Helical Spin Texture Research Articles

Spin texture and magnetic anisotropy of Co impurities in Bi2Se3topological insulators

Based upon first-principles methods, we investigate the magnetic anisotropy and spin texture of Co impurities embedded at the interlayer van der Waals (vdW) spacings and onto the topmost Se network of the topological insulator Bi${}_{2}$Se${}_{3}$. The interaction of the magnetic impurity with the surface spin texture breaks time-reversal symmetry, opening up a surface band gap. For a Co atom adsorbed onto the surface, the net magnetic moment is aligned perpendicular to the surface plane, with anisotropy energy of 6 meV. On the other hand, for the Co impurity at the vdW interlayers, the net magnetic moment is aligned in plane. While pristine Bi${}_{2}$Se${}_{3}$ presents a helical spin texture in the massless surface Dirac cone and states resonant within the valence band, the presence of the Co impurity reduces the planar spin helicity of now massive Dirac fermions.

Spatial fluctuations of helical Dirac fermions on the surface of topological insulators

Surfaces of topological insulators host a new class of states with Dirac dispersion and helical spin texture. Potential quantum computing and spintronic applications using these states require manipulation of their electronic properties at the Dirac energy of their band structure by inducing magnetism or superconductivity through doping and proximity effect. Yet, the response of these states near the Dirac energy in their band structure to various perturbations has remained unexplored. Here we use spectroscopic mapping with the scanning tunneling microscope to study their response to magnetic and non-magnetic bulk dopants in Bi2Te3 and Bi2Se3. Far from the Dirac energy helicity provides remarkable resilience to backscattering even in the presence of ferromagnetism. However, approaching the Dirac point, where the surface states' wavelength diverges bulk doping results in pronounced nanoscale spatial fluctuations of energy, momentum, and helicity. Our results and their connection with similar studies of Dirac electrons in graphene demonstrate that while backscattering and localization are absent for Dirac topological surface states, reducing charge defects is required for both tuning the chemical potential to Dirac energy and achieving high electrical mobility for these novel states.

Half-metallic surface states and topological superconductivity in NaCoO2from first principles

Based on first-principles calculations, we predict a half-metallic surface state in layered bulk insulator NaCoO${}_{2}$, with tunable surface hole concentration. The half-metallic surface has a single Fermi surface with a helical spin texture, similar to the surface state of topological insulators, but with the key difference of time-reversal symmetry breaking in the present case. We propose the realization of topological superconductivity and Majorana fermions when the half-metallic surface states are in proximity contact with a conventional superconductor.

Versatile helimagnetic phases under magnetic fields in cubic perovskiteSrFeO3

A helical spin texture is of great current interest for a host of novel spin-dependent transport phenomena. We report a rich variety of nontrivial, helimagnetic phases in the simple cubic perovskite SrFeO3 under magnetic fields up to 42 T. Magnetic and resistivity measurements revealed that the proper-screw spin phase proposed for SrFeO3 can be subdivided into at least five kinds of ordered phases. Near the multicritical point, an unconventional anomalous Hall effect was found to show up and was interpreted as due to a possible long-period noncoplanar spin texture with scalar spin chirality.

Direct Measurement of the Out-of-Plane Spin Texture in the Dirac-Cone Surface State of a Topological Insulator

We have performed spin- and angle-resolved photoemission spectroscopy of Bi(2)Te(3) and present the first direct evidence for the existence of the out-of-plane spin component on the surface state of a topological insulator. We found that the magnitude of the out-of-plane spin polarization on a hexagonally deformed Fermi surface of Bi(2)Te(3) reaches maximally 25% of the in-plane counterpart, while such a sizable out-of-plane spin component does not exist in the more circular Fermi surface of TlBiSe(2), indicating that the hexagonal deformation of the Fermi surface is responsible for the deviation from the ideal helical spin texture. The observed out-of-plane polarization is much smaller than that expected from the existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi(2)Te(3).

Helical spin textures in dipolar Bose-Einstein condensates

We numerically study elongated helical spin textures in ferromagnetic spin-1 Bose-Einstein condensates subject to dipolar interparticle forces. Stationary states of the Gross-Pitaevskii equation are solved and analyzed for various values of the helical wave vector and dipolar coupling strength. We find two helical spin textures which differ by the nature of their topological defects. The spin structure hosting a pair of Mermin-Ho vortices with opposite mass flows and aligned spin currents is stabilized for a nonzero value of the helical wave vector.

Spin textures in condensates with large dipole moments

We have solved numerically the ground states of a Bose-Einstein condensate in the presence of dipolar interparticle forces using a semiclassical approach. Our motivation is to model, in particular, the spontaneous spin textures emerging in quantum gases with large dipole moments, such as $^{52}\mathrm{Cr}$ or $\mathrm{Dy}$ condensates, or ultracold gases consisting of polar molecules. For a pancake-shaped harmonic (optical) potential, we present the ground-state phase diagram spanned by the strength of the nonlinear coupling and dipolar interactions. In an elongated harmonic potential, we observe a helical spin texture. The textures calculated according to the semiclassical model in the absence of external polarizing fields are predominantly analogous to previously reported results for a ferromagnetic $F=1$ spinor Bose-Einstein condensate, suggesting that the spin textures arising from the dipolar forces are largely independent of the value of the quantum number $F$ or the origin of the dipolar interactions.

Spontaneously Modulated Spin Textures in a Dipolar Spinor Bose-Einstein Condensate

Helical spin textures in a 87Rb F=1 spinor Bose-Einstein condensate are found to decay spontaneously toward a spatially modulated structure of spin domains. The formation of this modulated phase is ascribed to magnetic dipolar interactions that energetically favor the short-wavelength domains over the long-wavelength spin helix. The reduction of dipolar interactions by a sequence of rf pulses results in a suppression of the modulated phase, thereby confirming the role of dipolar interactions in this process. This study demonstrates the significance of magnetic dipole interactions in degenerate 87Rb F=1 spinor gases.