus WSeTe monolayers

Zhe Li, Haijun Cao, Lijuan Li, Huixia Fu, Mengxue Guan, Sheng Meng

TL;DR

This work demonstrates Floquet-engineered valley topology in anisotropic 1Tprime-WSe2 and Janus WSeTe, revealing a single $QSH ightarrow QAH$ transition in the former and a two-stage sequence via a valley-polarized intermediate in the latter under circularly polarized light with $math{ ext{hbar}} ext{omega}=1.0$ eV. It employs first-principles calculations, Wannier-based tight-binding, and Magnus-expanded Floquet theory to map spin-resolved band evolutions, edge-state signatures, and Berry-curvature distributions, uncovering $C_v=+1$ and $C=+2$ topological phases with robust global gaps. The study further predicts a pronounced anisotropic response under oblique light incidence, enabling directionally controlled manipulation of valley topology. Collectively, these findings establish design principles for light-controllable valleytronic and topological devices, potentially operable at room temperature.

Abstract

Valley topology has emerged as a key concept for realizing new classes of quantum states. Here, we investigate Floquet-engineered topological phase transitions in anisotropic 1T'-WSe$_2$ and its Janus derivative WSeTe monolayers, which exhibit valley-degenerate and valley-polarized characteristics, respectively. In 1T'-WSe$_2$, a single topological-phase-transition (TPT) occurs from the quantum-spin-Hall state (QSH) to the quantum anomalous Hall (QAH) state, involving one spin channel at both valleys simultaneously. In contrast, Janus WSeTe undergoes a two-stage Floquet-driven TPT that occurs within a single valley and sequentially involves two spin components. The intermediate phase manifests as a valley-polarized QAH (vp-QAH) state with a finite valley Chern number, while the final phase evolves into a high-Chern-number QAH state with distinct valley gaps. Furthermore, an in-plane anisotropic response of the TPTs is predicted under oblique light incidence, reflecting the intrinsic low-symmetry nature of the lattice. These findings provide a comprehensive understanding of Floquet-engineered valley-based topological properties and offer guidance for designing light-controllable valleytronic and topological devices.

Floquet-engineered Valley Topology with Anisotropic Response in 1T'-WSe$_2$ and Janus WSeTe monolayers

TL;DR

This work demonstrates Floquet-engineered valley topology in anisotropic 1Tprime-WSe2 and Janus WSeTe, revealing a single transition in the former and a two-stage sequence via a valley-polarized intermediate in the latter under circularly polarized light with eV. It employs first-principles calculations, Wannier-based tight-binding, and Magnus-expanded Floquet theory to map spin-resolved band evolutions, edge-state signatures, and Berry-curvature distributions, uncovering and topological phases with robust global gaps. The study further predicts a pronounced anisotropic response under oblique light incidence, enabling directionally controlled manipulation of valley topology. Collectively, these findings establish design principles for light-controllable valleytronic and topological devices, potentially operable at room temperature.

Abstract

Valley topology has emerged as a key concept for realizing new classes of quantum states. Here, we investigate Floquet-engineered topological phase transitions in anisotropic 1T'-WSe and its Janus derivative WSeTe monolayers, which exhibit valley-degenerate and valley-polarized characteristics, respectively. In 1T'-WSe, a single topological-phase-transition (TPT) occurs from the quantum-spin-Hall state (QSH) to the quantum anomalous Hall (QAH) state, involving one spin channel at both valleys simultaneously. In contrast, Janus WSeTe undergoes a two-stage Floquet-driven TPT that occurs within a single valley and sequentially involves two spin components. The intermediate phase manifests as a valley-polarized QAH (vp-QAH) state with a finite valley Chern number, while the final phase evolves into a high-Chern-number QAH state with distinct valley gaps. Furthermore, an in-plane anisotropic response of the TPTs is predicted under oblique light incidence, reflecting the intrinsic low-symmetry nature of the lattice. These findings provide a comprehensive understanding of Floquet-engineered valley-based topological properties and offer guidance for designing light-controllable valleytronic and topological devices.

