Spin Texture Of Surface States Research Articles

Reversal of spin-polarization near the Fermi level of the Rashba semiconductor BiTeCl

Spin–orbit coupling forms the physical basis for quantum materials with non-trivial topology and potential spintronics applications. The Rashba interaction is a textbook model of spin–orbit interactions, with charge carriers undergoing linear, isotropic spin-splitting in momentum space. Recently, non-centrosymmetric semiconductors in the family BiTeX (X = Cl, Br, I) have been identified as exemplary Rashba materials due to the strong splitting of their bulk bands, yet a detailed investigation of their spin textures, and their relationships to local crystal symmetry, is currently lacking. We perform high-efficiency spin-resolved photoemission spectroscopy to directly image the spin texture of surface states of BiTeCl, and we find dramatic deviations from idealized behavior, including a reversal of the spin-polarization near the Fermi level. We show that this behavior can be described by higher-order contributions to the canonical Rashba model with the surface states localized to individual trilayers of the crystal. Due to the prominence of these effects near the Fermi level, they should have a strong impact on the spin-dependent transport of carriers.

Theory for linear and nonlinear planar Hall effect in topological insulator thin films

Recently, the linear and nonlinear planar Hall effect (PHE) on the topological insulators (TIs) surface has been extensively studied in experiments. To explain this phenomenon, various microscopic mechanisms are proposed theoretically, and one has to employ different mechanisms to separately understand the linear and nonlinear PHE even for the same system. Here, we study the planar magnetic resistance effect in TI thin film and find that a peculiar anisotropic scattering and a spin valve structure with respect to the PHE can be caused by the tilt and shift of Dirac cones, respectively, which are induced by the combination of spin-momentum locking of surface states and an in-plane magnetic field. The tilt and shift effects can act as the origin of both the linear and nonlinear PHE by distorting the spin texture of surface states or forming the spin polarization. These two mechanisms interplay and dominate, respectively, in strong coupling (thin TI) and decoupling (thick TI) between bottom and top surfaces. For thick TI film, we show that both the linear and nonlinear PHEs induced by the tilt effect can recover the results observed in recent experiments. Our theory provides a perspective to understand the origin of both linear and nonlinear PHE observed in recent experiments.

Prediction of spin polarized Fermi arcs in quasiparticle interference in CeBi

We predict that CeBi in the ferromagnetic state is a Weyl semimetal. Our calculations within density functional theory show the existence of two pairs of Weyl nodes on the momentum path $(0,0,{k}_{z})$ at $15\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ above and $100\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ below the Fermi level. Two corresponding Fermi arcs are obtained on surfaces of mirror-symmetric (010)-oriented slabs at $E=15\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ and both arcs are interrupted into three segments due to hybridization with a set of trivial surface bands. By studying the spin texture of surface states, we find the two Fermi arcs are strongly spin polarized but in opposite directions, which can be detected by spin-polarized ARPES measurements. Our theoretical study of quasiparticle interference (QPI) for a nonmagnetic impurity at the Bi site also reveals several features related to the Fermi arcs. Specifically, we predict that the spin polarization of the Fermi arcs leads to a bifurcation-shaped feature only in the spin-dependent QPI spectrum, serving as a fingerprint of the Weyl nodes.

Electronic Coupling between Graphene and Topological Insulator Induced Anomalous Magnetotransport Properties.

It has been theoretically proposed that the spin textures of surface states in a topological insulator can be directly transferred to graphene by means of the proximity effect, which is very important for realizing a two-dimensional topological insulator based on graphene. Here we report the anomalous magnetotransport properties of graphene-topological insulator Bi2Se3 heterojunctions, which are sensitive to the electronic coupling between graphene and the topological surface state. The coupling between the pz orbitals of graphene and the p orbitals of the surface states on the Bi2Se3 bottom surface can be enhanced by applying a perpendicular negative magnetic field, resulting in a giant negative magnetoresistance at the Dirac point up to about -91%. An obvious resistance dip in the transfer curve at the Dirac point is also observed in the hybrid devices, which is consistent with theoretical predictions of the distorted Dirac bands with nontrivial spin textures inherited from the Bi2Se3 surface states.

Spin textures of topological surface states at side surfaces ofBi2Se3from first principles

