Experimental Study On The Quantum Coherence Transport Of Topological Insulator Bi₂Te₂Se Nanoribbons

As an exotic quantum condensed matter, the topological insulator (TI) is a bulk-insulating material with a Dirac-type conducting surface state (SS). Such dissipationless transport of topological SSs (TSSs) is protected by the time-reversal symmetry, which leads to the potential applications in spintronics and quantum computations. In the period when this work began, a lots of topological materials were discoveried by angle resolved photoemission spectroscopy and scanning tunneling microscopy. However, the topological symplectic transport of the Dirac fermions remains limited by the following two issues. On one hand, the transport of residual bulk carriers is dominant over the conduction contributions from TSSs. On the other hand, the coupling between the two TSSs on the opposite side of sample surfaces results in the topologically trivial transport. In this work, the bulk suppressed Bi2Te2Se crystals have been grown. By using a mechanical exfoliation method and the standard lift-off process, the Bi2Te2Se nanoribbon field-effect transistors are preparaed. We have carefully measured the low-temperature magnetotransport of these devices to investigate the quantum coherent transport of TSSs. The main results are summarized in the following.(1) We have observed the two-dimensional (2D) universal conductance fluctuations (UCFs) phenomenon in TIs for the first time, and its topological nature is demonstrated based on the investigations of UCF by angle-varying, in-plane field tuning and scaling analysis. As an important manifestation of mesoscopic electronic interference, UCFs are rarely considered in TIs. Firstly, we show that the random conductance fluctuations (CFs) in the magnetoconductance (MC) curves are originated from the UCF by changing the field sweeping directions and temperatures. Then, by considering the field-tilting MC measurements, we demonstrate the 2D nature of UCFs based on the fact that the CF peaks are solely dependent to the normal component of field. Furthermore, we show that such 2D UCFs are not from the quasi-2D bulk transport, because the UCF amplitudes keep unchanged while an in-plane magnetic field is applied to suppress the coherence of bulk carriers. We further extract the intrinsic UCF amplitudes by considering the self-averaging effect. The obtained UCF amplitudes are close to the theoretically expected value of TSSs and far below the theoretical values of a trivial 2D electron gas (2DEG). This excludes the contribution of the topological trivial 2DEG, and finally pindowns the topological nature of UCFs in the Bi2Te2Se nanoribbons.(2) Experimental evidence and control of the intersurface coupling-decoupling transition between the two TSSs. Weak antilocalization (WAL) is an effective tool to investigate the interactions of the conduction channels in TIs. a, one of the theoretical parameters of TI’s WAL theory, is an indicator to reveal the changes of the conduction channels in TIs. The low-field MC has been analyzed by the WAL theory for 18 Bi2Te2Se nanoribbons of different thicknesses (H). In order to obtain the MC of SSs, the bulk contributions have been subtracted. The results of a as a function of H reveal that, a abruptly drops from ～0.5 to ～0.25 while H exceeds the bulk dephasing length. The abrupt thickness-dependent change in a is interpreted by the coupling-decoupling transition between the top and bottom surfaces. The observation of α≈0.25 in the decoupling situation reveals the existence of at least one topologically-trivial surface 2DEG, which exhibits a weak localization. The a independence on the back-gate-voltage demonstrates that the trivial 2DEG must be located on the top surface, and the bottom surface is dominated by a TSS. Therefore, the thickness-dependent change in a is due to the coupling-decoupling transition between the hybrid 2DEG+TSS state localized at the top surface and the TSS localized at the bottom surface. We propose a bulk-mediated intersurface coherent coupling. The coupling conditions are advised:(ⅰ) electrons from one surface tunnel to the bulk, diffuse toward the vicinity of the opposite surface, and tunnel onto it. The total time of this process must be shorter than the phase relaxation time of TSSs; (ⅱ) the diffusion time in which an electron diffuses across a film of thickness H must be shorter than the bulk phase relaxation time. Breaking either of the two conditions will break the intersurface coupling. This hypothesis is further confirmed by additional MC measurements conducted under an in-plane magnetic field, which alters the intersurface coupling in a controllable way.(3) Statistical symmetry of TSSs. The UCF amplitude is sensitive to the statistical symmetry of an electronic system. For a single TSS, the applied magnetic field will drive the system from a Gaussian symplectic ensemble into a Gaussian unitary ensemble. It results a (?)2 fold increase of the UCF amplitude. The gate-tuned UCF of a Bi2Te2Se nanoribbon has been measured at various constant fields, where a decreasing of the UCF amplitude of (?)2 times is revealed. This is condratictary to the theoretical prediction. However, there are two TSSs and they are coherently coupled to each other since the sample’s thickness is shorter than its bulk dephasing length. This leads to a Gaussian orthogonal ensemble of the intersurface coupling system without an external field. In such situation, the UCF amplitude will decrease by (?)2 times with the field increasing. It is consistent with the experimental results. More work is still being caried out.