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Abstract:
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)' and A(1)'' modes, respectively. Most of the Raman bands stiffen
with decreasing thickness, 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 the bulk and few-layer samples with
increasing temperature.
