We report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) and Sn-doped Bi(2)Te(2)Se. In this process, the injected spins are converted into a charge current along the Hall direction due to the spin-momentum locking on the surface state.

The three-dimensional (3D) topological insulator is a novel quantum state of matter where an insulating bulk hosts a linearly dispersing surface state, which can be viewed as a sea of massless Dirac fermions protected by the time-reversal symmetry (TRS). Breaking the TRS by a magnetic order leads to the opening of a gap in the surface state 1 , and consequently the Dirac fermions become massive. It has been proposed theoretically that such a mass acquisition is necessary to realize novel topological phenomena 2,3 , but achieving a sufficiently large mass is an experimental challenge. Here we report an unexpected discovery that the surface Dirac fermions in a solid-solution system TlBi(S 1−x Se x ) 2 acquire a mass without explicitly breaking the TRS. We found that this system goes through a quantum phase transition from the topological to the non-topological phase, and, by tracing the evolution of the electronic states using the angle-resolved photoemission, we observed that the massless Dirac state in TlBiSe 2 switches to a massive state before it disappears in the non-topological phase. This result suggests the existence of a condensedmatter version of the 'Higgs mechanism' where particles acquire a mass through spontaneous symmetry breaking.Whether a band insulator is topological or not is determined by the parity of the valence-band wave function, which is described by the Z 2 topological invariant. Strong spin-orbit coupling can lead to an inversion of the character of valence-and conduction-band wave functions, resulting in an odd Z 2 invariant that characterizes the topological insulator 4,5 . All known topological insulators 6-14 are based on this band-inversion mechanism 4,5,[15][16][17][18] , but the successive evolution of the electronic state across the quantum phase transition (QPT) from trivial to topological has not been well studied in 3D topological insulators owing to the lack of suitable materials. TlBi(S 1−x Se x ) 2 is therefore the first system where one can investigate the 3D topological QPT (ref. 19). The advantage of this system is that it always maintains the same crystal structure (Fig. 1a), irrespective of the S/Se ratio. Low-energy, ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES), which has recently become available, is particularly suited to trace such a QPT in great detail.The bulk band structures of the two end members, TlBiSe 2 and TlBiS 2 , are shown in Fig. 1b, where one can see several common features, such as the prominent hole-like band at the binding energy E B of 0.5-1 eV and a weaker intensity at the Fermi level (E F ), both being centred at the point (Brillouin-zone centre). These features

We observed pronounced angular-dependent magnetoresistance (MR) oscillations in a high-quality Bi2Se3 single crystal with the carrier density of 5×10 18 cm −3 , which is a topological insulator with residual bulk carriers. We show that the observed angular-dependent oscillations can be well simulated by using the parameters obtained from the Shubnikov-de Haas oscillations, which clarifies that the oscillations are solely due to the bulk Fermi surface. By completely elucidating the bulk oscillations, this result paves the way for distinguishing the two-dimensional surface state in angulardependent MR studies in Bi2Se3 with much lower carrier density. Besides, the present result provides a compelling demonstration of how the Landau quantization of an anisotropic three-dimensional Fermi surface can give rise to pronounced angular-dependent MR oscillations.

We have performed angle-resolved photoemission spectroscopy on (PbSe)5(Bi2Se3)3m, which forms a natural multilayer heterostructure consisting of a topological insulator (TI) and an ordinary insulator. For m = 2, we observed a gapped Dirac-cone state within the bulk-band gap, suggesting that the topological interface states are effectively encapsulated by block layers; furthermore, it was found that the quantum confinement effect of the band dispersions of Bi2Se3 layers enhances the effective bulk-band gap to 0.5 eV, the largest ever observed in TIs. In addition, we found that the system is no longer in the topological phase at m = 1, pointing to a topological phase transition between m = 1 and 2. These results demonstrate that utilization of naturally-occurring heterostructures is a new promising strategy for realizing exotic quantum phenomena and device applications of TIs.PACS numbers: 75.70.Tj, Three-dimensional (3D) topological insulators (TIs) realize a topological quantum state associated with unusual metallic surface states which appear within the bulk-band gap [1,2]. The topological surface states are characterized by a Dirac-cone energy dispersion with a helical spin texture. Owing to the peculiar spin texture, the Dirac fermions in the TIs are immune to backward scattering by nonmagnetic impurities or disorder [3,4] and carry dissipationless spin current [5], holding promise for exploring fundamental physics, spintronics, and quantum computing [1,2]. However, there are a number of challenges that need to be overcome before TIs meet those promises. For example, while experimental realizations of novel topological phenomena depend crucially on the inherent robustness of the topological surface states against perturbations, it turned out to be difficult to maintain stable surface properties under ambient atmosphere [6,7]. Also, potential applications of TIs for a wide range of devices working at room temperature require a large bulk-band gap, but the gap value reported to date is ∼0.35 eV at most [1,8]. Such a situation has been a hindrance for realizing novel topological phenomena and device applications of TIs, calling for a conceptually new approach to the manipulation of materials properties of TIs.A commonly used strategy for such a manipulation is the chemical substitution of constituent elements, as has been widely tried in systems based on Bi 2 Se 3 and Bi 2 Te 3 [6, 9-14]. Another, potentially more effective approach is the heterostructure engineering where one can alter the stacking sequence of layers or insert different building blocks into the crystal, which may trigger gigantic quantum effects and/or new physical phenomena. However, this method has not been seriously explored in 3D TIs owing to a limited number of TI materials discovered to date.In this Letter, we demonstrate that utilization of naturally-occurring heterostructures in bulk crystals containing TI units is a promising pathway to overcome aforementioned problems. Specifically, we show highresolution angle-resolved photoemission...

