Thin Film Electronic Properties of Ternary Topological Insulator
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Thin Film Electronic Properties of Ternary Topological Insulator Jiwon Chang, Leonard F. Register, Sanjay K. Banerjee and Bhagawan Sahu Microelectronics Research Center, The University of Texas at Austin, Austin, TX 78758, U.S.A. ABSTRACT Using an ab initio density functional theory (DFT), we study thin film electronic properties of topological insulators (TIs) based on ternary compounds of Tl (thallium) and Bi (bismuth). We consider TlBiX2 (X=Se, Te) and Bi2X2Y (X, Y=Se, Te) compounds. Here we discuss the nature of surface states, their locations in the Brillouin Zone (BZ) and their interactions within the bulk region. Our calculations suggest a critical film thickness to maintain the Dirac cone which is smaller than that in binary Bi-based compounds. Atomic relaxations are found to affect the Dirac cone in some of these compounds. We discuss the penetration depth of surface states into the bulk region. INTRODUCTION Recently, two promising 3D TI materials, TlBiX2 (X = Se,Te) and Bi2X2Y (X,Y = Se,Te) have been predicted to have near perfect Dirac cones, in terms of less entanglement of bulk and surface states, and experiments have supported these predictions [1–3]. Moreover, Bi-based ternary compounds offer high-bulk resistivity, due to the structurally perfect nature of the crystals. In these promising advances, a comprehensive theoretical study of the thin film structures of these ternary 3D TIs is necessary to help better understand these materials. These materials can have intrinsic size limits to protect the metallic surface bands whose states can spread inside the bulk region, and atomic rearrangements can have profound effect on the Dirac cone. We address these issues here using DFT and compare our calculations with available experimental results as well as with the properties of binary Bi-based TIs. THEORY We used DFT calculations with projector-augmented wave basis [4] and a generalized gradient approximation to the exchange-correlation potential [5] for computing the thin film electronic properties of both compounds. Spin-orbit coupling (SOC) was invoked in the calculation as implemented in the numerical method [6]. We built the bulk hexagonal cell and the corresponding thin films of both the compounds by using experimental lattice parameters. For computing surface band structure of both Tl- and Bi-based ternary TIs thin films, we used kinetic energy cutoff of 400 eV and k-mesh size of 9 × 9 × 1 on the surface BZ. DISCUSSION Surface states in bismuth-based ternary compounds The building block of Bi-based ternary TIs is quintuple layer (QL), five atomic layers stacked along the crystallographic z direction in the order of Se(Te)-Bi-Te(Se)-Bi-Se(Te). Different film thicknesses corresponding to the different number of QLs were considered with a
maximum of 4QLs. The choice thickness is guided by the necessity to maintain the metallic nature of the surface bands. We first construct the thin film from experimental parameters [7] and then study the thin film properties without and with relaxing the atomic posit
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