First-principles study of HgTe/CdTe heterostructures under perturbations preserving time-reversal symmetry

The observation of the quantum spin Hall effect without the need of an external magnetic field in HgTe/CdTe heterostructures triggered the study of materials exhibiting persistent spin-polarized electronic currents at their interfaces. These Dirac-like spin states are predicted to be topologically protected against perturbations preserving time-reversal symmetry. However, nonmagnetic (time-reversal preserving) perturbations will certainly affect these interface states. In this work, the density functional theory is used to characterize the topologically protected states of the (001) HgTe/CdTe heterostructure as well as to understand the influence of external adiabatic parameters as pressure and electric fields on these states. The intrasite Hubbard U term is seen to be important to correctly describe the HgTe bulk band structure. It is shown that, differently from the three-dimensional topological insulators, the HgTe/CdTe interface states present fully in-plane Rashba-like spin texture. Further, biaxial external pressures and electric fields perpendicular to the interfaces are seen to change the energetics and dispersion of the protected states, modifying the energy ordering of the crossing of the polarized interface states inside the band structure, and altering their Fermi velocities while not changing the topological quantum phase. These adiabatic variables can then be used to tune the topologically protected states with respect to the Fermi level.