The three-dimensional topological insulator is a quantum state of matter characterized by an insulating bulk state and gapless Dirac cone surface states. Device applications of topological insulators require a highly insulating bulk and tunable Dirac carriers, which has so far been difficult to achieve. Here we demonstrate that Bi 2-x sb x Te 3-y se y is a system that simultaneously satisfies both of these requirements. For a series of compositions presenting bulk-insulating transport behaviour, angle-resolved photoemission spectroscopy reveals that the chemical potential is always located in the bulk band gap, whereas the Dirac cone dispersion changes systematically so that the Dirac point moves up in energy with increasing x, leading to a sign change of the Dirac carriers at x~0.9. such a tunable Dirac cone opens a promising pathway to the development of novel devices based on topological insulators.

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 have performed spin-and angle-resolved photoemission spectroscopy of Bi2Te3 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 Bi2Te3 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 TlBiSe2, 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 existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi2Te3.PACS numbers: 75.70.Tj The topological insulators (TIs) materialize a new state of quantum matter where an unusual gapless metallic state appears at the edge or the surface of a band insulator due to a topological principle. The surface state (SS) of three-dimensional TIs is characterized by a Diraccone dispersion which has been shown to have a helical spin structure where the spin vector points parallel to the surface and perpendicular to the momentum k, as shown in Fig. 1(a) [1,2]. Because of this helicity in the spin direction and the protection by the time-reversal symmetry, the Dirac fermions in the TIs are immune to the backward scattering [3,4] and are not very sensitive to nonmagnetic impurities or disorder. This peculiar situation provides a platform for novel topological phenomena such as the emergence of Majorana fermions in the proximity-induced superconducting state, and indeed many theoretical models or experimental results were developed or interpreted relying essentially on this simple helical spin structure [1,3,4]. However, it is unclear at the moment to what extent such a simple spin texture is adequate. In fact, recent theoretical studies predicted that when the Fermi surface (FS) of the surface Dirac state is hexagonally deformed, the spin structure starts to obtain a finite out-of-plane (OP) component [5][6][7][8]. It was also predicted [6,8] that such a component can be as large as the in-plane (IP) counterpart in Bi 2 Te 3 which shows the strongest hexagonal FS warping among known TIs [5,9]. The deviation from the simple helical spin texture would be a key to better understanding the peculiar SS of the TIs, whereas an experimental verification has not yet been successfully made.In this Letter, we demonstrate that such an OP spin component is indeed present in Bi 2 Te 3 , by determining for the first time the detailed IP and OP spin texture at various k locations of the Dirac-cone SS using the low-energy, spin-and angle-resolved photoemission spectroscopy (SR-ARPES) [10]. We also elucidated that the magnitude of the OP spin polarization is related to the strength of the hexagonal warping of the FS. We discuss the present result in relation to theoretical ...

Hydrogen peroxide (H 2 O 2 ) in the range of several tens to several hundreds of micromoles per liter is usually added to the process water in advanced oxidation processes (AOPs). In this study, a spectrophotometric method using copper(II) ion and 2,9-dimethyl-1,10-phenanthroline (DMP) for measuring H 2 O 2 concentration was compared with other methods [i.e., spectrophotometric methods using titanium oxalate and N,N-diethyl-p-phenylenediamine (DPD) and a fluorometric method using p-hydroxyphenyl acetic acid (POHPAA)]. Particular attention was paid to sensitivities and effects of coexisting substances. The most sensitive method was the fluorometric method, followed in order by DPD, DMP, and the titanium oxalate colorimetric method; their detection limits in 1-cm cells were 0.16, 0.77, 0.80, and 29 µM, respectively. Therefore, the DMP method was found to be reasonably sensitive when applied to AOPs. Also, the DMP reagent is commercially available, and the absorbance of Cu(DMP) 2 + , a reaction product of the DMP method, was not affected by reaction time. In the DMP method, copper(II)-DMP complexes react with humic acid, and colored chemicals are produced. However, the slopes of the calibration curves of H 2 O 2 containing up to 10 mg of C L -1 from humic acid did not change significantly as compared to that in ultrapure water. The effect of chlorine on the DMP method was not observed up to at least 23 µM (0.8 mg of Cl L -1 ) of free chlorine, although the DPD and fluorometric methods are known to be interfered by chlorine. From this study, it was concluded that the DMP method is suitable to be used in AOPs.

In Japan, N-nitrosodimethylamine (NDMA) formation associated with ozonation at a relatively high concentration has been reported only at a small number of water treatment plants (WTPs) in the western part of Japan for which the source water is the Yodo River. In the present study, the formation of relatively high concentrations of NDMA was found upon ozonation of water samples taken from sewage treatment plants (STPs) located upstream of the water intake points of the WTPs in the Yodo River basin. NDMA concentrations before and after ozonation were 16-290 and 14-280 ng/L, respectively. At least some of the STPs investigated receive industrial effluents. At one STP in this area, an extremely high concentration of NDMA (10,000ng/L) was found in one influent water sample after ozonation. To identify potential NDMA precursors upon ozonation in the influent at this STP, the concentrated extracts of the influent were fractionated by high-performance liquid chromatography (HPLC). Ultraperformance liquid chromatography coupled with tandem mass spectrometry (UPLC/MS/MS) identified 4,4'-hexamethylenebis(1,1-dimethylsemicarbazide) (HDMS) and 1,1,1',1'-tetramethyl-4,4'-(methylene-di-p-phenylene)disemicarbazide (TMDS) as precursors of NDMA on ozonation of the influent. Both HDMS and TMDS are used as antiyellowing agents in polyurethane fibers and as light stabilizers in polyamide resins. Their contributions to NDMA production on ozonation of water samples at STPs were up to 17%. The remaining unidentified NDMA precursors may be hydrophilic compounds that were not trapped by the cartridges used for extraction of the water samples. HDMS and TMDS were frequently present in surface waters and STP effluents in the Yodo River basin and were also detected in surface waters from several other areas in Japan.