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

The recent discovery of non-saturating giant positive magnetoresistance has aroused much interest in Td-WTe₂. We have investigated 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 and its structural link with the metallic 1 T form provides an understanding of its structural stability. There is a metal-to-insulator switch-over in the electrical conductivity and a change in the sign of the Seebeck coefficient around 373 K. Lattice vibrations of Td-WTe₂ have been analyzed using first-principles 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 calculated Raman tensors, we assign the intense bands at 165 cm⁻¹ and 212 cm⁻¹ to the $A_1^{\prime }$ and $A_1^{\prime \prime }$ modes, respectively. Most of the Raman bands stiffen with decreasing thickness, and the ratio of the integrated intensities of the $A_1^{\prime \prime }$ to $A_1^{\prime }$ bands decreases in the few-layer sample, while all the bands soften in both the bulk and few-layer samples with increasing temperature. (paper)