Silver chalcogenide and indium arsenide nanostructures

Abstract

In the past few decades, spin-orbit coupling is considered as minor perturbation in the condensed matter physics. However, theoretical predictions and material researches have brought new perspective on the spin-orbit coupling studies. The materials with strong spin-orbit coupling make possible to manipulate the spin degree of freedom by only electric field. Furthermore, the materials called topological insulator with strong spin-orbit coupling have recently attracted attention because of their unique properties. The spin-orbit coupling inherent to topological insulators causes the spin orientation of the surface electrons to be locked perpendicular to their translational momentum, resulting in the suppression of electron backscattering from nonmagnetic perturbations. These exotic properties caused by strong spin-orbit coupling could develop for spintronics, quantum computing applications and fundamental studies such as developing Majorana fermions.

Topological insulator and strong spin-orbit coupling materials are mostly composed of heavy atoms. The chalcogenide and arsenic compounds are those kinds of materials in condensed matters. In the sense of dimension, the nanostructured materials give various opportunities to investigate unique properties of strong spin-orbit coupling materials because of their large surface to volume ratio and quantum confinement effect. Among diverse materials, we synthesized single crystalline $\beta -Ag_2Se$, $Ag_2Se_xTe_{1-x}$, $Bi_2Se_3$ nanostructures by chemical vapor transport. By measuring electrical properties in low temperature, we confirmed that $\beta -Ag_2Se$ is new class of 3D topological insulator. The topological surface states were verified by measuring electronic transport properties including the weak antilocalization effect, Aharonov-Bohm oscillations, and Shubnikov-de Haas oscillations. First-principles band calculations revealed that the band inversion in $\beta -Ag_2Se$ is caused by strong spin-orbit coupling and Ag-Se bonding hybridization. These extensive investigations provide new meaningful information about silver-chalcogenide TIs that have anisotropic Dirac cones, which could be useful for spintronics applications.

We also performed that the hybrid junction between $\beta -Ag_2Se$ nanowire and Al superconducting electrodes. This Josephson junction showed strong Josephson coupled super current. We observed critical current $I_c$ = $28.8 \mu A$ at the base temperature T = 300 mK. The temperature dependence of $I_c$ showed the convex-shape, inferring that the $\beta -Ag_2Se$ nanowire and Al were strongly coupled. The stochastic switching current distribution showed that the macroscopic quantum tunneling of the Josephson phase particle was maintained at T = 0.8 K, confirming the strongly coupled Josephson junction. This study could provide useful information for developing Josephson phase qubit.

We performed the electrical and mechanical measurement of InAs nanowires which also have strong spin-orbit coupling. The measured InAs nanowires were grown by two different methods, one is metal-organic chemical vapor deposition and the other is molecular beam epitaxy. The InAs nanowire grown by metal-organic chemical vapor deposition had relatively low resistance that make possible to measure the magneto-motive mechanical signal with quality factor Q ~ 126000. The mechanical resonance of InAs nanowire grown by molecular beam epitaxy was measured by capacitive coupled between nanowire and bottom gate. The field effect transistor characteristic of InAs nanowire amplified the mechanical signal. They also showed coupling between the mechanical resonance and their electronic structures. These demonstrations could provide deeper understanding of relation between mechanics and electronics.