Mercury is a peculiar metal, which can be found as a liquid in its native form in the nature. Its distinct properties arise from the strong relativistic effects and the ensuing lanthanide contraction.[1] Mercury chalcogenides, such as HgTe, manifest the characteristic physics of mercury by having the Gamma₆  band (arising from Hg 6s orbitals) lower than the Gamma₈ one (arising from 5p orbitals of Te). This is contrary to other metal chalcogenides such as CdSe, and makes HgTe have a so-called negative band gap (i.e. a semi-metal). Because quantum confinement restores the usual band order, it has been used to open gaps controllably from the THz regime up to the near infrared, which makes HgTe nanocrystals the most widely tunable colloidal material.[2]

In this presentation we explain the electronic structure and optical properties of HgTe nanoplatelets.[3,4] Optical measurements carried out on these systems reveal scarce qualitative differences with respect to their CdTe counterparts. We show this is because the small thickness (3.5 monolayers) suppresses any
trace of band inversion.[4] Next we investigate, by means of an eight-band k·p Hamiltonian and configuration interaction methods, how the response should change when increasing the thickness.

We predict that for thicknesses beyond 2 nm, the physics of HgTe starts being governed by the Gamma₆-Gamma₈ band mixing, which leads to important departures with respect to the expectations of quantum confinement. A prominent example is the formation of hybrid states in the conduction band, where the charge density is not localized in the volume, but partly migrates towards the surface of the platelet. Because this does not happen to the valence band states, we foresee the possibility of building indirect excitons without the need of using type-II heterostructures.

Around 6 nm the Gamma₆-Gamma₈ band inversion takes place, which is indeed close to the critical thickness reported in epitaxial HgTe/CdTe quantum wells.[5] At this point a quantum phase transition from regular to topological insulator takes place. A few hints are given on the topological magnetoelectric effect that could be achieved in this regime. Namely, we show through axion electrodynamics equations that electrical charges could induce magnetic fields in the nanostructure. These could be modulated through the vertical and lateral confinement of the platelet.[6]