Two-dimensional THz spectroscopy of nonlinear phononics in the topological insulator $\mathrm{MnBi}_2\mathrm{Te}_4$

TL;DR

This study uses two-dimensional THz spectroscopy to investigate nonlinear phononics in the topological insulator MnBi2Te4, revealing how THz fields induce coherent energy transfer between phonon modes.

Contribution

It demonstrates a novel application of 2D THz spectroscopy combined with modeling to elucidate multi-stage phonon excitation mechanisms in a topological insulator.

Findings

Coherent energy transfer between Raman and IR phonons observed.

Multi-stage excitation process identified and characterized.

THz electric field effectively excites specific phonon modes.

Abstract

The interaction of a single-cycle THz electric field with the topological insulator $\mathrm{MnBi}_2\mathrm{Te}_4$ triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise non-interacting Raman-active $E_g$ and infrared (IR)-active $E_u$ phononic modes. Two-dimensional (2D) THz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multi-stage process underlying the excitation of the Raman-active $E_g$ phonon. In this process, the THz electric field first prepares a coherent IR-active $E_u$ phononic state and subsequently interacts with this state to efficiently excite the $E_g$ phonon.