Ultrafast Electronic Dynamics of a Weyl Semimetal MoTe$_2$ Revealed by Time and Angle Resolved Photoemission Spectroscopy

TL;DR

This study uses time-resolved photoemission spectroscopy to investigate the ultrafast electronic dynamics of the Weyl semimetal MoTe$_2$, revealing unoccupied states, Weyl node energy, and two key relaxation timescales related to electron-phonon interactions.

Contribution

It provides the first detailed measurement of ultrafast relaxation dynamics near the Weyl node in MoTe$_2$, including relaxation times and electron-phonon coupling parameters.

Findings

Identified the Weyl node at 70 meV above E_F.

Observed two intrinsic relaxation timescales: 430 fs and 4.1 ps.

Estimated electron-phonon coupling constant as approximately 32 meV^2.

Abstract

A Weyl semimetal is a new type of topological quantum phase with intriguing physics near the Weyl nodes. Although the equilibrium state of Weyl semimetals has been investigated, the ultrafast dynamics near the Weyl node in the nonequilibrium state is still missing. Here by performing time and angle resolved photoemission spectroscopy on type-II Weyl semimetal MoTe$_2$, we reveal the dispersion of the unoccupied states and identify the Weyl node at 70 meV above E$_F$. Moreover, by tracking the ultrafast relaxation dynamics near the Weyl node upon photo-excitation with energy, momentum and temporal resolution, two intrinsic recovery timescales are observed, a fast one of 430 fs and a slow one of 4.1 ps, which are associated with hot electron cooling by optical phonon cascade emission and anharmonic decay of hot optical phonons respectively. The electron population shows a metallic response, and the two temperature model fitting of the transient electronic temperature gives an electron-phonon coupling constant of $\lambda\langle\Omega^2\rangle\simeq32$ $\textrm{meV}^2$. Our work provides important dynamic information for understanding the relaxation mechanism of a Weyl semimetal and for exploiting potential applications using ultrafast optical control.