Non-equilibrium States and Interactions in the Topological Insulator and Topological Crystalline Insulator Phases of NaCd4As3

TL;DR

This study investigates the non-equilibrium electronic dynamics of NaCd4As3 in its topological crystalline insulator and topological insulator phases using ultrafast laser pulses, revealing phase-specific electronic responses and couplings.

Contribution

It provides the first experimental comparison of light-induced non-equilibrium properties in TCI and TI phases of NaCd4As3, highlighting their distinct electronic and structural interactions.

Findings

Significant band edge shift (>150 meV) in TI phase after excitation.

Strong coupling between electronic and structural orders in TI phase.

No immediate excited state observed in TCI phase due to low density of states.

Abstract

Topological materials are of great interest because they can support metallic edge or surface states that are robust against perturbations, with the potential for technological applications. Here we experimentally explore the light-induced non-equilibrium properties of two distinct topological phases in NaCd4As3: a topological crystalline insulator (TCI) phase and a topological insulator (TI) phase. This material has surface states that are protected by mirror symmetry in the TCI phase at room temperature, while it undergoes a structural phase transition to a TI phase below 200 K. After exciting the TI phase by an ultrafast laser pulse, we observe a leading band edge shift of >150 meV, that slowly builds up and reaches a maximum after ~0.6 ps, and that persists for ~8 ps. The slow rise time of the excited electron population and electron temperature suggests that the electronic and structural orders are strongly coupled in this TI phase. It also suggests that the directly excited electronic states and the probed electronic states are weakly coupled. Both couplings are likely due to a partial relaxation of the lattice distortion, which is known to be associated with the TI phase. In contrast, no distinct excited state is observed in the TCI phase immediately or after photoexcitation, which we attribute to the low density of states and phase space available near the Fermi level. Our results show how ultrafast laser excitation can reveal the distinct excited states and interactions in phase-rich topological materials.