We investigate the spin and spin-orbital textures and electronic structures of topologically protected surface states at side surfaces of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ by using slab models within density-functional theory. This is motivated by recent experiments on nanowires, nanoribbons, and nanoplates of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ with side surfaces. In particular, two representative surfaces normal to the (111) surface, such as $(1\overline{1}0)$ and $(11\overline{2})$ surfaces, are examined, in the presence of time-reversal symmetry and inversion symmetry. The $(1\overline{1}0)$ surface lying in the mirror plane has twofold $({\mathrm{C}}_{2})$ rotational symmetry, whereas the $(11\overline{2})$ surface has only mirror symmetry. For the $(1\overline{1}0)$ surface, we find that a Dirac cone with strongly anisotropic Fermi velocity is formed at $\mathrm{\ensuremath{\Gamma}}$ with the Dirac point at the Fermi level, and that the spin texture reveals features of Rashba-type combined with Dresselhaus-type spin-orbit coupling. For the $(11\overline{2})$ surface, a Dirac cone is found at either $\mathrm{\ensuremath{\Gamma}}$ or the $Y$ point (along the mirror symmetry axis) below the Fermi level. In this case, the spin texture of the surface states strikingly differs from that of the (111) and $(1\overline{1}0)$ surfaces: (i) the in-plane spin polarization dominantly aligned perpendicular to the [111] direction or the mirror symmetry axis, (ii) the Dresselhaus-type spin texture, and (iii) significant out-of-plane spin polarization away from the mirror symmetry axis. Our findings distinctively differ from the previous works based on the effective bulk model Hamiltonian. Our calculated spin and spin-orbital textures and band structures can be observed by spin-resolved angle-resolved photoemission spectroscopy.

Surface-State Spin Textures and Mirror Chern Numbers in Topological Kondo Insulators.

The recent discovery of topological Kondo insulators has triggered renewed interest in the well-known Kondo insulator samarium hexaboride, which is hypothesized to belong to this family. In this Letter, we study the spin texture of the topologically protected surface states in such a topological Kondo insulator. In particular, we derive close relationships between (i) the form of the hybridization matrix at certain high-symmetry points, (ii) the mirror Chern numbers of the system, and (iii) the observable spin texture of the topological surface states. In this way, a robust classification of topological Kondo insulators and their surface-state spin texture is achieved. We underpin our findings with numerical calculations of several simplified and realistic models for systems like samarium hexaboride.

Three-dimensional topological insulators in the supercubane-like lattice: Phase diagram, surface state and spin texture

We study a tight-binding model of the supercubane-like lattice with spin–orbit coupling. By evaluating the Z2 topological indices, we find that the supercubane-like lattice can support strong topological insulator and the phase diagrams of the lattice with different filling fractions are present. Strong topological insulators with Z2 invariants (1;000) and (1;111) can be realized for 1/8 filling fraction and semimetals can be obtained for 1/8, quarter and half filling fractions. We analyze and discuss the characteristics of these topological insulators and their surface states. Spin textures of surface states are also evaluated for 11¯1 slab geometry.

Controlling the Spin Texture of Topological Insulators by Rational Design of Organic Molecules.

We present a rational design approach to customize the spin texture of surface states of a topological insulator. This approach relies on the extreme multifunctionality of organic molecules that are used to functionalize the surface of the prototypical topological insulator (TI) Bi2Se3. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin texture of Bi2Se3. The theoretical predictions are then verified in angular-resolved photoemission experiments. We show that, by tuning the strength of molecule-TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created. These tailored interface properties-passivation, spin-texture tuning, and creation of hybrid interface states-lay a solid foundation for interface-assisted molecular spintronics in spin-textured materials.

Helical spin texture of surface states in topological superconductors

Surface states of topological noncentrosymmetric superconductors (NCSs) exhibit intricate helical spin textures, i.e., the spin orientation of the surface quasiparticles is coupled to their momentum. Using quasiclassical theory, we study the spin polarization of the surface states as a function of the spin–orbit interaction and superconducting pairing symmetry. We focus on two- and three-dimensional fully gapped and nodal NCSs. For the case of nodal systems, we show that the spin polarization of the topological flat bands is controlled by the spin polarization of the bulk normal states at the bounding gap nodes. We demonstrate that the zero-bias conductance in a magnetic tunnel junction can be used as an experimental test of the surface-state spin polarization.

Surface states of topological insulators

We develop an effective bulk model with a topological boundary condition to study the surface states of topological insulators. We find that the Dirac point energy, the band curvature and the spin texture of surface states are crystal face-dependent. For a given face on a sphere, the Dirac point energy is determined by the bulk physics that breaks p-h symmetry in the surface normal direction and is tunable by surface potentials that preserve T symmetry. Constant energy contours near the Dirac point are ellipses with spin textures that are helical on the S/N pole, collapsed to one dimension on any side face, and tilted out-of-plane otherwise. Our findings identify a route to engineering the Dirac point physics on the surfaces of real materials.

Design of strain-engineered quantum tunneling devices for topological surface states

Strain-dependent charge and spin transport on a topological insulator (TI) surface are investigated by combining first-principles calculations with quantum tunneling theory. It is shown that the Dirac point of helical surface states can be significantly shifted by applying compressive uniaxial strain. As an example of strain engineering applications based on this effect, a strain-induced quantum tunneling nanostructure is designed, where the tunneling conductance and the spin texture of surface states can be sensitively modulated by strain. Our work suggests that various local strain patterns can be integrated to manipulate surface states in all-TI-based spintronic nanodevices.