Unconventional spin texture in a noncentrosymmetric quantum spin Hall insulator

Abstract

We proposed that the simultaneous presence of both Rashba and band inversion can lead to a Rashba-like spin-splitting formed by two bands with the same in-plane helical spin texture. Because of this unconventional spin texture, the backscattering is forbidden in edge and bulk conductivity channels. We propose a new non-centrosymmetric honeycomb-lattice quantum spin Hall (QSH) insulator family formed by the IV, V, and VII elements with this property. The system formed by Bi, Pb and I atoms is mechanically stable and has both a large Rashba spin-splitting of 60 meV and a large nontrivial band gap of 0.14 eV. Since the edge and the bulk states are protected by the TR symmetry, contrary to what happens in most doped QSH insulators, the bulk states do not contribute to the backscattering in the electronic transport, allowing the construction of a spintronic device with less energy loss. The main objective of spintronics is to understand the mechanisms by which it is possible to achieve efficient control of both spin configurations and spin currents [1,2]. In the last decade, the way to achieve this objective has experienced a breakthrough due to i ) the discovery and understanding of mechanisms to generate spin currents in conductors with magnetic order and in paramagnetic conductors/semiconductors [3][4][5], ii ) the experimental observation of theoretically proposed spin injector systems [6][7][8], and iii ) the synthesis of 2D materials with long spin relaxation time [1,9]. The generation of spin currents, spin injections and spin conservation are mediated by the spin-orbit coupling (SOC) mainly via Rashba effect and/or nontrivial topological phases [10][11][12][13], such as the quantum spin Hall (QSH) effect [14]. Therefore, the search for systems experiencing these properties is a primary concern for the development of spintronics.QSH insulators support helical metallic edge states, forming topological Dirac fermions protected by the timereversal (TR) symmetry on an insulating bulk [6,7]. The topological transition from trivial insulating to topological insulators is evidenced as a band inversion at the TR invariant k-point mediated by the SOC. The topological band dispersion has been experimentally characterized via angle-resolved photoemission spectroscopy (ARPES) and local scanning tunneling microscopy (STM) in 3D topological insulators [6], and via transport measurements in HgTe/CdTe quantum wells [15,16]. On the other hand, the Rashba effect, arising from the lack of inversion symmetry, leads to parallel spin-polarized band dispersion curves with opposite in-plane chiral spin texture [17], allowing the control of the spin direction through an electric field [10,12,13]. These dispersion curves and Fermi contours have been characterized by spectroscopic measurements for many surfaces and interfaces [18][19][20][21]. Large Rashba spin-splitting are found in materials formed by heavy elements with strong intrinsic SOC such as Bi, Pb, W, among others [21][22][23][24][25]. In this work, we look at the consequence...

From DFT to machine learning: recent approaches to materials science–a review

“…DFT has been used for almost every kind of system ranging from atomic [306], molecular [307,308], and chemical systems, extended solids, surfaces [309], defects [310], 0D [311][312][313], 1D [314][315][316][317][318][319][320], and 2D [321] systems. In terms of properties, structural [322], electronic/transport [323,324], thermal [325][326][327], electron-phonon [328], optical [329], catalytic [330], magnetic [331][332][333], topological [334][335][336], and many others have been studied.…”

Spin-Polarization Control Driven by a Rashba-Type Effect Breaking the Mirror Symmetry in Two-Dimensional Dual Topological Insulators

“…The Hamiltonian describing QSHIs with inversion symmetry breaking (Eq. 5) leads to parabolic bands with the same helical in-plane spin texture, forbidding the backscattering in bulk states [31]. This unconventional spin topology was also observed in the non-topological alloy Bi/Cu(111) [38].…”

“…Thus, HF IV-V hexagonal-lattices materials, formed by two triangular sub-lattices, one consisting of an IV-VII dimer and the other of atoms V (See Fig 1), have no magnetic moment. Therefore, this systems are predicted to be mechanically stable QSHIs [31]. Since the functionalization breaks the mirror symmetry M z , the symmetry operation satisfied by HF-compounds are only R 3 , T , and M x , which is schematically represented in Fig.…”

“…For this purpose, we first i ) construct a tightbinding effective model for the Na 3 Bi compound [21], and ii ) predict that the TR-symmetry protected edges states in HF-honeycomb lattice materials Ref. 31 are also protected by mirror planes perpendicular to the layer, which is different from the conventional TCIs. Using the proposed models, we then study the in-and out-plane electric field breaking the mirror symmetry in Na 3 Bi and HF-hexagonal compounds, verifying the change of the arXiv:1811.11014v2 [cond-mat.mes-hall] 12 Jan 2019 spin-polarization in TR-symmetry protected edge states.…”

“…Similarly, in the inverted order, the VBM has a p-orbital character, mainly dominated by p z -Bi orbitals, whereas the CBm at Γ mainly consists of p x,y -Bi orbitals. Hence, the Hamiltonian in the full SOC basis {|Bi 1/2 , ±1/2 , |Bi 3/2 , ±1/2 } reads [31]…”