Paper Structure

This paper contains 9 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Structural Symmetries and Topological Phases
  4. Floquet-Driven Topological Phase Transitions
  5. Spin-resolved Bands & Topological Edge States
  6. Angle-dependent Anisotropic Response
  7. Laser-Parameter Phase Diagram
  8. Discussion
  9. Conclusion

Figures (5)

  • Figure 1: Illustrations of different TPT processes for monolayer 1T$^\prime$-WSe$_2$ and Janus WSeTe. (a) Side view structure of 1T$^\prime$-WSe$_2$, where the purple and light-green balls represent W and Se atoms, respectively. (b) Illustration of the TPT process in 1T$^\prime$-WSe$_2$ under irradiation of R-CPL. The light-brown region indicates the position of the 2D bulk states, with the red and blue edges corresponding to Chern-number contributions of --0.5 and +0.5, respectively. The black oblique line within the gap denotes the chiral edge state, where the positive or negative slope indicates positive or negative chirality. (c) and (d) are similar to (a) and (b), respectively, but depict the conditions for Janus WSeTe. The gray balls in (c) represent Te atoms.
  • Figure 2: Spin-resolved band evolutions near the two valleys of monolayer 1T$^\prime$-WSe$_2$ and Janus WSeTe, with the light frequency set to $\hbar\omega$ = 1.0 eV. (a) Illustration of the BZ of 1T$^\prime$-WSe$_2$, where the high-symmetry lines $\Gamma$-$X$, $\Gamma$-$Y$ and $\Gamma$-$M$ are represented by solid black lines, while the line $\Gamma$-$-Y$ is indicated by a dashed black line. The positions of the two valleys are also marked. (b)-(f) sequentially depict zoomed-in spin-resolved band evolutions along the $K'$-$\Gamma$-$K$ path. Red and blue bubbles represent contributions from spin components “1” and “2”, respectively. The light intensity is set to $A_0$ = 0 V/$c$, 40 V/$c$, 65 V/$c$, 80 V/$c$ and 120 V/$c$. (g)-(l) are analogous to (a)-(f) but correspond to Janus WSeTe, with light intensities selected as 0 V/$c$, 40 V/$c$, 70 V/$c$, 83 V/$c$ and 120 V/$c$ sequentially.
  • Figure 3: Developments of LDOS patterns and selected Berry curvature distributions for monolayer 1T$^\prime$-WSe$_2$ and Janus WSeTe, with the light frequency set to $\hbar\omega$ = 1.0 eV. (a) LDOS patterns projected onto the $X$ edge of 1T$^\prime$-WSe$_2$, with light intensities chosen as $A_0$ = 0 V/$c$, 40 V/$c$, 65 V/$c$, 80 V/$c$ and 120 V/$c$ corresponding to the panels from left to right. The color gradient transitions from black to red, yellow, then to white, indicating an increase in LDOS values. The topological state associated with each panel is indicated at the top. (b) Berry curvature distributions of 1T$^\prime$-WSe$_2$ within one 2D BZ. The color gradient from green to red (blue) represents the enhancement of Berry curvature with positive (negative) chirality. The light intensity is set to 70 V/$c$, corresponding to the QAH state. (c) and (d) are analogous to (a) and (b), respectively, but relate to Janus WSeTe. In subfigure (c), the light intensities are 0 V/$c$, 40 V/$c$, 70 V/$c$, 83 V/$c$ and 120 V/$c$. In subfigure (d), the light intensity is selected corresponding to the vp-QAH state.
  • Figure 4: In-plane anisotropic response of TPTs based on Janus WSeTe, with the light frequency set to $\hbar\omega$ = 1.0 eV. (a) The illustration of the incident polar angle ($\theta$) is shown in the side view of Janus WSeTe, with the red arrow indicating the direction of the incident light. (b) presents a similar view, but from the top perspective of Janus WSeTe, where it depicts the azimuthal angle ($\varphi$). The blue and red arrows represent the incident light at azimuthal angles of 90$^{\circ}$ and 180$^{\circ}$, respectively. The black frame envelopes a 2$\times$2 in-plane supercell of Janus WSeTe. Panels (c) and (d) show the evolution of the gap at the $K'$ valley around the first and second TPT points, respectively, as a function of light intensity. In these panels, the red and blue lines correspond to azimuthal angles of 90$^{\circ}$ and 180$^{\circ}$, with $\theta$ set to 60$^{\circ}$. Panels (e) and (f) are similar to (c) and (d), except that the polar angle is set to 80$^{\circ}$.
  • Figure 5: Contour distributions of global gaps and valley gaps based on various laser parameters. Panels (a)-(c) are related to 1T$^\prime$-WSe$_2$, with subfigure (a) providing the distribution of the global gap, while subfigures (b) and (c) depict the gaps around the $K$ and $K'$ valleys, respectively. Panels (d)-(f) correspond to Janus WSeTe and are analogous to (a)-(c). In each contour distribution, the color gradient transitions from blue to green, yellow, and then to red, indicating an increase in gap values, meanwhile the maroon region represents an ultra-large gap exceeding 60 meV. The laser parameters vary along the $x$-axis (light frequency) and $y$-axis (light intensity). Additionally, the white dashed curve indicates the first TPT point, while the black dashed curve signifies the second TPT point, which is present only in Janus WSeTe.