Photoinduced Dirac semimetal in ZrTe5

TL;DR

This study demonstrates that femtosecond laser pulses can induce a transient 3D Dirac semimetal state in ZrTe$_5$, revealing a new method to engineer topological phases with potential applications in ultrafast optoelectronics.

Contribution

It shows how ultrafast photoexcitation can create a transient Dirac semimetal state in ZrTe$_5$, combining ultrafast electron diffraction and DFT analysis to reveal the electronic and structural dynamics.

Findings

Transient Dirac semimetal state induced by femtosecond laser pulse

Long relaxation time (~160 ps) of the photoinduced state

Gap closure and emergence of massless Dirac fermions

Abstract

Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli, such as photoexcitation, which offers prospects of their integration into future devices compatible with optical communication and information technology. Here, we use MeV Ultrafast Electron Diffraction (UED) to show how a transient three-dimensional (3D) Dirac semimetal state can be induced by a femtosecond laser pulse in a topological insulator ZrTe$_5$. We observe marked changes in Bragg diffraction, which are characteristic of bond distortions in the photoinduced state. Using the atomic positions refined from the UED, we perform density functional theory (DFT) analysis of the electronic band structure. Our results reveal that the equilibrium state of ZrTe$_5$ is a topological insulator with a small band gap of $\sim$25 meV, consistent with angle-resolved photoemission (ARPES) experiments. However, the gap is closed in the presence of strong spin-orbit coupling (SOC) in the photoinduced transient state, where massless Dirac fermions emerge in the chiral band structure. The time scale of the relaxation dynamics to the transient Dirac semimetal state is remarkably long, $\tau \sim$160 ps, which is two orders of magnitude longer than the conventional phonon-driven structural relaxation. The long relaxation is consistent with the vanishing density of states in Dirac spectrum and slow spin-repolarization of the SOC-controlled band structure accompanying the emergence of Dirac fermions.