Thermoelectric transport in topological insulator Bi 2 Te 2 Se bulk crystals
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Thermoelectric transport in topological insulator Bi2Te2Se bulk crystals Yang Xu1,2, Helin Cao1,2, Ireneusz Miotkowski1, Yong P. Chen1,2 Department of Physics, Purdue University, West Lafayette, IN 47907 U.S.A 2 Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907 U.S.A 4 School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 U.S.A 1
ABSTRACT Bi2Te2Se (BTS221) bulk crystals were recently discovered as an intrinsic 3D topological insulator. We have synthesized this material, and studied the transport properties of BTS221 from the thermoelectrics perspective. Temperature (T) dependent resistivity measurement indicates surface dominant transports in our sample at low T. We also report Seebeck measurement between 50K to room T. INTRODUCTION There dimensional (3D) Topological insulator (TI), a new state of quantum matter, has attracted considerable attention mostly because of its unique topological protected surface states (TSS). On the surface of 3D TI materials, there exist non-trivial gapless surface states protected by time-reversal symmetry (TRS), whereas the bulk of TI resembles a normal band insulator. The TSS give rise to 2D helical Dirac fermions with linear energy momentum (E-k) dispersion. Due to the helical nature (resulting in spin-momentum locking) of the Dirac cone(s) on TSS, the topological protection reduces backscattering which requires spin flip (breaking the TRS), and is thus unlikely without magnetic impurities. Therefore, the TSS channels are largely immune to (nonmagnetic) structural imperfection. Because of the characteristic properties, TSS are promising for not only novel physics, but also nano-electronics[1],[2],[3],[4],[5]. Among other applications, TI materials are also expected to offer opportunities to enhance energy efficiency of thermoelectric (TE) devices[6][7]. We note that most of the current heavily studied TI materials, such as Bi2Se3 and Bi2Te3, are also widely-commercialized TE materials with high figure of merit (ZT~1, at ordinary temperatures, ~270 to 400 K). However, both Bi2Se3 and Bi2Te3 have substantial bulk conduction due to a significant amount of unintentional doping in the bulk. This has been one major challenge for the transport and device studies of TI, as a highly conducting bulk would “short-out” the TSS conduction and mask the transport features associated with TSS. Recently, a new TI material, Bi2Te2Se (BTS221) has been synthesized and demonstrated to have a dominant surface transport at low temperature (T) [8][9]. Here we report our preliminary thermal power (Seebeck coefficient) measurements on bulk BTS221 crystals. Since our BTS221 has been shown to be an intrinsic TI, with bulk insulating and surface conduction dominating at low T [10], studying the Seebeck coefficient of BTS221 at various temperatures (tuning from bulk dominated transport to surface dominated transport as T lowers) will likely provide much insights about thermoelectric transport of TSS and distinguish surface from bulk contributions.
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