Optical switching of ferro-rotational charge-density wave states

TL;DR

This work demonstrates ultrafast optical switching in 1T-TaS$_{2}$ that drives a surface 2D-heterochiral CDW state, featuring coexisting opposite ferro-rotational chiralities. Using high-coherence LEED and femtosecond optical quenches, the authors show threshold-dependent formation and metastability of 2D-chiral domains and reveal a corresponding CDW moiré at the surface. Complementary DFT uncovers an emergent $13\times13$ kagome moiré lattice with a double-ring charge texture and a near-$E_{F}$ conducting network, predicting a metallic pathway at the heterochiral interfaces. The combination of surface-sensitive diffraction and theory suggests a structural and electronic degree of freedom in CDW systems that can be manipulated via twist-like stacking, with potential implications for twist-angle engineering of correlated states. Overall, the study connects ultrafast switching, moiré-derived kagome physics, and surface interface engineering in a prototypical strongly correlated material.

Abstract

Tailored optical excitations can steer a system along non-equilibrium pathways to metastable states with specific structural or electronic properties. The light-induced hidden state of 1T-TaS$_{2}$, with its strongly enhanced conductivity and exceptionally long lifetime, represents a unique model system for studying the ultrafast switching of correlated electronic states. We use surface-sensitive electron diffraction in combination with a femtosecond optical quench to reveal the coexistence of both charge-density-wave (CDW) 2D chiralities as a structural characteristic of the hidden state, corresponding to coexisting ferro-rotational CDW states. Density functional theory (DFT) simulations of interfaces between opposite CDW 2D chiralities predict a higher-level, fractal-type moir'{e} superstructure with a kagome band structure near the Fermi energy. More broadly, these findings suggest that heterochiral interfaces in CDW systems provide an additional structural degree of freedom, expanding the possibilities for electronic control via twist-angle engineering.

Figures (13)

