Synergistic chemical and optical switching of chiral symmetry breaking in 1\textit{T}-TaS$_2

Abstract

Optical control of symmetry-breaking quantum phases is a central goal in quantum materials, yet deterministic switching is often hindered by the stability of single-domain ground states. The chiral structure of the charge density wave (CDW) in 1T-TaS$_2$ provides a natural platform for such control, but the pristine material remains locked in a single chirality. Here we show that combining chemical doping with femtosecond optical excitation enables efficient and non-thermal switching of the chiral CDW state and reveal its microscopic mechanism. Ti substitution stabilizes a ground state with coexisting chiral domains, creating a tunable energy landscape for optical manipulation. Femtosecond photoexcitation then induces asymmetric and anisotropic switching from dominant to minority chiral domains, characterized by in-plane domain growth and a redistribution toward an achiral configuration. The switching occurs on a timescale comparable to a coherent phonon oscillation ($\sim$2 THz), revealing a phonon-mediated pathway that proceeds through a transient domain-wall state. Our results establish a broadly applicable strategy for engineering and controlling chiral order parameters through combined chemical and ultrafast optical tuning.

Figures (4)

• Figure 1: Static control of chiral-domain coexistence via Ti doping.a, Schematic of the mirror-symmetry-breaking $\alpha$-NC (blue) and $\beta$-NC (red) CDW states. The Star-of-David motifs represent the atomic clusters forming the NC-CDW structure, and dots denote Ta atoms. Black dashed lines indicate the crystallographic axes, while colored solid lines mark the corresponding NC-CDW wave vectors. b, Logarithmically scaled X-ray diffraction intensity maps in the $H$–$K$ plane ($L=6.33$) measured at 300 K, 306 K, and 311 K. Dashed lines indicate the reciprocal lattice directions $a^*$ and $b^*$. The white solid line in the 300 K panel marks the momentum-space cut used to obtain the peak profiles shown in c. c, Temperature evolution of the diffraction intensity along the line cut indicated in b during cooling. The green curve corresponds to the profile at 300 K. The intensity of the $\alpha$-NC peak is approximately 2.5 times that of the $\beta$-NC peak. d, Phase diagram of 1T-Ti$_x$Ta$_{1-x}$S$_2$ as a function of Ti concentration $x$. The onset temperature of the NC-CDW phase (orange circles) and the disappearance temperature of the IC-CDW phase (blue circles) during cooling are shown. e, Low-temperature intensity ratio $I_\alpha/I_\beta$ between the $\alpha$ and $\beta$ chiral domains as a function of doping. Each point represents the average ratio obtained from several tens of diffraction peak pairs (see Methods). f, Schematic free-energy landscape of the CDW phases at room temperature for different doping levels. From left to right: dominance of a single chirality in the undoped system, coexistence of $\alpha$ and $\beta$ phases at intermediate doping, and suppression of the NC-CDW phase at higher doping.
• Figure 2: Microscopic dynamics of photo-driven chiral-domain switching.a, Schematic of the ultrafast pump–probe X-ray diffraction setup. An 800 nm near-infrared (NIR) laser pulse serves as the pump, and 10.0 keV X-rays serve as the probe. Ta and S atoms are represented by red and yellow spheres, respectively. Diffracted X-rays are recorded by a two-dimensional area detector. The inset shows the orientation of the detector plane in momentum space relative to the measured reflection at $Q_{1\alpha}=(-0.684,-2.068,4/3)$. The detector plane can be decomposed into in-plane ($q_{\mathrm{in}}$) and out-of-plane ($q_{\mathrm{out}}$) momentum components, a convention used throughout the analysis. b, Distribution of measured CDW superlattice peaks in momentum space. The tracked reflections $Q_{1\alpha}$, $Q_{1\beta}$, and $Q_{2\beta}$ (highlighted by colored circles) are located near the $(-1,-2,4/3)$ Bragg reflection. c, In-plane ($q_{\mathrm{in}}$) line profiles of the $Q_{1\alpha}$ (blue) and $Q_{1\beta}$ (red) peaks at equilibrium ($t_d=-0.5$ ps, light gray) and under photoexcitation ($t_d=0.85$ ps, colored lines). d, Normalized time evolution of the integrated intensities of the $Q_{1\alpha}$ (blue) and $Q_{1\beta}$ (red) reflections at an absorbed fluence of 0.88 mJ cm$^{-2}$. Thin lines represent the coherent phonon oscillations, while bold lines show the background dynamics after subtracting the oscillatory component. e, Fourier-transform spectra of the intensity dynamics in d, revealing a pronounced amplitude-mode (AM) phonon at $\sim$2 THz. f, Time evolution of the in-plane peak width ($\sigma_{in}$ obtained from Gaussian fits; see Methods) for $Q_{1\alpha}$ and $Q_{1\beta}$ at the same absorbed fluence, normalized to their equilibrium values. Vertical dashed lines mark characteristic delay times of 0, 250, and 500 fs.
• Figure 3: Anisotropy and characteristic timescales of the light-induced phase transition.a–c, Fluence dependence of the normalized peak intensity (a), in-plane peak width (b), and out-of-plane peak width (c) for the $Q_{1\alpha}$ (top row) and $Q_{2\beta}$ (bottom row) reflections measured at fixed pump–probe delay times. d, Time evolution of the normalized intensity of the $Q_{1\alpha}$ reflection at an absorbed fluence of 1.28 mJ cm$^{-2}$. The time axis is plotted on a logarithmic scale. Vertical colored lines mark characteristic timescales (0.5, 2, 80, and 800 ps) that separate distinct dynamical regimes. e, Time evolution of the $Q_{1\alpha}$ peak position along the out-of-plane direction (upper panel, in units of $c^*$) and the corresponding normalized out-of-plane peak width (lower panel). The vertical lines denote the same characteristic timescales as in d.
• Figure 4: Microscopic mechanism of photoinduced chiral-domain switching.a, Microscopic processes and characteristic timescales of the switching dynamics. Upper panel: schematic evolution of the domain-intensity distribution at different delay times, together with the corresponding dynamical timescales. Solid blue lines indicate spectral-weight transfer associated with domain conversion, with the arrow highlighting the initial photo-driven switching. Lower panel: schematic evolution of the in-plane domain configuration. b, Schematic illustration of the light-induced chiral phase transition. c, Proposed real-space microscopic pathway for chiral-domain switching. d, Analogy between Ti doping and photo-doping in redistributing the chiral domains. The diagram summarizes the evolution of the domain population, the in-plane correlation length, and the out-of-plane correlation length of the NC and IC CDW phases. e, Calculated energy barrier separating chiral domains and domain walls. The blue and green curves show the dependence of the barrier on pump fluence and Ti doping concentration, respectively.