| Grant number: | 18/05565-0 |
| Support Opportunities: | Scholarships in Brazil - Post-Doctoral |
| Start date: | May 01, 2018 |
| End date: | November 30, 2019 |
| Field of knowledge: | Physical Sciences and Mathematics - Physics - Condensed Matter Physics |
| Principal Investigator: | Adalberto Fazzio |
| Grantee: | Antonio Cláudio Michejevs Padilha |
| Host Institution: | Centro Nacional de Pesquisa em Energia e Materiais (CNPEM). Ministério da Ciência, Tecnologia e Inovação (Brasil). Campinas , SP, Brazil |
| Associated research grant: | 17/02317-2 - Interfaces in materials: electronic, magnetic, structural and transport properties, AP.TEM |
Abstract Topological Insulators (TI) are insulators with metallic edges when interfaced with vacuum or trivial insulator (TI'). The wavefunction acquires geometric phases distinct from TI' when run over a closed cycle in the Brillouin zone (BZ), defining topological invariants leading to such edges.The first known system of this kind, the Quantum Hall effect (QH) presents perfect metallic wires at the edges, separated from the insulator when a strong magnetic field, perpendicular to the plane where the electrons are restricted, is applied. This effect contradicted the idea that a system with an energy gap can't transport electrons. This seemingly paradox was solved by Laughlin, and Thaouless, Kohmoto, Nighting and Nijs (TKNN) by relating the wavefunction phases with the transversal conductivity Ãxy, bringing the concept of topology into the characterization of matter.The first BZ is a closed manifold, and by the Gauss-Bonnet theorem, is characterized by a topological invariant. The conditions for the existence of topological QH phases were believed to be the intense magnetic field and the 2D geometry. However Kane and Mele removed the former when studying graphene (2D material) by including the spin-orbit coupling (SOC), but since its gap is tiny (~0,01 meV) the experimental realisation was unfeasible. Alternatively, Molemkamp et al. obtained the same effect with the heterostructure CdTe/HgTe, where the metallic state is different from QH. Since [H, LûS ] = 0, time reversal symmetry (TRS) is preserved and electrons with opposite spins flow in opposite directions in the edges. This is the Quantum Spin Hall (QSH) and the material is TI, i.e. a condition for the material to be TI are the band inversions due to the SOC. Thus, materials containing heavier elements and with small gap are possible TI.What about 3D materials? Bi2Se3 and Bi2Te3 [Nat. Phys. 5, 398 (2009), Nat. Phys. 5, 438, (2009) and Science 325, 178 (2009)] present topologically protected surface states, given that a perturbation obey TRS. In this case there is a spin texture in the plane and a linear dispersion E x K - the so-called Dirac cones (DC). At TI-TI' and TI-semiconductor (TI-SC) interfaces, it was noticed some phenomena in the TI' part, such as DC unfolding. Currently we are studying interfaces [Nat. Commun. 6, 7630 (2015)] in particular TI-GaAs, and we have shown that the TI' acquires a spin texture due to interactions with topological states at the interface. Our simple model reproduced ab-initio (DFT) results, describing the TI-SC interaction: surface states (DC), by vF '(k x Ã) z, and frontier bands of TI' by ('2/2m* |k|2 +) , where is the relative position between the Dirac point and the valence band maximum.Recently it has been found a class of metals with topology distinct from TI', the "topological Weyl semimetal" (TWS). TWS present similar properties to the bandstructure of 3D graphene and TI. When a degenerate state is lifted by TRS break or inversion, some crossing bands remain gapless in the semimetal and exhibit linear dispersion in the 3 k directions, from the Weyl point, analogous to the graphene. These materials present aspects of TI, differing in the protections. In the case of TWS, TRS is violated, but point symmetries such as inversion are preserved, leading to Rashba and Anomalous QH effects. Stability of the Weyl nodes is connected to the topological protection inherent to the bandstructure. Such topology has non-usual consequences, such as the appearance of the Fermi Arc (FA) [Science 349, 613 (2015)]. The phenomenon has been observed experimentally in TaAs by Arpes and recently NbP, NbAs, and TaP have been predicted/confirmed.The project consists in a systematic study of TWS surfaces. (i) Cation and anion terminations and influence on the FA. (ii) Properties when in disordered surfaces, which will be generated by Monte Carlo methods. (iii) Electronic transport. TWS have a great potential in spintronics and quantum computing. (AU) | |
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