Ultrafast dynamics of three-dimensional Kane plasmons in the narrow-bandgap Hg$_{0.8}$Cd$_{0.2}$Te

TL;DR

This study uses ultrafast terahertz spectroscopy to explore the dynamics of bulk plasmons in narrow-bandgap Hg0.8Cd0.2Te, revealing how plasmon frequency scales with carrier density and confirming the presence of massless Kane fermions.

Contribution

It introduces a room-temperature method to analyze band dispersion in narrow-gap semiconductors, demonstrating the unique plasmon scaling in Kane fermions without high magnetic fields.

Findings

Plasma frequency scales with the cube root of carrier density in MCT.

The method distinguishes between massless and massive band dispersions.

Massless Kane fermion behavior persists despite deviations from the gapless condition.

Abstract

We report on an ultrafast terahertz spectroscopic study on the dynamics of free carriers and the pertinent bulk plasmons in Hg$_{0.8}$Cd$_{0.2}$Te (MCT) film, a narrowband semiconductor accommodating three dimensional massless Kane fermions. The ultrabroadband terahertz source enables the investigation of the lightly doped equilibrium state in the presence of plasmon-phonon hybridization through the heavily doped excited state, primarily dominated by plasmons. Without the recourse to the resource consuming cryogenic high magnetic field spectroscopy that hinges on observable related to the interband transition, we show that the massless band dispersion can instead be conveniently perceived by the room temperature study of the intraband transition through the determination of the plasmon carrier density relationship. We found the plasma frequency in MCT scales with the cube root of carrier density, in contrast with the square root scaling in the conventional massive fermion system of parabolic band dispersion. This work also answers the curious question of whether the MCT can maintain its massless Kane fermion character in case the strict gapless condition is deviated from. The method presented herein provides a convenient approach to identifying the landscape of both massless and massive band dispersion.