Ultrafast optical switching to a heterochiral charge-density wave state

Abstract Optical control of correlated electronic states promises unprecedented tunability of novel functional materials [1-3]. Tailored optical excitations can steer a system along non-equilibrium pathways to metastable states with specific structural or electronic properties [4-6]. A much-desired feature is the reproducible and ultrafast switching to functional states. The light-induced hidden state of 1T-TaS2, with its strongly enhanced conductivity and exceptionally long lifetime, represents a unique model system for studying the switching of correlated electronic states using femtosecond optical stimuli. [4-7] However, despite intense investigation, the switching mechanism and the structural origins of the distinctive electronic properties of the hidden state have not been fully uncovered. Here, we use surface-sensitive electron diffraction in combination with a femtosecond optical quench to reveal coexistent charge-density wave chiralities as a new structural feature of the hidden state. We find that a single-pulse optical quench produces a state with long-range structural order and different weights of the two chiral enantiomorphs of the charge-density wave. Harnessing a double-pulse optical quench [5, 7-9], we trace the origin of the mixed chirality to the transient electronic excitation of the host crystal. The coexistent long-range-order of both chiralities suggests the presence of extended heterochiral charge-density wave interfaces, which results in a higher-level, fractal-type moiré superstructure. Density functional theory simulations for such a charge-density wave moiré superstructure reveal multiple flat bands, Dirac cones, and a kagome electronic subsystem around the Fermi energy. Our findings shed light on novel electronic properties gained by chiral interface engineering, and create avenues for light-induced moiré superstructures in quasi-two-dimensional materials.