Differential Conductance Fluctuation of Curved Nanographite Sheets in the Mesoscopic Regime

Excess conductance fluctuations with peculiar temperature-dependence from 1.4 to 250 K were observed in curved nano-graphite sheets with an electrode gap length of 300 and 450 nm, whereas the conductance fluctuation is greatly suppressed above 4.2 K when the electrode gap length increases to 800 and 1000 nm. The former is discussed in the context of the presence of a small energy bandgap in the nano-graphite sheets, while the latter is attributed to the crossover from coherent transport to diffusive transport regime.Comment: 12 pages, 3 figure

THEORETICAL AND EXPERIMENTAL STUDIES OF COMPOUND SEMICONDUCTOR ALLOYS

Ph.DDOCTOR OF PHILOSOPH

Gate-tuned quantum oscillations of topological surface states in β-Ag2Te

We report the strong experimental evidence of the existence of topological surface states with large electric field tunability and mobility in β-Ag2Te. Pronounced 2D Shubnikov-de Haas oscillations have been observed in β-Ag2Te nanoplates. A Berry phase is determined to be near π using the Landau level fan diagram for a relatively wide nanoplate while the largest electric field ambipolar effect in topological insulator so far (~2500%) is observed in a narrow nanoplate. The π Berry phase and the evolution of quantum oscillations with gate voltage (Vg) in the nanoplates strongly indicate the presence of topological surface states in β-Ag2Te. Moreover, the mobility of the narrow Ag2Te nanoplate is about several thousand cm2s−1V−1. Our results suggest that β-Ag2Te has the potential to become an important material in the investigation of topological insulators.Published versio

Domain wall motions in perpendicularly magnetized CoFe/Pd multilayer nanowire

Current induced domain wall (DW) motion has been investigated in a 600-nm wide nanowire using multilayer film with a structure of Ta(5 nm)/Pd(5 nm)/[CoFe(0.4 nm)/Pd(1.2 nm)]$_{15}$/Ta(5 nm)in terms of anomalous Hall effect measurements. It is found that motion of DWs can be driven by a current density as low as 1.44$\times$10$^{11}$ A.m$^{-2}$. The effect of the Oersted field ($H_{Oe}$) and spin transfer torque field ($H_{ST}$), which are considered as effective fields for DW motion, has been quantitatively separated from the dependence of depinning fields on the current. The results show that the motion of the walls was essentially dominated by the non-adiabaticity with a high non-adiabatic factor $\beta$ up to 0.4.Comment: 14 pages, 4 figures, original research article (Accepted for publication in Journal of Physics D: Applied Physics

Origin and transport properties of two-dimensional electron gas at ZnMgO/ZnO interface grown by MOVPE

In this study, the origin and transport properties of high mobility carriers of two-dimensional electron gas (2DEG) at ZnMgO/ZnO interface have been investigated. It is found that the observed experimental dependence of carrier sheet density of 2DEG on Z

Interfacial and bulk polaron masses in Zn<inf>1-</inf><inf>x</inf>Mg<inf>x</inf>O/ZnO heterostructures examined by terahertz time-domain cyclotron spectroscopy

10.1063/1.4921469Applied Physics Letters1062

Evidences of thermoelectrically driven unidirectional magnetoresistance from a single Weyl ferromagnet Co2MnGa

Weyl ferromagnets, with large anomalous Hall (and Nernst) effects, are an ideal playground to study unconventional transport phenomena. Here, we report a sizable unidirectional magnetoresistance with a ratio of up to 7.73 × 10−5 per current density of 1 MA cm−2 in single-layer epitaxial Co2MnGa films. Surprisingly, the nonlinear signal has an isotropic crystallographic axis dependence and scales almost linearly with the film thickness. Both features cannot be explained by the spin transport from an intrinsic band structure, but rather agree with the current induced transverse thermoelectric effect. By employing a 1D heat transfer model to account for the temperature gradient, we derived an analytical expression of this thermoelectrically driven unidirectional magnetoresistance, from which a upper bound of transverse thermopower Sxy = 3.70 ± 1.10 µV K−1 can be obtained. Our work provides direct evidences of thermoelectric voltages in the nonlinear transport signals that may be extended to other material systems as well