Weak antilocalization in (111) thin films of a topological crystalline insulator SnTe

Bulk-surface coupling in dual topological insulator Bi1Te1and Sb-doped Bi1Te1single crystals via electron-phonon interaction

Recently,Bi1Te1has been proved to be a dual topological insulator (TI), a new subclass of symmetry-protected topological phases, and predicted to be higher order topological insulator (HOTI). Being a dual TI (DTI), Bi₁Te₁is said to host quasi-1D surface states (SSs) due to weak TI phase and topological crystalline insulating SSs at the same time. On the other hand, HOTI supports topologically protected hinge states. So,Bi1Te1is a unique platform to study the electrical signature of topological SS (TSS) of fundamentally different origins. Though there is a report of magneto-transport measurements on large-scale Bi₁Te₁thin films, the Bi₁Te₁single crystal is not studied experimentally to date. Even the doping effect in a DTI Bi₁Te₁is missing in the literature. In this regard, we performed the perpendicular and parallel field magneto-transport measurement on the exfoliated microflake of Bi₁Te₁and Sb-doped Bi₁Te₁single crystals, grown by the modified Bridgmann method. Ourmetallicsample shows the weak anti-localization behavior analyzed by the multi-channel Hikami-Larkin-Nagaoka equation. We observed the presence of a pair of decoupled TSS. Further, we extracted the dephasing index (β) from temperature (T)-dependence of phase coherence length (Lϕ), following the power law equation (Lϕ∝T-β). The thickness-dependent value ofβindicates the transition in the dephasing mechanism from electron-electron to electron-phonon interaction with the increase in thickness, indicating the enhancement in the strength of bulk-surface coupling. Sb-doped system shows weakened bulk-surface coupling, hinted by the reduced dephasing indices.

Weak Antilocalization in Polycrystalline SnTe Films Deposited by Magnetron Sputtering

Previous works on weak antilocalization (WAL) of SnTe were mostly carried out in MBE-grown films, where the signals of WAL usually coexist with a large parabolic background of classical magnetoresistance. In this article, we present our study on WAL in polycrystalline SnTe films deposited by magnetron sputtering. Due to the polycrystalline nature and the relatively low mobility of the films, the background of conventional magnetoresistance was greatly suppressed, and clean WAL signals, which are well described by the Hikami–Larkin–Nagaoka equation, were obtained at low temperatures. A close analysis of the WAL data shows that the number of transport channels contributing to WAL increases monotonously with decreasing temperatures, reaching N=2.8 at T=1.6 K in one of the devices, which indicates the decoupling of Dirac cones at low temperatures. Meanwhile, as the temperature decreases, the temperature dependence of phase coherence length gradually changes from lϕ∼T−1 to lϕ∼T−0.5, suggesting that the dominant mechanism of phase decoherence switches from electron–phonon scattering to electron–electron scattering. Our results are helpful for understanding the quantum transport properties of SnTe.

Two-Dimensional Weak Antilocalization Signatures Due to Quantum Coherent Transport in Nanocrystalline SnTe

Nanostructured topological crystalline insulators (TCIs) in the presence of exotic surface states with spin momentum locking reported in individual nanostructures are predicted to hold a great promise for spintronics and quantum computing applications. However, practical application demands a strategy with large-scale production and integration for device applications. In this work, we demonstrate through prominent signatures of weak antilocalization (WAL), arising predominantly from destructive quantum interference on robust surface states, that a correlated TCI phase is possible in the nanobulk assembly of carefully nanostructured quasi-two-dimensional SnTe (edge-to-edge length ∼ 382 nm) synthesized by a simple, rapid, and scalable microwave-assisted solvothermal method. Hikami-Larkin-Nagaoka analysis (T^-0.71), as well as the temperature dependence of resistivity, illustrates an interplay of both conductions from 2D channels and 3D EEI effects as the precursor for the observed WAL at low temperatures (2-6 K). Interestingly, the enhanced thermoelectric power of the sample of ∼45 μV/K, with a p-type carrier concentration of ∼10¹⁸/cm³ at 300 K, makes this SnTe nanocrystalline assembly more attractive as a multifunctional material for large-scale technological applications.

Weak Localization and Antilocalization in Topological Materials with Impurity Spin-Orbit Interactions

Topological materials have attracted considerable experimental and theoretical attention. They exhibit strong spin-orbit coupling both in the band structure (intrinsic) and in the impurity potentials (extrinsic), although the latter is often neglected. In this work, we discuss weak localization and antilocalization of massless Dirac fermions in topological insulators and massive Dirac fermions in Weyl semimetal thin films, taking into account both intrinsic and extrinsic spin-orbit interactions. The physics is governed by the complex interplay of the chiral spin texture, quasiparticle mass, and scalar and spin-orbit scattering. We demonstrate that terms linear in the extrinsic spin-orbit scattering are generally present in the Bloch and momentum relaxation times in all topological materials, and the correction to the diffusion constant is linear in the strength of the extrinsic spin-orbit. In topological insulators, which have zero quasiparticle mass, the terms linear in the impurity spin-orbit coupling lead to an observable density dependence in the weak antilocalization correction. They produce substantial qualitative modifications to the magnetoconductivity, differing greatly from the conventional Hikami-Larkin-Nagaoka formula traditionally used in experimental fits, which predicts a crossover from weak localization to antilocalization as a function of the extrinsic spin-orbit strength. In contrast, our analysis reveals that topological insulators always exhibit weak antilocalization. In Weyl semimetal thin films having intermediate to large values of the quasiparticle mass, we show that extrinsic spin-orbit scattering strongly affects the boundary of the weak localization to antilocalization transition. We produce a complete phase diagram for this transition as a function of the mass and spin-orbit scattering strength. Throughout the paper, we discuss implications for experimental work, and, at the end, we provide a brief comparison with transition metal dichalcogenides.

Low-Dimensional Topological Crystalline Insulators

Topological crystalline insulators (TCIs) are recently discovered topological phase with robust surface states residing on high-symmetry crystal surfaces. Different from conventional topological insulators (TIs), protection of surface states on TCIs comes from point-group symmetry instead of time-reversal symmetry in TIs. The distinct properties of TCIs make them promising candidates for the use in novel spintronics, low-dissipation quantum computation, tunable pressure sensor, mid-infrared detector, and thermoelectric conversion. However, similar to the situation in TIs, the surface states are always suppressed by bulk carriers, impeding the exploitation of topology-induced quantum phenomenon. One effective way to solve this problem is to grow low-dimensional TCIs which possess large surface-to-volume ratio, and thus profoundly increase the carrier contribution from topological surface states. Indeed, through persistent effort, researchers have obtained unique quantum transport phenomenon, originating from topological surface states, based on controllable growth of low-dimensional TCIs. This article gives a comprehensive review on the recent progress of controllable synthesis and topological surface transport of low-dimensional TCIs. The possible future direction about low-dimensional TCIs is also briefly discussed at the end of this paper.