Local Real-Space View of the Achiral 1T−TiSe2 2×2×2

Abstract

The transition metal dichalcogenide 1T-TiSe_{2}-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds.

Spontaneous gyrotropic electronic order in a transition-metal dichalcogenide

“…By contrast, a particularly unconventional scenario is the gyrotropic order [4][5][6][7][8], where chirality spontaneously emerges across a phase transition as the electron system breaks the relevant symmetries of an originally achiral lattice. Such a gyrotropic order, proposed as the quantum analogue of the cholesteric liquid crystals, has attracted significant interest [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, to date, a clear observation and manipulation of the gyrotropic order remain challenging.…”

“…In classical liquids, this is known as the cholesteric liquid crystal. In quantum materials, despite fundamental interest [4][5][6][7][8][9][10][11][12][13][14][15][16], a clear observation of the gyrotropic order remains lacking.For a condensed matter system, one route to obtain chirality is to look for low-symmetry materials that naturally crystalize in lattice-structures that are free of any mirror, inversion and roto-inversion symmetries, i.e., chiral crystals [3]. Representative chiral crystal materials include quartz, tellurium, carbon nanotubes and twisted bilayer graphene.…”

“…Such a gyrotropic order [4][5][6][7][8] was suggested in 1T -TiSe 2 [9][10][11] and cuprates [12,22] based on scanning tunneling microscopy (STM) and polar Kerr rotation measurements. However, later studies [13,23,24] including a recent STM experiment [13] on 1T -TiSe 2 argued that the existing experimental evidence remains inconclusive. Moreover, the manipulation of the emergent chirality has not been achieved.…”

Direct Visualization of Trimerized States in 1T′−TaTe2

“…Striped contrast in the HT phase was proposed to represent chains of Ta dimers instead of trimers [20], however, the stripes are also consistent with the surface Te atoms undergoing the bulk distortion [21,22]. Mapping the bulk structure and disentangling lattice displacements from charge modulations are thus required for resolving the nature of density waves in 1T 0 -TaTe 2 and layered TMDs in general [23][24][25].…”

Attractive Coulomb interaction, temperature-dependent hybridization, and natural circular dichroism in 1T−TiSe2

“…Thus, it is important to experimentally investigate the presence or absence of correlation effects as well as the role of hybridization in the electronic structure of 1T -TiSe 2 . Further, it has been reported that 1T -TiSe 2 exhibits a chiral CDW order [26], but this is still under debate [27]. Very recently, a study showed gyrotropic order in 1T -TiSe 2 , in the form of chiral order induced by midinfrared circularly polarized light in an originally achiral lattice [28].…”