Emerging weak antilocalization effect in Ta_(0.7)Nb_(0.3)Sb_(2)semimetal single crystals

Weak antilocalization(WAL)effect is commonly observed in low-dimensional systems,three-dimensional(3D)topological insulators and semimetals.Here,we report the growth of high-quality Ta_(0.7)Nb_(0.3)Sb_(2)single crystals via the chemical vapor transport(CVT).Clear sign of the WAL effect is observed below 50 K,probably due to the strong spin–orbital coupling in 3D bulk.In addition,it is worth noting that a relatively large MR of 120%appears under 1 T magnetic field at T=2 K.Hall measurements and two-band model fitting results reveal high carrier mobility(>1000 cm^(2)·V^(–1)·s^(–1)in 2–300 K region),and off-compensation electron/hole ratio of~8:1.Due to the angular dependence of the WAL effect and the fermiology of the Ta_(0.7)Nb_(0.3)Sb_(2)crystals,interesting magnetic-field-induced changes of the symmetry of the anisotropic magnetoresistance(MR)from two-fold(≤0.6 T)to four-fold(0.8–1.5 T)and finally to two-fold(≥2 T)are observed.This phenomenon is attributed to the mechanism shift from the low-field WAL dominated MR to WAL and fermiology co-dominated MR and finally to high-field fermiology dominated MR.All these signs indicate that Ta_(0.7)Nb_(0.3)Sb_(2)may be a topological semimetal candidate,and these magnetotransport properties may attract more theoretical and experimental exploration of the(Ta,Nb)Sb_(2)family.

Effects of post-annealing on crystalline and transport properties of Bi_(2)Te_(3) thin films

A well-established method is highly desirable for growing topological insulator thin films with low carrier density on a wafer-level scale. Here, we present a simple, scalable method based on magnetron sputtering to obtain high-quality Bi_(2) Te_(3) films with the carrier density down to 4.0 × 10^(13) cm^(-2). In contrast to the most-used method of high substrate temperature growth, we firstly sputtered Bi_(2) Te_(3) thin films at room temperature and then applied post-annealing. It enables the growth of highly-oriented Bi_(2) Te_(3) thin films with larger grain size and smoother interface. The results of electrical transport show that it has a lower carrier density as well as a larger coherent length(~ 228 nm, 2 K). Our studies pave the way toward large-scale, cost-effective production of Bi_(2) Te_(3) thin films to be integrated with other materials in wafer-level scale for electronic and spintronic applications.

Impurity effect on surface states of Bi(111) ultrathin films

The surface impurity effect on the surface-state conductivity and weak antilocalization（WAL） effect has been investigated in epitaxial Bi（111） films by magnetotransport measurements at low temperatures. The surface-state conductivity is significantly reduced by the surface impurities of Cu, Fe, and Co. The magnetotransport data demonstrate that the observed WAL is robust against deposition of nonmagnetic impurities, but it is quenched by the deposition of magnetic impurities which break the time reversal symmetry. Our results help to shed light on the effect of surface impurities on the electron and spin transport properties of a 2D surface electron systems.

Two-dimensional quantum transport of multivalley （111） surface state in topological crystalline insulator SnTe thin films

The magneto-transport properties of （111）-oriented single-crystal thin films of SnTe were investigated. SnTe （111） thin films were epitaxially grown on a BaF2 substrate by molecular beam epitaxy. By optimizing the growth conditions and the thickness of the films, the bulk carrier density could be reduced, making it possible to detect the surface transport. Magneto-conductance （MC） measurements show a cusp-like feature around zero magnetic field, which is attributed to the weak-antilocalization effect of the transport in the topological surface state. Detailed analysis of this negative MC reveals a reduced number of transport channels contributing to the surface transport, suggesting strong coupling between Dirac valleys on the SnTe （111） surface, as a characteristic feature of the transport in the multivalley structure of topological crystalline insulators.