Semiconductor-like photocarrier dynamics in Dirac Semimetal Cd3As2 film Probed with transient Terahertz Spectroscopy

TL;DR

This study investigates the nonequilibrium photocarrier dynamics in the Dirac semimetal Cd3As2 using time-resolved terahertz spectroscopy, revealing semiconductor-like relaxation behavior and a narrow energy gap around the Dirac node.

Contribution

It provides the first temperature-dependent analysis of photocarrier relaxation in Cd3As2, demonstrating semiconductor-like behavior and estimating a narrow energy gap using a Rothwarf-Taylor model.

Findings

Relaxation time increases from 4.7 ps at 5 K to 7.5 ps at 220 K.

Photocarrier relaxation exhibits a single exponential decay.

A narrow energy gap of ~35 meV is identified around the Dirac node.

Abstract

The topological three-dimensional Dirac semimetal Cd3As2 has drawn great attention for the novel physics and promising applications in optoelectronic devices operating in the infrared and THz regimes. Among the extensive studies in the past decades, one intriguing debate is the underlined mechanism that governing the nonequilibrium carrier dynamics following photoexcitation. In this study, the temperature dependent photocarrier dynamics in Cd3As2 film has been investigated with time-resolved terahertz spectroscopy. The experimental results demonstrate that photoexcitation results in abrupt increase in THz photoconductivity, and the subsequent relaxation shows a single exponential relaxation for various temperatures and pump fluences. The relaxation time increase from 4.7 ps at 5 K to 7.5 ps at 220 K, while the lifetime remains almost constant of ~7.5 ps with temperature above 220 K. A Rothwarf-Taylor model was employed to fit the temperature dependent relaxation time, and a narrow energy gap of ~35 meV is obtained, which occurs around the Dirac node. Our THz spectroscopy results demonstrate that the photocarrier relaxation in Cd3As2 shows a semiconductor-like behavior, rather than hot carrier scatterings in graphene and most of metals.