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

We performed a materials investigation into the properties of the THz conductivity spectra in the ternary alloy Bi₂(Te₍₁—ₓ₎Seₓ)₃ 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 Bi₂(Te₍₁—ₓ₎Seₓ)₃ results in the prominent E1uphonon (occurring between 1.5 and 1.9 THz) red-shifting on cooling less than the binary Bi2Te3and Bi2Se3samples. We also find that the E1uphonon couples to an electronic continuum at low temperatures (T⩽40K), regardless of Bi₂(Te₍₁—ₓ₎Seₓ)₃stoichiometry 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.