Two-dimensional (2D) materials are of particular interest to scientists, as they offer many potential applications thanks to unique electronic properties linked to quantum effects, such as “charge density waves” (CDWs). Due to periodic spatial oscillations of the material's electrons, some of these CDWs exhibit in-plane chirality*. Chirality also finds applications in several sectors, including optical materials, electronics, photonics, catalysis and biomedical applications.
Recently, a team including scientists of the TEMPO beamline and the Laboratoire des Solides Irradiés (Palaiseau) has shown that in-plane chirality of a CDW can be controlled by applying a shear stress across a phase transition. These results pave the way for new functional devices in which chiral properties can be set on demand.
There are many applications for chirality: optical materials, electronics, photonics, catalysis, biomedical applications… For example, chiral compounds and metamaterials are used to modulate light intensity, which is essential in optical communication systems. Multiple and high-speed gates can be obtained combining chiral degrees of freedom with nonlinear optics. Or chiral materials can instead be employed for asymmetric synthesis of specific enantiomers and biosensors, in a chemical context.
More generally, solid state materials can also display a Ferro-Rotational Order (FRO) with chiral structure of individual planes. This is indeed the case in the 2D transition metal dichalcogenide 1T-TaS2, where an in-plane chirality comes with a Charge Density Wave (CDW) of large amplitude. Here we shows that the FRO of the CDW can be stabilized in any of the two possible configurations by combining the shear stress with an annealing cycle. By heating the sample above ~ 355 K, the periodic lattice distortion goes from Nearly commensurate (NC) to InCommensurate (IC) and any Ferro-Rotational Order is lost. Cooling under shear stress, provides a way to select a desired in-plane chirality, and opens the perspective of realizing chiral devices compatibles with high frequency optoelectronics.
Results
First, the CDW has been cooled under the application of clockwise shear stress. The resulting chirality has been determined by collecting ARPES data at the TEMPO beamline. Figure 1 shows an ARPES intensity map extracted at energy -0.5 eV. The distribution of spectral weight is strongly asymmetric with respect to the Γ-M direction, holding higher intensity on the right-hand side in the first Brillouin Zone (BZ).

Figure 1: ARPES measurements under clockwise torque. (a) Image of the sample holder with mounted 1T-TaS2 crystal. (b) Photoelectron intensity map extracted at -0.5 eV from the Fermi level. (c) Photoelectron intensity map extracted along the dashed line in previous panel. (d) Ferro-Rotational Order with α character.
This is confirmed by the intensity maps extracted along a cut parallel to M-K, and crossing at 0.8 of Γ-M. The redistribution of spectral weight corresponds to the α FRO. Next the same protocol is repeated, but applying counterclockwise couple instead of a clockwise one. The ARPES data measured in this case have been reported in Figure 2. Clearly, the asymmetry of spectral weight distribution has reversed with respect to the one of Figure 1, proving that the CDW is in the β FRO.

Figure 2: ARPES measurements under unterclockwise torque. (a) Image of the sample holder with mounted 1T-TaS2 crystal. (b) Photoelectron intensity map extracted at -0.5 eV from the Fermi level. (c) Photoelectron intensity map extracted along the dashed line in previous panel. (d) Ferro-Rotational Order with β character.
The scientists propose that the FRO control is linked to the microscopic structure of the NC-CDW phase. The latter can be modeled as a honeycomb network of metallic domain walls with vortex-antivortex alternation at the nodes. The free energy of opposite FRO would be identical if the density of vortex and antivortex is exactly balanced. Suppose however that shear stress breaks this balance while crossing the IC to NC phase transition. The final density of such topological defects would depend on their handiness, so to favor the development of one FRO with respect to the other.
Conclusions
The join application of shear stress and temperature cycling can reliably control the in-plane chirality of the CDW in 1T-TaS2. In the future, the application of tunable stress also to thin flakes combined with structure and electronic investigation spatially resolved, would allow investigating links between FRO and local sources of deformation or topological defects. Furthermore, we are considering the possibility of switching FRO via eddy currents, opening the possibility of a purely electronic control of future chiral devices.
* Chirality / enantiomer: An object is “chiral” if it exists as two non-superimposable forms which are mirror image of each other. This is the case of hands (or feet!), propellers, but also of many molecules – the two forms are called left or right enantiomer.