Effects of structural ordering on infrared active vibrations within Bi2(Te(1−x)Sex)3

Abstract

We performed a materials investigation into the properties of the THz conductivity spectra in the ternary alloy ${\mathrm{Bi}}_{2}{({\mathrm{Te}}_{(1\ensuremath{-}x)}{\mathrm{Se}}_{x})}_{3}\phantom{\rule{4pt}{0ex}}$ as a function of selenium fraction, $x$, and temperature. We find that the reduction in crystalline anharmonicity caused by the preferential ordering of the $x=1/3$ phase of ${\mathrm{Bi}}_{2}{({\mathrm{Te}}_{(1\ensuremath{-}x)}{\mathrm{Se}}_{x})}_{3}\phantom{\rule{4pt}{0ex}}$results in the prominent ${E}_{u}^{1}\phantom{\rule{4pt}{0ex}}$phonon (occurring between 1.5 and 1.9 THz) red-shifting on cooling less than the binary ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}\phantom{\rule{4pt}{0ex}}$and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}\phantom{\rule{4pt}{0ex}}$samples. We also find that the ${E}_{u}^{1}\phantom{\rule{4pt}{0ex}}$phonon couples to an electronic continuum at low temperatures ($T\ensuremath{\leqslant}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), regardless of ${\mathrm{Bi}}_{2}{({\mathrm{Te}}_{(1\ensuremath{-}x)}{\mathrm{Se}}_{x})}_{3}\phantom{\rule{4pt}{0ex}}$stoichiometry or the Hall mobility of the topological insulator crystal. These results highlight the role that these optical modes play in the electronic and thermal transport within this ternary alloy and pave the way for exploring the interesting phonon dynamics within these topological insulator and thermoelectric materials.