A combined experimental and theoretical study of the electronic and vibrational properties of bulk and few-layer Td-WTe2

TL;DR

This study combines experimental and theoretical approaches to investigate the structural, electronic, vibrational, and thermoelectric properties of bulk and few-layer Td-WTe2, revealing insights into its stability, phase transitions, and vibrational modes.

Contribution

It provides a comprehensive analysis of Td-WTe2's properties, including spin-orbit effects, Raman spectra, and temperature-dependent behavior, advancing understanding of this material's potential applications.

Findings

Spin-orbit coupling influences semi-metallic behavior.

Identified five key Raman-active modes and assigned their symmetries.

Observed temperature and layer-dependent shifts in vibrational spectra.

Abstract

The recent discovery of non-saturating giant positive magnetoresistance in Td-WTe2 has aroused great interest in this material. We have studied the structural, electronic and vibrational properties of bulk and few-layer Td-WTe2 experimentally and theoretically. Spin-orbit coupling is found to govern the semi-metallic character of Td-WTe2. Its structural link with the metallic 1T form provides an understanding of its structural stability. We observe a metal to insulator transition and a change in the sign of the Seebeck coefficient around 373 K. Lattice vibrations in Td-WTe2 have been analyzed by first principle calculations. Out of the 33 possible zone-center Raman active modes, five distinct Raman bands are observed around 112, 118, 134, 165 and 212 cm-1 in bulk Td-WTe2. Based on symmetry analysis and the calculated Raman tensors, we assign the intense bands at 165 cm-1 and 212 cm-1 to the A_1^' and A_1^" modes respectively. We have examined the effect of temperature and the number of layers on the Raman spectrum. Most of the bands of Td-WTe2 stiffen, and the ratio of the integrated intensities of the A_1^" to A_1^' bands decreases in the few-layer sample, while all the bands soften in both bulk and few-layer samples with increasing temperature.