Discovery of transient topological crystalline order in optically driven SnSe

TL;DR

This paper reports the transient induction of topological crystalline order in SnSe using ultrafast optical excitation, revealing Dirac-like surface states and symmetry changes that enable topological phases in a conventional semiconductor.

Contribution

It demonstrates the first observation of light-induced topological crystalline order in a semiconductor via femtosecond excitation and spectroscopy.

Findings

Transient Dirac-like surface states observed within the band gap.

Femtosecond excitation increases lattice symmetry, enabling topological order.

Spectral features are consistent with mirror-symmetry-protected topological surface states.

Abstract

Ultrafast optical excitation provides a powerful route for accessing emergent quantum phases far from equilibrium, enabling transient light-induced phenomena such as magnetism, ferroelectricity, and superconductivity. However, extending this approach to induce topological phases, especially in conventional semiconductors, remains challenging. Here, we report the observation of a thermally inaccessible, transient topological crystalline order in the layered semiconductor SnSe, a trivial insulator with a sizable (~ 0.8 eV) band gap, induced by femtosecond above-gap excitation. Time- and angle-resolved photoemission spectroscopy directly reveals the sub-picosecond emergence of Dirac-like linear dispersions within the band gap. Their features, including a high Fermi velocity (~ 2.5x10^5 m/s), multiple Dirac points away from high-symmetry momenta, and independence from probe photon energy, are consistent with mirror-symmetry-protected surface states of a topological crystalline insulator. The observed spectral dynamics, combined with density functional theory calculations, indicate that the femtosecond excitation transiently increases lattice symmetry, enabling topological crystalline order to emerge. Our discovery opens new avenues for ultrafast optical control of topological quantum phases in semiconductors, with potential applications in quantum and spintronic devices.