We have performed spin-and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)2Te4 (Pb124) and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators (Pb124, Bi2Te3, Bi2Se3, and TlBiSe2) and elucidated that the out-of-plane spin polarization Pz is systematically correlated with the warping strength. However, the magnitude of Pz is found to be only half of that expected from the k·p theory when the warping is strong, which points to the possible role of many-body effects. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.PACS numbers: 75.70.Tj, Three-dimensional topological insulators (TIs) realize a novel quantum state of matter where a distinct topology of bulk wave functions produces a gapless surface state (SS) which disperses across the bulk insulating gap [1][2][3]. The topological SS is characterized by a Diraccone band dispersion with an in-plane spin helical texture arising from the strong spin-orbit coupling (SOC). This peculiar spin texture plays an essential role in realizing various quantum phenomena associated with non-trivial topology [4][5][6], and also in manipulating spin polarization and spin current in TIs [7].It is known that the spin texture of the Dirac-cone SS is closely related to the shape of the Fermi surface (FS) [8,9], as proposed by the k·p perturbation theory for TIs with C 3v symmetry [8]. Theoretically, the k cubic term in the surface SOC causes a hexagonal warping of the FS as well as a finite out-of-plane spin polarization (P z ) with C 3 symmetry. Such a deviation from an ideal isotropic Dirac cone has been thought to cause a variety of intriguing physical properties. For instance, the FS warping is responsible for the strong quasiparticle interference as seen by scanning tunneling microscopy [10,11]. Also, the warping could trigger the spin density wave and create an energy gap at the Dirac point under transverse magnetic field [8].The out-of-plane spin polarization has been observed by spin-and angle-resolved photoemission spectroscopy (ARPES) for Bi 2 Te 3 [12,13] in which the pronounced FS warping leads to the finite P z component along thē Γ-K direction in the surface Brillouin zone (BZ). While previous spin-resolved ARPES experiments suggested a correspondence between the FS warping and the out-ofplane spin polarization, no systematic investigation on the relationship has been performed. This is largely due to the small magnitude of the P z value compared to the in-plane counterpart and the limitation of materials suitable for observing the out-of-plane spin polarization. In addition, the spin-polarization values reported from spinresolved ARPES have not been very consistent between experiments even for the same ...

Electron scattering in the topological surface state (TSS) of the topological insulator Bi1.5Sb0.5Te1.7Se1.3 was studied using quasiparticle interference observed by scanning tunneling microscopy. It was found that not only the 180° backscattering but also a wide range of backscattering angles of 100°-180° are effectively prohibited in the TSS. This conclusion was obtained by comparing the observed scattering vectors with the diameters of the constant-energy contours of the TSS, which were measured for both occupied and unoccupied states using time- and angle-resolved photoemission spectroscopy. The robust protection from backscattering in the TSS is good news for applications, but it poses a challenge to the theoretical understanding of the transport in the TSS.

We have performed angle-resolved photoemission spectroscopy on Pb(Bi(1-x)Sb(x))2Te4, which is a member of lead-based ternary tellurides and has been theoretically proposed as a candidate for a new class of three-dimensional topological insulators. In PbBi2Te4, we found a topological surface state with a hexagonally deformed Dirac-cone band dispersion, indicating that this material is a strong topological insulator with a single topological surface state at the Brillouin-zone center. Partial replacement of Bi with Sb causes a marked change in the Dirac carrier concentration, leading to the sign change of Dirac carriers from n type to p type. The Pb(Bi(1-x)Sb(x))2Te4 system with tunable Dirac carriers thus provides a new platform for investigating exotic topological phenomena.

We performed systematic spin-and angle-resolved photoemission spectroscopy of TlBi(S1−xSex)2 which undergoes a topological phase transition at x ∼ 0.5. In TlBiSe2 (x = 1.0), we revealed a helical spin texture of Dirac-cone surface states with an intrinsic in-plane spin polarization of ∼ 0.8. The spin polarization still survives in the gapped surface states at x > 0.5, although it gradually weakens upon approaching x = 0.5 and vanishes in the non-topological phase. No evidence for the out-of-plane spin polarization was found irrespective of x and momentum. The present results unambiguously indicate the topological origin of the gapped Dirac surface states, and also impose a constraint on models to explain the origin of mass acquisition of Dirac fermions.PACS numbers: 75.70.Tj, Three-dimensional topological insulators (TIs) exhibit a novel quantum state with metallic topological surface state (SS) which disperses across the bulk band gap generated by a strong spin-orbit coupling. Specifically, the topological SS is characterized by a linearly dispersing Dirac-cone energy band with the helical spin texture [1-4], which hosts massless Dirac fermions protected by time-reveal symmetry (TRS). This peculiar SS of TIs provides a platform for fascinating quantum phenomena such as the robustness against nonmagnetic impurities / disorder [5,6] and the emergence of Majorana fermions [7]. Breaking the TRS by a magnetic field or a magnetic order lifts the band degeneracy at the Dirac point and causes an energy gap called a Dirac gap in the topological SS, turning the massless Dirac fermions into a massive state. Theoretically, realization of the massive state is a prerequisite to novel topological phenomena such as the topological magnetoelectric effect and halfinteger quantum Hall effect [8,9], but experimentally, the role of TRS-breaking magnetic impurities in triggering the Dirac-gap opening in the topological SS is under intense debate [10][11][12][13].