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

Abstract

We have performed a materials investigation into the properties of the THz conductivity spectra in the ternary alloy Bi 2 (Te (1−x) Se x ) 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 Bi 2 (Te (1−x) Se x ) 3 results in the prominent E 1 u phonon (occurring between 1.5 and 1.9 THz) red-shifting on cooling less than the binary Bi 2 Te 3 and Bi 2 Se 3 samples. We also find that the E 1 u phonon couples to an electronic continuum at low temperatures (T ⩽ 40 K), regardless of Bi 2 (Te (1−x) Se x ) 3 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.

Exploring dynamics, disorder, and anharmonicity in THz spectra of crystalline materials

“…33 These materials have a very strong degenerate phonon which can be easily tuned by varying the value of x by varying the ratio of tellurium and selenium fluxes during the molecular-beam epitaxy (MBE) growth. 34 Of particular interest in this work is that a) The prominent phonon, somewhat counter-intuitively, red-shifts upon cooling while other Raman active phonons do not 34,35 and b) we observe a reduction in crystalline anharmonicity caused by the preferential ordering with the x = 1 3 showing the lowest anharmonicity. 34 The fine control of alloy fraction during growth and the interesting temperature response, therefore, make this an excellent material to explore how anharmonicity (and defects) influence the spectral properties of this ternary alloy.…”

“…34 Of particular interest in this work is that a) The prominent phonon, somewhat counter-intuitively, red-shifts upon cooling while other Raman active phonons do not 34,35 and b) we observe a reduction in crystalline anharmonicity caused by the preferential ordering with the x = 1 3 showing the lowest anharmonicity. 34 The fine control of alloy fraction during growth and the interesting temperature response, therefore, make this an excellent material to explore how anharmonicity (and defects) influence the spectral properties of this ternary alloy. TI samples were grown on [0001] oriented Al 2 O 3 substrates by co-deposition of evaporated bismuth, selenium, and tellurium which has been described in detail previously.…”

“…TI samples were grown on [0001] oriented Al 2 O 3 substrates by co-deposition of evaporated bismuth, selenium, and tellurium which has been described in detail previously. 34 The THz-TDS 16 measurements were undertaken between 3 K and 300 K in an Oxford Instruments Microstat-HE cryostat. Additional measurements were taken of a blank Al 2 O 3 substrate without TI film as a reference where the THz conductivity of the film can then be determined.…”

“…In the TI samples discussed in Section 3 the substitution of Se with Te is used to influence both the central frequency and anharmonicity 34 of the key phonon mode, which could be viewed as the influence of defects in the crystal structure on the measured spectral parameters. When looking at molecular systems as discussed in Section 2 we tend to assume that we consider a perfect static crystal with no defects.…”

“…Experimental and Static calculations related to the TI samples can be found in the Leeds Data Repository. 52 Additional data related to the QHA simulations of Bi 2 Se 3 can be found from Zenodo. 53 The calculated l-cysteine data are openly available from Zenodo.…”