We study the spin-orbit interaction (SOI) in InAs/GaSb and InAs quantum wells. We show through temperatureand gate-dependent magnetotransport measurements of weak antilocalization that the dominant spin-orbit relaxation mechanism in our low-mobility heterostructures is Elliott-Yafet and not Dyakonov-Perel in the form of the Rashba or Dresselhaus SOI as previously suggested. We compare our findings with recent work on this material system and show that the SOI length lies within the same range. The SOI length may be controlled using an electrostatic gate, opening up prospects for developing spintronic applications.

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.

The Dresselhaus spin orbit interaction is expected to perturb the quantum spin Hall phase predicted to arise within InAs/GaSb coupled quantum wells. As such, to gain a greater understanding of this spin-orbit interaction, the spin orbit coupling in two InAs/GaSb coupled quantum wells, grown along the [001] axis, is investigated along 3 different in-plane crystallographic axes. Due to the crystallographic axis dependence of the Dresselhaus spin orbit coupling, we can deconvolute this coupling from the axis-invariant Rashba spin orbit coupling. We find that the Dresselhaus parameter is robust against an external gate bias and small changes in growth conditions, with an associated Dresselhaus parameter of (0.20±0.07)10-11 eVm being measured across all samples and top gate bias conditions. In addition we show that the asymmetries associated with the coupled quantum well structure, leading to the Rashba spin orbit coupling, are likely to play a dominant role in determining the spin orbit interaction experienced by a quantum spin Hall state as the system is tuned towards charge neutrality.

InAs/GaSb coupled quantum well heterostructures are important semiconductor systems with applications ranging from spintronics to photonics. Most recently, InAs/GaSb heterostructures have been identified as candidate two-dimensional topological insulators, predicted to exhibit helical edge conduction via fully spin-polarised carriers. We study an InAs/GaSb double quantum well heterostructure with an AlSb barrier to decouple partially the 2D electrons and holes, and find conduction consistent with a 2D hole gas, with an effective mass of 0.235±0.005 m0, existing simultaneously with hybridised carriers with an effective mass of 0.070±0.005 m0, where m0 is the bare electron mass.2

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