# Nonequilibrium Photocarrier and Phonon Dynamics in Dirac Semimetal NiTe2 Microcrystals Probed by Ultrafast Reflectivity Spectroscopy

## TL;DR

This study explores the ultrafast dynamics of electrons and phonons in NiTe2 microcrystals, a 3D Dirac semimetal, using time-resolved spectroscopy to understand their behavior for potential optoelectronic applications.

## Contribution

The paper provides new insights into the nonequilibrium dynamics of photocarriers and phonons in microcrystalline NiTe2, a prototypical 3D Dirac semimetal.

## Key findings

- The transient reflectivity kinetics of NiTe2 are described by a triple-exponential decay function.
- A sub-picosecond relaxation component is attributed to electron-optical phonon coupling.
- An 8 ps process is linked to electron–hole recombination, and a 20–30 ps decay to anharmonic phonon decay.

## Abstract

Topological 3D Dirac semimetals are characterized by bulk Dirac cone band crossings and nontrivial topological surface states, giving rise to a wealth of exotic physical properties and attracting considerable attention in recent years. Understanding the nonequilibrium dynamics of Dirac semimetals in micro-size provides critical guidance for the design of micro- and nanoscale optoelectronic and ultrafast photonic devices. In this work, we employ time-resolved microscopic transient spectroscopy to investigate the nonequilibrium photocarrier and lattice dynamics in microcrystalline Dirac semimetal NiTe2, a prototypical 3D Dirac semimetal. Following photoexcitation at 390 nm, the transient reflectivity kinetics of NiTe2 can be well described with a triple-exponential decay function. The fastest relaxation component occurs on a sub-picosecond timescale and increases with pump fluence, which originates from electron-optical phonon coupling. An intermediate relaxation process with a characteristic time of ~8 ps is attributed to electron–hole recombination, while a slower decay component on the order of ~20–30 ps can be assigned to the anharmonic decay of optical phonons into acoustic phonons. Polarization-resolved measurements reveal nearly in-plane isotropic transient responses, which are insensitive to the polarization of probe light. These findings contribute to the physical insights for the development of future photonics and optoelectronic devices based on topological Dirac semimetals.