Wydarzenia

Seminarium Międzynarodowego Laboratorium Silnych Pól Magnetycznych i Niskich Temperatur PAN

Surface States Transport in Topological Insulator Bi_0.83Sb_0.17 Nanowires

Leonid Konopko and Albina Nikolaeva

Ghitu Institute of Electronic Engineering and Nanotechnologies, Academy of Sciences of Moldova, Chisinau, Moldova

We investigate the transport properties of topological insulator (TI) Bi_0.83Sb_0.17 nanowires. Single-crystal nanowire samples with diameters ranging from 75 nm to 1.1 μm are prepared using high frequency liquid phase casting in a glass capillary; cylindrical single crystals with (1011) orientation along the wire axis are produced. Bi_0.83Sb_0.17 is a narrow-gap semiconductor with an energy gap at the L point of the Brillouin zone, ΔE = 21 meV. The resistance of the samples increases with decreasing temperature, but a decrease in resistance is observed at low temperatures. This effect is a clear manifestation of TI properties (i.e., the presence of a highly conducting zone on the TI surface). When the diameter of the nanowire decreases, the energy gap ΔE grows as 1/d (for diameter d = 1.1μm and d = 75 nm ΔE = 21 and 45 meV, respectively), which proves the presence of the quantum size effect in these samples. We investigate the magnetoresistance of Bi_0.83Sb_0.17 nanowires at various magnetic field orientations. Shubnikov-de Haas oscillations are observed in Bi_0.83Sb_0.17 nanowires at T = 1.5 K, demonstrating the existence of high mobility (μS = 26,700−−47,000 cm²V^-1s^-1) two-dimensional (2D) carriers in the surface areas of the nanowires, which are nearly perpendicular to the C3 axis. From the linear dependence of the nanowire conductance on nanowire diameter at T = 4.2 K, the square resistance Rsq of the surface states of the nanowires is obtained (Rsq = 70 Ohm). In thin Bi_0.83Sb_0.17 nanowires (d ≤ 100 nm) at low temperatures (1.5 K ≤ T < 5 K), we discovered the Aharonov–Bohm (AB) oscillations of longitudinal magnetoresistance with a period of one flux quantum, Φ0 (ΔB =Φ0/S, where S is the cross-sectional area of the nanowire). The period ΔB depends on the inclination angle α of the magnetic field according to the law ΔB=ΔBparallel/cosα. Recent efforts to detect and manipulate Majorana fermions in solid state devices have employed TI nanowires proximity coupled to superconducting leads (SC). This combination holds some promises for the fundamental physics and applications. For investigation TI/SC interface we have prepared Bi_0.83Sb_0.17 glass coated microwire samples using superconducting alloy In₂Bi (T_c=5.6 K) for making contact of one side of microwires with copper leads and gallium for making contact of another side of microwires with copper leads. The equidistant in transverse magnetic field (up to 1 T) magnetoresistance oscillations at the TI/SC interface have been observed at various temperatures (4.2 K – 1.5 K). In 1.7 µm Bi_0.83Sb_0.17 sample magnetoresistance oscillations are characterized by a period of ΔB=46 mT. The observed oscillations cannot be referred to the Shubnikov de Haas or AB oscillations.