Robustness of topological surface states against strong disorder observed inBi2Te3nanotubes

Abstract

Three dimensional topological insulators are characterized by Dirac-like conducting surface states, the existence of which has been confirmed in relatively clean metallic samples by angle-resolved photoemission spectroscopy, as well as by anomalous Aharonov-Bohm oscillations in the magneto-resistance of nanoribbons. However, a fundamental aspect of these surface states, namely their robustness to time-reversal invariant disorder, has remained relatively untested. In this work, we have synthesized thin nanotubes of Bi 2 Te 3 with extremely insulating bulk at low temperatures due to disorder. Nonetheless, the magneto-resistance exhibits quantum oscillations as a function of the magnetic field along the axis of the nanotubes, with a period determined by the cross-sectional area of the outer surface. Detailed numerical simulations based on a recursive Green function method support that the resistance oscillations are arising from the topological surface states which have substantially longer localization length than that of other non-topological states. This observation demonstrates coherent transport at the surface even for highly disordered samples, thus providing a direct confirmation of the inherently topological character of surface states. The result also demonstrates a viable route for revealing the properties of topological states by suppressing the bulk conduction using disorder.3

Evidence of robust 2D transport and Efros-Shklovskii variable range hopping in disordered topological insulator (Bi2Se3) nanowires

“…(9) comes from the bulk channel. 29 A closer inspection of Figure 4(a) suggests that the experimental conductance deviates from the VRH fit at ~ 42K (shown in the blue shaded region). The deviation from VRH transport mechanism and the sharp decrease in conductance can be attributed to the high EEI in TIs at low temperatures, which is discussed after the hopping mechanism.…”

“…Previous works have shown similar kind of TSS contribution close to one conductance quantum. 29,59 Also, a comparison between the total conductance and bulk conductance is plotted in inset of Figure 4(b), which clearly reflects that the bulk contribution is very low compared to the total conductance, which is evident from the low value Figure 4(d) indicates that the contribution from bulk conductance is very large and almost equal to the total conductance. Thus, the TSS contribution is very less than one conductance quantum and is shown by the fluctuating values of ~ 0.3 2 ℎ ⁄ in inset of Figure 4(d).…”

“…Previously, Ref. [29] stated that due to strong disorder in the system bulk conductance is suppressed and TSS contribution is enhanced significantly for TI nanotubes with very less cross-sectional area. A similar kind of phenomenon is observed in our case for smaller width nanowire W3.…”

“…23,[26][27][28] In a recent study, it was demonstrated that introducing strong disorder in nanoscale TI samples might suppress the bulk conduction, while TSS remains immune to disorder. 29 Another report observed the change in band structure and TSS relocation to lower quintuple layers in presence of impurity and adsorbate addition. 20,21 The Universal conductance fluctuation (UCF) in weakly disordered TI specimens has also been a subject of intense research in recent years and different studies were performed to show the UCF arising from SS, the role of underlying symmetries (time reversal symmetry and spin rotational symmetry) in characterizing the UCF in 2D TIs, etc.…”

Observation of quantum oscillations in FIB fabricated nanowires of topological insulator (Bi2Se3)

“…Note that most of such studies have been performed on very high crystalline quality nanostructures of TIs and synthesized using methods like chemical vapour deposition (CVD), electrochemical deposition, vapour-liquid-solid (VLS), etc where robustness of TSS is less accessible assuming no deformation or doping in the material. Recently a study was carried out on Bi 2 Te 3 nanotubes grown by a solution phase method where robustness of TSS against strong disorder was first time demonstrated [11]. Here we use FIB fabricated nanowires of Bi 2 Se 3 where doping of Ga ions, deformation and cracks due to ion milling process and impurities introduced during electrode deposition by gas injection system could test the robust nature of surface states in TI materials and shows FIB fabrication technique as an efficient method for fabricating TI nanostructures.…”