• Figure 1: LEED images of the laser-induced 2D-heterochiral CDW state.a, Sketch of the optical excitation sequence, with quadrants representing the LEED images in d. b, Experimental setup for single- and double-pulse quenches monitored by LEED. c, The two 2D-chiralities of the 1T-TaS$_{2}$ CDW superstructure, rotated by $\pm13.9^{\circ}$ relative to the host lattice (black line). d, Sections of LEED images in the sequence from the C-phase to the 2D-heterochiral CDW state and back to a 2D-monochiral state. A single-pulse quench of the C-phase (top-left panel, sample #1, pulse fluence: 1.7 mJ cm$^{\textrm{-}2}$) generates first- and second-order $\beta$ superstructure peaks (solid and dashed circles, respectively, in the top-right panel). Annealing with a laser pulse train reverses the sample to a 2D-monochiral state (bottom-left panel) indistinguishable from the initial C-phase. A difference image (bottom-right panel) highlights the emergence of $\beta$ peaks and the suppression of $\alpha$ peaks in the 2D-heterochiral state. e, Close-up LEED images of the C-phase and the 2D-heterochiral state. Left column: area denoted by the black dashed rectangle in d. Right column: close-ups of main lattice and superstructure peaks (white solid rectangles in the bottom-left panel), normalized to individual peak heights. Main lattice peaks stem from the undistorted lattice, whereas $\alpha$ and $\beta$ peaks are associated with the CDW-coupled periodic lattice distortion. f, Lineouts along main-$\alpha$ and main-$\beta$ directions (bottom-left panel in e) illustrate that the spot profiles of the main lattice and $\alpha$ peaks remain nearly unchanged after the optical quench.
• Figure 2: Generation conditions for the 2D-heterochiral CDW state.a, Normalized intensity changes of $\alpha$ (blue squares) and $\beta$ (red circles) peaks as a function of the single-pulse quench fluence (sample #2). Both traces exhibit a threshold (black vertical line) and saturation behavior (red horizontal line). b, Normalized $\beta$ peak intensity after the single-pulse quench. Measurements of the 13 samples are fitted with error functions. The obtained threshold fluences vary from 0.7 mJ cm$^{\textrm{-}2}$ (#3a) to 2.2 mJ cm$^{\textrm{-}2}$ (#5), and saturation levels range from 2 % (#3c) to about 7 % (#6), with two samples (#1 and #5) showing no signs of saturation. Considering all data, an average threshold fluence of 1.2 mJ cm$^{\textrm{-}2}$ and saturation intensity ratio $I_{\beta}/I_{\alpha} \approx 4\,\%$ are determined. c, Normalized intensity changes as a function of the annealing pulse fluence (sample #3a). The $\alpha$ trace does not fully return to 0 % for annealing fluences up to 1 mJ cm$^{\textrm{-}2}$, possibly due to some irreversible changes of the sample. d, Double-pulse optical quench (sample #4). Normalized intensity changes as a function of the pulse separation $\tau$ show an enhancement for $|\tau| \le 400\,\textrm{fs}$ (shaded area). At a combined pulse fluence of 1.14 mJ cm$^{\textrm{-}2}$, the $\beta$ peak intensity generated by the double-pulse quench varies between 4.9 % ($\tau = 0\,\textrm{ps}$) and 1.2 % ($\tau = -2\,\textrm{ps}$). (All data obtained by averaging the intensities of 15 CDW superstructure peaks in each LEED image. Data sets #3a, #3b, #3c taken on the same sample but with slightly different surface conditions and laser pulse durations.)
• Figure 3: Proposed generation mechanism and possible structural texture for coexistent CDW 2D-chiralities.a, Suggested mechanism for the generation of a 2D-heterochiral hidden state with translational domains. The illustration depicts simplified free-energy surfaces as a function of the order parameter $\varphi$ of the homogeneous 2D-monochiral C phase, and along other coordinates of the high-dimensional configuration space (schematically indicated as multiple axes). The optically-induced displacive excitation of the CDW causes a rapid evolution of the system towards an inverted CDW state which is energetically highly unfavorable. Collision with this steep barrier leads to a forced symmetry breaking into a distribution of translational and 2D-chiral domains. Concurrent with electronic cooling and structural relaxation, this branching finally results in the formation of a metastable state with quenched disorder. b, From the relative diffraction intensities of $\alpha$ and $\beta$ peaks, the experiments imply a random distribution of both 2D-chiralities in the surface layer. Recent STM measurements corroborate this scenario Ravnik2023. Layers in both textures may be covered with translational domains, for which measured diffraction peak widths yield a minimum structural correlation length of about $\geq 20$ nm. This implies extended interfaces between CDWs of opposite 2D-chirality at or near the surface.
• Figure 4: Emergent kagome system in the CDW moiré superstructure.a, Construction of the CDW moiré unit cell (see also depiction in Fig. S6b). (Left) The structure considered consists of two 1T-TaS$_{2}$ trilayers of opposite CDW 2D-chirality, $\alpha$ (blue) and $\beta$ (red). (Right) The moiré structure is described by a super-hexagram unit cell containing 13 Ta hexagrams in each layer. Circles denoted by capital letters A to M indicate the center of the Ta hexagrams and the local stacking order of both layers Lee2019, with darker filling color representing higher electrostatic energy. The inner and outer rings of the super-hexagram are marked by orange and purple, respectively. b, Kagome structural order emerging from the coupling of both layers. A comparison between atomic positions in the CDW moiré superstructure and the C-phase shows that displacements of the Ta hexagram centers (depicted by arrows) break the translational symmetry of Ta hexagram, resulting in a $13 \times 13$ kagome superlattice. The concentric rings mark the inner and outer rings of the super-hexagrams. The Ta atoms at the center of the super-hexagrams experience no displacement. c, Energy- and band-resolved electron density distributions. The total electron density for bands from -$90$ meV to $60$ meV around $E_{\textrm{F}}$ develops a double-ring texture (left, red shading), while the kagome electronic subsystem (right, blue shading) features a conducting network (yellow arrows). Both charge orders follow the same kagome superlattice (black lines) as the atomic structure. Electron density of $\alpha$ and $\beta$ layers are shown in blue and red, respectively. Isosurface values are adjusted for optimal clarity of the charge pattern. d, Total density of states (DoS) and e, band structure of the 2D-heterochiral CDW state. Several flat bands and a kagome electronic subsystem (highlighted in blue) are visible near $E_{\textrm{F}}$.
• Figure S1: Lineouts of the main lattice, $\alpha$, and $\beta$ peaks. a, Lineouts of the main lattice and $\alpha$ peaks, compared between the C-phase and the H-state. As the sample switches from the C-phase to the H-state after the optical quench, the diffraction intensities of the main lattice and $\alpha$ peak are slightly reduced, but there is negligible change in the linewidth. b, Lineout comparison in the H-state. The lineout of the emergent $\beta$ peak is similar to that of the $\alpha$ peak. Lineout angles for the $\alpha$ and $\beta$ peaks are along the radial directions, i.e., the main-$\alpha$ and main-$\beta$ directions (cf. main text Fig. 1e, left column). The lineout angle for the main lattice peak is along the horizontal direction.