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...

We report the first observation of a topological surface state on the (111) surface of the ternary chalcogenide TlBiSe 2 by angle-resolved photoemission spectroscopy. By tuning the synchrotron radiation energy we reveal that it features an almost ideal Dirac cone with the Dirac point well isolated from bulk continuum states. This suggests that TlBiSe 2 is a promising material for realizing quantum topological transport. DOI: 10.1103/PhysRevLett.105.146801 PACS numbers: 73.20.Àr, 79.60.Ài Three-dimensional topological insulators, which harbor massless helical Dirac fermions in a bulk energy gap [1][2][3][4], provide fertile ground to realize new phenomena in condensed matter physics, such as a magnetic monopole arising from the topological magnetoelectric effect and Majorana fermions hosted by hybrids with superconductors [5,6]. All of them can hardly be achieved with trivial 2D electron gas of the semiconductor heterostructures or graphene. The topological insulator phase has been predicted to exist in a number of materials, such as Bi 1Àx Sb x , Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 [4]. The experimental realization of the 1st and 2nd generation of the 3D topological insulators has opened a way for applications of the quantum matter [7][8][9][10][11][12].In particular, Bi 2 Se 3 has been regarded as the most promising candidate because of its single and less-warping Dirac cone than in Bi 2 Te 3 . However, recent magnetotransport measurements showed that the bulk conductance dominates even in low carrier samples [13][14][15], which raises the question of possible scattering channels responsible for the reduced surface mobility. Band-structure calculations [9,16] predict that the Dirac point (DP) of the surface state in Bi 2 Se 3 is close to the bulk valence band (BVB) maximum. As a consequence, the electron scattering channel from surface states to bulk continuum states opens, and the topological transport regime collapses. Thus, it is important to extend the search for 3D topological insulators with an ideal and isolated helical Dirac cone to a wider range of materials. Recent first principles studies suggested a variety of candidates with nontrivial electronic states ranging from oxide materials, in which the electron correlation plays a role in addition to the spin-orbit coupling [17], to Heusler-type alloys with an uniaxial strain [18,19].Recently, thallium-based ternary compounds have been proposed as a new family of 3D topological insulators [20][21][22]. In contrast to the layered binary chalcogenides, with their inert surface due to the weak bonding between the layers, in the ternary compounds the broken bonds may give rise to trivial surface states. The theoretical studies [21,22] have indeed revealed the presence of such surface states in addition to the topological ones. This calls for an angle-resolved photoemission spectroscopy (ARPES) experiment with broadly tunable photon energy, which allows us to unambiguously separate out two-dimensional electron states in this new class of ternary compound...