There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.

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The valley degree of freedom of electrons is attracting growing interest as a carrier of information in various materials, including graphene, diamond and monolayer transition-metal dichalcogenides. The monolayer transition-metal dichalcogenides are semiconducting and are unique due to the coupling between the spin and valley degrees of freedom originating from the relativistic spin-orbit interaction. Here, we report the direct observation of valley-dependent out-of-plane spin polarization in an archetypal transition-metal dichalcogenide--MoS2--using spin- and angle-resolved photoemission spectroscopy. The result is in fair agreement with a first-principles theoretical prediction. This was made possible by choosing a 3R polytype crystal, which has a non-centrosymmetric structure, rather than the conventional centrosymmetric 2H form. We also confirm robust valley polarization in the 3R form by means of circularly polarized photoluminescence spectroscopy. Non-centrosymmetric transition-metal dichalcogenide crystals may provide a firm basis for the development of magnetic and electric manipulation of spin/valley degrees of freedom.

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A hexagonal deformation of the Fermi surface of Bi 2 Se 3 has been for the first time observed by angleresolved photoemission spectroscopy. This is in contrast to the general belief that Bi 2 Se 3 possesses an ideal Dirac cone. The hexagonal shape is found to disappear near the Dirac node, which would protect the surface state electrons from backscattering. It is also demonstrated that the Fermi energy of naturally electron-doped Bi 2 Se 3 can be tuned by 1% Mg doping in order to realize the quantum topological transport. DOI: 10.1103/PhysRevLett.105.076802 PACS numbers: 73.20.Àr, 79.60.Ài After the theoretical prediction and experimental realization of two-dimensional topological insulators in the HgTe=CdTe quantum well [1-4], a spectroscopic discovery of a three-dimensional topological insulator by probing the odd number of massless Dirac cones has generated a great interest in this new state of quantum matter [5][6][7][8][9]. Unlike the conventional Dirac fermions as found in graphene, this novel electronic state possesses helical spin textures protected by time-reversal symmetry, which could realize the quantum spin transport without heat dissipation. This new state of matter has been predicted to exist in a number of materials, for example, in Bi 1Àx Sb x , Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 [10]. Among them, stoichiometric Bi 2 Se 3 is theoretically predicted to be a 3D topological insulator with a single Dirac cone within a substantial bulk energy gap (0.3 eV), which makes it the most suitable candidate for the high-temperature spintronics application [10]. However, in the actual situation, the bulk conduction band is energetically lowered and crosses the Fermi energy through natural electron doping from vacancies or antisite defects, which allows bulk electron conduction. In order to avoid the bulk electron conduction and realize the quantum spin Hall phase, the Fermi energy must be tuned by additional doping [11,12].In ideal topological insulators with perfect linear dispersion, the surface state electrons should be protected from backscattering by nonmagnetic impurities between timereversal partners with opposite momenta because of their opposite spin configurations. However, recent scanning tunneling microscopy experiments for the Bi 2 Te 3 surface show a clear quasiparticle interference pattern as a result of backscattering nearby the step edge or at the point defect on the surface [13,14]. Theoretically, it is pointed out that the hexagonal Fermi surface warping would also induce the quasiparticle interference pattern [15]. It is generally believed that, owing to a large band gap (0.35 eV), which exceeds the thermal excitation energy at room temperature, Bi 2 Se 3 features a nearly ideal Dirac cone, in contrast to Bi 2 Te 3 [16,17]. In the present Letter, we show by a precise angle-resolved photoemission spectroscopy (ARPES) measurement that the Fermi surface of naturally electrondoped Bi 2 Se 3 is hexagonally deformed, while the constant energy contour is circular-shaped near the Dirac point...