Electronic structures and photoemission spectroscopy

Abstract:

Chiral materials have attracted significant research interests as they exhibit intriguing physical properties, such as chiral optical response, spin-momentum locking, and chiral induced spin selectivity. Recently, layered transition metal dichalcogenide 1T-TaS_{2} has been found to host a chiral charge density wave (CDW) order. Nevertheless, the physical consequences of the chiral order, for example, in electronic structures and the optical properties, are yet to be explored. Here, we report the spectroscopic visualization of an emergent chiral electronic band structure in the CDW phase, characterized by windmill-shaped Fermi surfaces. We uncover a remarkable chirality-dependent circularly polarized Raman response due to the salient in-plane chiral symmetry of CDW, although the ordinary circular dichroism vanishes. Chiral Fermi surfaces and anomalous Raman responses coincide with the CDW transition, proving their lattice origin. Our Letter paves a path to manipulate the chiral electronic and optical properties in two-dimensional materials and explore applications in polarization optics and spintronics.

Abstract:

Ultrathin films of intrinsic magnetic topological insulator MnBi₂Te₄ exhibit fascinating quantum properties such as the quantum anomalous Hall effect and the axion insulator state. In this work, we systematically investigate the evolution of the electronic structure of MnBi₂Te₄ thin films. With increasing film thickness, the electronic structure changes from an insulator type with a large energy gap to one with in-gap topological surface states, which is, however, still in drastic contrast to the bulk material. By surface doping of alkali-metal atoms, a Rashba split band gradually emerges and hybridizes with topological surface states, which not only reconciles the puzzling difference between the electronic structures of the bulk and thin-film MnBi₂Te₄ but also provides an interesting platform to establish Rashba ferromagnet that is attractive for (quantum) anomalous Hall effect. Our results provide important insights into the understanding and engineering of the intriguing quantum properties of MnBi₂Te₄ thin films.