Cálculos de primeiros princípios em isolantes topológicos: HgTe/CdTe

Abstract

The observation of the quantum spin Hall effect in the HgTe/CdTe heterostructure triggered the study of materials exhibiting a spin polarized electronic current at their surfaces/ interfaces. These states are topologically protected against perturbations preserving time reversal symmetry and presenting a linear dispersion, forming a Dirac cone. However, non-magnetic perturbations (that preserve time reversal symmetry) will certainly affect these surface/interface states. In this work we user the density functional theory to characterize the topologically protected states of the (001) HgTe/CdTe heterostructure. We observed that for a correct description of the HgTe band structure we use a GGA+U method. The topological states showed a Rashba-like in-plane spin texture. We analyzed the effects of external pressures and electric fields in the HgTe/CdTe heterostructures. We show that these perturbations modify the energetics and dispersion of the protected states, although not destroying the topological phase. Also, we study defects like antisite, vacancy and a Fe magnetic impurity at the interface of the (001) HgTe/CdTe heterostructure. We show that the antisite and the vacancy do not affect the spin polarization nor the energy dispersion of the protected states. On the other hand, the magnetic impurity significantly affects the topological states, degrading the spin polarization for the states close to the magnetic impurity and inducing out-of-plane spin components. Further, we study the (001) HgTe surface for different thicknesses of the HgTe sample, and with different terminations (Hg and Te). To the (001) HgTe samples with a thickness of 38 Å , the spin polarized states do not show a linear dispersion, however, when the thickness is increased we observe the formation of spin-polarized surface states with linear dispersion, characterizing the formation of a Dirac cone. Also, we show that biaxial pressures modify the energy dispersion of the spin polarized states. Finally, we study materials that turn topological insulators under external pressures as the anti-perovskite structures Sr3BiN and Ca3BiN, using the self-consistent GW method. We show that these materials present an inversion of the Bi-pz and Bi-s band edge states when subjected to biaxial tensile stress. We conclude that these materials can be characterized Topological Insulators under pressure.