The electronic structure of ultrathin Bi͑001͒ films on Si͑111͒-7 ϫ 7 was studied by spin and angle-resolved photoemission spectroscopy. We directly observed a clear momentum-dependent spin splitting and polarization of the surface-state bands. The spin structure was antisymmetric with respect to the ⌫ point as predicted by theory, and the obtained in-plane spin polarization was as high as ±0.5. The qualitative features of the observed spin polarization are discussed in comparison with the spin-polarized band structure obtained by first-principles calculations. DOI: 10.1103/PhysRevB.76.153305 PACS number͑s͒: 79.60.Bm, 68.35.Ϫp, 73.20.Ϫr, 85.75.Ϫd Spintronics, which aims at the utilization of the spin degree of freedom, has attracted wide interest due to its potential in realizing new functionalities in electronic devices. 1Spin manipulation is the key factor in spintronics, and the conventional style was to develop novel ferromagnetic materials.2 Recently, on the other hand, it was found that spin-split two-dimensional electron gases can be formed in asymmetric quantum wells controlled by an electric field even for nonmagnetic materials.3 This is called the Rashba effect, which is a combined effect of the spin-orbit interaction and structural inversion asymmetry ͑SIA͒. 4At the crystal surface, the same effect occurs and spinorbit split band structures have been found for Au͑111͒ ͑Refs. 5 and 6͒ and W͑110͒-H ͑Ref. 7͒ surfaces. This splitting is caused by the spin-orbit coupling Hamiltonian, H soc = ͑ប /4m e 2 c 2 ͒ ជ · ٌ͑V ϫ p ជ͒, where ជ is the spin of electrons, V the one-electron potential, and p ជ the momentum. 8 Usually in the bulk, the time-reversal symmetry ͓E͑k ជ , ↑ ͒ = E͑−k ជ , ↓ ͔͒ and the space-inversion symmetry ͓E͑k ជ , ↑ ͒ = E͑−k ជ , ↑ ͔͒ lead to the Kramers degeneracy. However, at the crystal surface, due to the SIA in the surface-normal direction, the degeneracy will be lifted. The spin orientation of such states is perpendicular both to the momentum p ជ and to the electric field ٌV, meaning an in-plane spin polarization antisymmetric about k ជ = 0, as the electric field is perpendicular to the surface. 8Bismuth ͑Bi͒ is a very heavy element and its electronic structure is highly influenced by the spin-orbit interaction.9 It was recently shown from angle-resolved photoemission spectroscopy ͑ARPES͒ measurements that the surface states of Bi crystals are highly metallic in contrast to the semimetallic nature of bulk Bi ͑Fig. 1͒. [10][11][12][13] Additionally, it was suggested by ab initio calculations that they will show large Rashba splitting due to the significant spin-orbit coupling.12,13 Furthermore, in a recent theoretical study, 14 two-dimensional Bi bilayers were predicted to show the quantum spin Hall ͑QSH͒ effect, and it was said that these surface states may have some relations with the edge modes that characterize the QSH phase. The spin property of the highly metallic surface states of semimetallic Bi films also has importance in application to spintronics. However, although some insi...

Bi2Se3 is an important semiconductor thermoelectric material and a prototype topological insulator. Here we report observation of Shubnikov-de Hass oscillations accompanied by quantized Hall resistances (R(xy)) in highly doped n-type Bi2Se3 with bulk carrier concentrations of few 10(19) cm(-3). Measurements under tilted magnetic fields show that the magnetotransport is 2D-like, where only the c-axis component of the magnetic field controls the Landau level formation. The quantized step size in 1/R(xy) is found to scale with the sample thickness, and average ~e(2)/h per quintuple layer. We show that the observed magnetotransport features do not come from the sample surface, but arise from the bulk of the sample acting as many parallel 2D electron systems to give a multilayered quantum Hall effect. In addition to revealing a new electronic property of Bi2Se3, our finding also has important implications for electronic transport studies of topological insulator materials.

Through soft X-ray absorption spectroscopy, hard X-ray Raman scattering, and theoretical simulations, we provide the most in-depth and systematic study of the phase transformation and (de)lithiation effect on electronic structure in Li(x)FePO(4) nanoparticles and single crystals. Soft X-ray reveals directly the valence states of Fe 3d electrons in the vicinity of Fermi level, which is sensitive to the local lattice distortion, but more importantly offers detailed information on the evolution of electronic states at different electrochemical stages. The soft X-ray spectra of Li(x)FePO(4) nanoparticles evolve vividly with the (de)lithiation level. The spectra fingerprint the (de)lithiation process with rich information on Li distribution, valency, spin states, and crystal field. The high-resolution spectra reveal a subtle but critical deviation from two-phase transformation in our electrochemically prepared samples. In addition, we performed both first-principles calculations and multiplet simulations of the spectra and quantitatively determined the 3d valence states that are completely redistributed through (de)lithiation. This electronic reconfiguration was further verified by the polarization-dependent spectra collected on LiFePO(4) single crystals, especially along the lithium diffusion direction. The evolution of the 3d states is overall consistent with the local lattice distortion and provides a fundamental picture of the (de)lithiation effects on electronic structure in the Li(x)FePO(4) system.

Ultrafast time-resolved differential reflectivity of Bi 2 Se 3 crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes. The deduced relaxation timescale and the sign of the reflectivity change suggest that electron-phonon interactions and defect-induced charge trapping are the underlying mechanisms for the three processes. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy.

No abstract