Strong surface scattering in ultrahigh-mobilityBi2Se3topological insulator crystals

Abstract

While evidence of a topologically nontrivial surface state has been identified in surface-sensitive measurements of Bi 2 Se 3 , a significant experimental concern is that no signatures have been observed in bulk transport. In a search for such states, nominally undoped single crystals of Bi 2 Se 3 with carrier densities approaching 10 16 cm −3 and very high mobilities exceeding 2 m 2 V −1 s −1 have been studied. A comprehensive analysis of Shubnikov-de Haas oscillations, Hall effect, and optical reflectivity indicates that the measured electrical transport can be attributed solely to bulk states, even at 50 mK at low Landau-level filling factor, and in the quantum limit. The absence of a significant surface contribution to bulk conduction demonstrates that even in very clean samples, the surface mobility is lower than that of the bulk, despite its topological protection.

Ambipolar Surface State Thermoelectric Power of Topological Insulator Bi2Se3

“…Further tuning of carrier density is accomplished using a Si back gate and 300 nm SiO 2 gate dielectric. We showed previously [22] that in such thin Bi 2 Se 3 slabs in the TI regime that the top and bottom surfaces are strongly capacitively coupled through the high-dielectric-constant bulk (ε ~100 [20]) and couple nearly equally to the back gate. to negative voltage, reflected also in a shift to negative voltage of the zeroes in S. This thermal activation effect was explained by us in detail in a previous work [26].…”

“…Further tuning of carrier density is accomplished using a Si back gate and 300 nm SiO 2 gate dielectric. We showed previously [22] that in such thin Bi 2 Se 3 slabs in the TI regime that the top and bottom surfaces are strongly capacitively coupled through the high-dielectric-constant bulk (ε ~100 [20]) and couple nearly equally to the back gate.…”

“…The 3D TIs have an insulating bulk but metallic surface states with spin-momentum helicity which cannot be localized by nonmagnetic disorder 4,5 . Recent theoretical calculations 6,7 show that significant enhancement of ZT is expected in 3D TIs particularly in thin film geometry where top and bottom surfaces can hybridize and form a surface gap 8 .Thermoelectric power is also of great interest in understanding the electronic transport properties of Dirac electronic systems 9 , thus measurement of the thermoelectric properties of TI surface states can elucidate details of the electronic structure of the ambipolar nature of 3 topological metallic surface states that cannot be probed by conductance measurements alone 10 .Although there have been theoretical investigations of thermal and thermoelectric properties of Dirac materials including graphene and TIs 6,7,[11][12][13][14][15] and thermoelectric transport in carbon nanotubes 16 and graphene 9,17-19 have been studied experimentally, it has been difficult to experimentally observe clear surface thermoelectric transport in TI materials due to the high level of bulk doping present in as-grown TI crystals 20,21 .Here we experimentally demonstrate the first measurement of thermoelectric power of topological insulator Bi 2 Se 3 dominated by metallic surface states. We show a clear ambipolar electric field effect indicated by the sign change of the thermoelectric power across the charge neutrality point where the charge transport exhibits minimum conductivity.…”

Transport properties of topological insulators: Band bending, bulk metal-to-insulator transition, and weak anti-localization

“…To get a feeling for how the bulk E F changes as a function of bulk charge defect density, N BD , we have plotted E F = ℏ 2 /(2m*)(3π 2 N BD ) 2/3 in Fig. 1, where we assumed an isotropic Fermi surface with the effective mass given in terms of the electron mass to be m* ≈ 0.15m e , a typical value for Bi 2 Se 3 and Bi 2 Te 3 [14]. Here and for all the following discussions, unless explicitly specified, the temperature is assumed to be absolute zero, which is a valid assumption for the commonly measured transport properties of TIs.…”

Catalyst-Free Growth of Millimeter-Long Topological Insulator Bi2Se3 Nanoribbons and the Observation of the π-Berry Phase

“…6 Angle-resolved photoemission spectroscopy (ARPES) measurements on single crystals have elucidated its two-dimensional (2D) quantum physics of the surface states. [7][8][9] Since bulk transport can mask surface conductance even in samples with low bulk carrier densities, 8,10,11 electrical transport experiments to delineate the surface and bulk contributions to the Fermi surface are often challenged and remain controversial. One approach to reveal the surface effect in such topological insulators is to reduce the sample size to the nanoscale, thereby significantly enhancing the surface-to-volume ratio.…”