Search

Chirality is ubiquitous in nature, and populations of opposite chiralities are surprisingly asymmetric at fundamental levels.sup.1,2. Examples range from parity violation in the subatomic weak force to homochirality in biomolecules. The ability to achieve chirality-selective synthesis (chiral induction) is of great importance in stereochemistry, molecular biology and pharmacology.sup.2. In condensed matter physics, a crystalline electronic system is geometrically chiral when it lacks mirror planes, space-inversion centres or rotoinversion axes.sup.1. Typically, geometrical chirality is predefined by the chiral lattice structure of a material, which is fixed on formation of the crystal. By contrast, in materials with gyrotropic order.sup.3-6, electrons spontaneously organize themselves to exhibit macroscopic chirality in an originally achiral lattice. Although such order--which has been proposed as the quantum analogue of cholesteric liquid crystals--has attracted considerable interest.sup.3-15, no clear observation or manipulation of gyrotropic order has been achieved so far. Here we report the realization of optical chiral induction and the observation of a gyrotropically ordered phase in the transition-metal dichalcogenide semimetal 1T-TiSe.sub.2. We show that shining mid-infrared circularly polarized light on 1T-TiSe.sub.2 while cooling it below the critical temperature leads to the preferential formation of one chiral domain. The chirality of this state is confirmed by the measurement of an out-of-plane circular photogalvanic current, the direction of which depends on the optical induction. Although the role of domain walls requires further investigation with local probes, the methodology demonstrated here can be applied to realize and control chiral electronic phases in other quantum materials.sup.4,16. Optical chiral induction and spontaneous gyrotropic electronic order are realized in the transition-metal chalcogenide 1T-TiSe.sub.2 by using illumination with mid-infrared circularly polarized light and simultaneous cooling below the critical temperature., Author(s): Su-Yang Xu [sup.1] , Qiong Ma [sup.1] , Yang Gao [sup.2] , Anshul Kogar [sup.1] , Alfred Zong [sup.1] , Andrés M. Mier Valdivia [sup.1] , Thao H. Dinh [...]

The electrical Hall effect is the production, upon the application of an electric field, of a transverse voltage under an out-of-plane magnetic field. Studies of the Hall effect have led to important breakthroughs, including the discoveries of Berry curvature and topological Chern invariants.sup.1,2. The internal magnetization of magnets means that the electrical Hall effect can occur in the absence of an external magnetic field.sup.2; this 'anomalous' Hall effect is important for the study of quantum magnets.sup.2-7. The electrical Hall effect has rarely been studied in non-magnetic materials without external magnetic fields, owing to the constraint of time-reversal symmetry. However, only in the linear response regime--when the Hall voltage is linearly proportional to the external electric field--does the Hall effect identically vanish as a result of time-reversal symmetry; the Hall effect in the nonlinear response regime is not subject to such symmetry constraints.sup.8-10. Here we report observations of the nonlinear Hall effect.sup.10 in electrical transport in bilayers of the non-magnetic quantum material WTe.sub.2 under time-reversal-symmetric conditions. We show that an electric current in bilayer WTe.sub.2 leads to a nonlinear Hall voltage in the absence of a magnetic field. The properties of this nonlinear Hall effect are distinct from those of the anomalous Hall effect in metals: the nonlinear Hall effect results in a quadratic, rather than linear, current-voltage characteristic and, in contrast to the anomalous Hall effect, the nonlinear Hall effect results in a much larger transverse than longitudinal voltage response, leading to a nonlinear Hall angle (the angle between the total voltage response and the applied electric field) of nearly 90 degrees. We further show that the nonlinear Hall effect provides a direct measure of the dipole moment.sup.10 of the Berry curvature, which arises from layer-polarized Dirac fermions in bilayer WTe.sub.2. Our results demonstrate a new type of Hall effect and provide a way of detecting Berry curvature in non-magnetic quantum materials.The nonlinear Hall effect is observed in bilayer WTe.sub.2 in the absence of a magnetic field, providing a direct measure of the dipole moment of the Berry curvature., Author(s): Qiong Ma [sup.1] , Su-Yang Xu [sup.1] , Huitao Shen [sup.1] , David MacNeill [sup.1] , Valla Fatemi [sup.1] , Tay-Rong Chang [sup.2] , Andrés M. Mier Valdivia [sup.1] [...]