Activation of new Raman modes by inversion symmetry breaking in type II Weyl semimetal candidate T'-MoTe2

TL;DR

This study reveals that inversion symmetry breaking in T'-MoTe2 induces new Raman modes, providing insights into its phase transition and Weyl semimetal properties through combined experimental and theoretical analysis.

Contribution

The paper demonstrates the activation of new Raman modes due to inversion symmetry breaking in T'-MoTe2, linking vibrational changes to topological Weyl semimetal characteristics.

Findings

New Raman bands appear in the low-temperature T'or phase.

Inversion symmetry breaking activates shear Raman modes.

Raman modes at 130 cm-1 and 12 cm-1 monitor symmetry breaking.

Abstract

We synthesized distorted octahedral(T') molybdenum ditelluride (MoTe2) and investigated its vibrational properties with Raman spectroscopy, density functional theory and symmetry analysis. Compared to the results from high temperature centrosymmetric monoclinic (T'mo) phase, four new Raman bands emerge in the low temperature orthorhombic (T'or) phase, which was recently predicted to be a type II Weyl semimetal. Crystal-angle-dependent, light-polarization-resolved measurements indicate that all the observed Raman peaks belong to two categories: those vibrating along the zigzag Mo atomic chain (z-mode) and those vibrating in the mirror plane (m-mode) perpendicular to the zigzag chain. Interestingly the low energy shear z-mode and shear m-mode, absent from the T'mo spectra, become activated when sample cooling induces a phase transition to the T'or crystal structure. We interpret this observation as a consequence of inversion-symmetry breaking, which is crucial for the existence of Weyl fermions in the layered crystal. Our temperature dependent Raman measurements further show that both the high energy m-mode at 130 cm-1 and the low energy shear m-mode at 12 cm-1 provide useful gauges for monitoring the broken inversion symmetry in the crystal.