Description-H1

H1. Topological Insulation, hidden polarization and other spin-orbit effects:

  Spin-orbit coupling (SOC), an important relativistic correction to the time-independent Hamiltonian in quantum mechanics, enacts fascinating phenomena in materials containing heavy elements. Topological insulators (TIs) are nonmetallic bulk compounds whose surface possesses passivation-resistant linearly dispersed metallic energy bands. To have such surface states requires the inversion in the order of bulk valence and conduction bands at special, time reversal invariant wave vectors in the Brillouin zone. TIs have recently attracted interest due to their proven ability to realize novel quantum phases including quantum spin Hall effect, quantum anomalous Hall effect, and topological superconductivity, and due to their promise of future potential applications in spintronics7 and quantum informations. The required band inversion3,9 is generally achieved by introducing high atomic number (Z) heavy elements, manifesting strong relativistic effects – both band gap reducing scalar relativistic Mass-Darwin effect, as well as degeneracy splitting spin-orbit coupling (SOC)10. The required incorporation of high-Z elements such as Hg, Tl, Bi, and Pu means that the corresponding compounds tend to be (even in the absence of doping) narrow gap semiconductors or semimetals rather that electrical insulators, and limits the available material pool relevant for discovery of such functionality. At the same time, documented bond energies of semiconductors and semimetals compounds show that as the bonded elements become heavy, their outer s electrons lose the ability of forming extended chemical bonds, leading to significant reduction in bond energies. This tendency exposes such TIs to structural defects that are readily formed in weakly bonded heavy element compounds, a tendency that could limit their integration into device technologies.

  Not surprisingly, attempts to broaden the range of TIs by converting non-TI compounds into TI compounds plays an important role in the search and discovery of new TIs.Transforming normal insulators (NIs) to TIs has been generally considered via two methods: (a) Manipulating electronically or mechanically the band structure of a single compound by external electric field or by applied external strain. (b) Designing a combination of different material building blocks into superlattices or quantum wells, affecting band inversion via heterostructure effects such as built-in electric field and quantum confinement. Both approaches could create TI from the current menu of non-TI compounds and in principle do not require any heavy, high-Z atoms, thus possibly relieving such systems from defects associated with weak bonds and expanding the material base of TIs.

  An important factor that is closely related to the future realization of novel TI proposals is their thermodynamic stability in the specific crystal structures that would be TIs. In the short history of TI search, many theoretical predictions of TI compounds did not assess whether the postulated structures predicted to be TIs are also the stable, or near-stable forms of these systems. Indeed, some theoretically predicted would be TIs are energetically not even close to their competing non TI phases (such as phase separation or atom exchange). Actually, the conditions of simultaneously having thermodynamically stable compounds with certain structures and inverted conduction and valence bands, can be contraindicated

  Our groups work in this field involved theoretical discovery of new TI’s ; assessment of the stability of such cases; design of methods to convert normal insulators to topological insulators.In addition to the widely studied SOC-induced Rashba and Dresselhaus spin polarizations in materials lacking inversion symmetry, hidden forms of spin polarization in centrosymmetric materials has been recently proposed and observed in subsequent experiments.