Coherent Dirac plasmons in topological insulators

TL;DR

This study investigates ultrafast coherent Dirac plasmons in topological insulators, revealing their transient behavior and coherence time of approximately 200 femtoseconds through time-resolved spectroscopy and modeling.

Contribution

It provides the first detailed analysis of the coherence dynamics of Dirac plasmons in topological insulators using ultrafast reflectivity and the two-temperature model.

Findings

Fano-like asymmetric phonon line shape observed in spectra.

Dirac plasmon coherence time estimated at approximately 200 fs.

Transient plasmon-phonon quantum interference characterized.

Abstract

We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in Sb$_2$Te$_3$ topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the $A^2_{1g}$ mode, which is attributed to quantum interference between continuum-like coherent Dirac-plasmons and phonons. By analyzing the time-dependent asymmetric line shape using the two-temperature model (TTM), it was determined that a Fano-like resonance persisted up to $\approx$1 ps after photo-excitation with a relaxation profile dominated by Gaussian decay at $\leq$200 fs. The asymmetry parameter could be well described by the TTM for $\geq$200 fs, therefore suggesting the coherence time of the Dirac plasmon is $\approx$200 fs.