Peculiar Linear Dispersive Bands Observed in Angle-Resolved Photoemission Spectra of Tl-Based Ternary Chalcogenide TlGaTe 2 Kojiro Mimura  , Takahiko Ishizu, Satoru Motonami, Kazuki Wakita 1 , Masashi Arita 2 , Sadig Hamidov 3 , Zakir Chahangirli 3 , Yukihiro Taguchi, Hirofumi Namatame 2 , Masaki Taniguchi 2;4 , Guseyn Orudzhev 3 , and Nazim Mamedov 3 Department of Mathematical Sciences, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan 1 Department of Electrical, Electronics and Computer Engineering, Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan 2 Hiroshima Synchrotron Radiation Center (HSRC), Hiroshima University, Higashihiroshima, Hiroshima 739-0046, Japan 3 Institute of Physics, Azerbaijan National Academy of Science, Baku AZ-1143, Azerbaijan 4 Department of Physical Sciences, Graduate School of Science, Hiroshima University, Higashihiroshima, Hiroshima 739-8526, Japan Received November 1, 2010; accepted December 24, 2010; published online May 20, 2011 Electronic energy bands of the Tl-based ternary chalcogenide TlGaTe 2 with a quasi one-dimensional crystalline structure have been studied by means of high resolution angle-resolved photoemission spectroscopy (ARPES) in order to check for dispersive structures similar to the Dirac cone observed in the surface bands of Bi-based binary chalcogenides. Two linear dispersive structures which are not reproduced in band calculations for bulk material have been observed in the energy band along the –N direction perpendicular to the chains. These dispersions form a cross-type structure that is centered at the  point and extends along the –H–T direction parallel to the chains, reflecting, in our opinion, one-dimensional features of surface morphology of TlGaTe 2 . The cross-type structure, the energy position of which linearly varies with excitation photon energy, is observed only for high-grade quality surfaces of TlGaTe 2 . It is therefore assumed that the observed peculiar dispersive structure is caused by the Dirac-type dispersion of high-lying surface conduction bands and that ARPES detects the joint density of states. # 2011 The Japan Society of Applied Physics 1. Introduction Topological insulators have been receiving a great deal of attention as new quantum phases of solids. Such insulating phases have already been observed in a number of com- pound materials. For example, binary Bi 1x Sb x , Bi 2 Se 3 , and Bi 2 Te 3 have been solidly verified to be three-dimensional (3D) topological insulators. 1–5) In these systems, spin–orbit coupling gives rise to a robust conducting edge-state, topologically isolated from an electrically insulating bulk state. Topological edge states form the odd number of non-degenerated Dirac cones protected by time-reversal symmetry from scattering by non- magnetic impurities, crystalline defects and distortion of the surface. Non-dissipative spin transport via the edge or surface states can be realized opening the way to spintronics and quantum-computing applications. 6) Very recently, a low-temperature rhombohedral ordered phase of Tl-based III–V–VI 2 ternary chalcogenides has been predicted to be topologically nontrivial and a terminated surface with a single Dirac cone has been identified through first-principles calculations. 7) In spite of their quasi-one-dimensional, chain crystal structure, Tl-based I–III–V 2 ternary chalcogenides with chemical formula TlMeX 2 (MeX = GaTe, InSe, and InTe) also show 3D character of the electronic band structure, 8) similar to Tl-based III–V–VI 2 materials. It is, therefore, reasonable to extend the search for 3D topological insulators to the former family of ternary thallium chalcogenides, which show negative differential resistance 9–11) and inter- esting thermoelectric properties. 12,13) The body-centered tetragonal structure of TlMeX 2 with D 18 4h space group is shown in Fig. 1. 14) The structure is built- up of the edge-sharing MeX 4 tetrahedrons along the crystal- lographic c-axis, and of the Tl-atoms positioned between the chains. Recently, we have already studied the electronic structures of TlInSe 2 15–17) and TlGaTe 2 17) by angle-resolved photoemission spectroscopy (ARPES) and hard X-ray photoemission spectroscopy, focusing on the influence of incommensurate phase transition on electronic energy bands of these materials. Here we extend the experimental approach to TlGaTe 2 beyond the limits of the previous ARPES studies and check for dispersive structures like a Dirac cone in the electronic spectrum of this material. The origin of the observed peculiar dispersive structures as well as their shape in the two-dimensional (2D) wave number space is discussed in terms of the available band structure calculations. 2. Experimental Procedure Single crystalline TlGaTe 2 was grown by a modified Bridgeman method, as reported before. 12) The ARPES experiments were carried out by using a hemispherical electron energy analyzer (VG-SCIENTA R4000) at the circular undulator beamline BL-9A of HiSOR, HSRC, Hiroshima University. 18,19) Clean (110) surfaces were obtained by in situ cleaving the sample under the pressure below 3  10 9 Pa. The overall energy resolution in the employed excitation photon energy (h) range from 8.9 to Tl Me = Ga, In X = Se, Te a a b b c Fig. 1. (Color online) Crystal structure of TlMeX 2 (Me = Ga, In; X = Se, Te). MeX 4 tetrahedron is given in yellow.  E-mail address: mimura@ms.osakafu-u.ac.jp Japanese Journal of Applied Physics 50 (2011) 05FC05 05FC05-1 # 2011 The Japan Society of Applied Physics REGULAR PAPER DOI: 10.1143/JJAP.50.05FC05