Electrical, Thermoelectric, and Phase Coherent Transport in Two-Dimensional Materials
Over the past few years there has been a growing interest in layered two dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and three dimensional topological insulators (TIs). In this thesis, we experimentally study electrical, thermoelectric, and phase coherent transport in these 2D materials and work on three main projects. First, we investigate the low frequency (f) flicker (also called 1/f) noise of single-layer graphene devices on h-BN along with those on SiO2/Si. We observe that the devices fabricated on h-BN have on average one order of magnitude lower noise amplitude compared with devices fabricated on SiO2/Si despite having comparable mobility at room temperature, a result that can be associated with the lower densities of impurities and trap sites in h-BN. Our study demonstrates that the use of h-BN as a substrate or dielectric can be a simple and efficient noise reduction technique valuable for electronic applications of graphene and other 2D materials. Secondly, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of single- and few-layer MoS2 . We observe that the electrical conductivity (sigma) increases as we reduce the thickness of MoS2 and peaks at about two layers, with six times larger conductivity than the bulk. We also show that the thermoelectric power factor (PF) increases with decreasing thickness then drops abruptly from double-layer to single-layer MoS2 , a feature, which according to our theoretical modeling, is due to a change in the energy dependence of the electron mean-free-path. Lastly, we focus on Josephson effects and phase coherent transport in bulk-insulating topological insulator BiSbTeSe2 flakes and nanoribbons (TINRs) with superconducting Nb contacts. We observe an ambipolar field effect critical current (IC) and multiple Andreev reflections (MAR), indicating high quality of the junctions including the TI-superconductor interfaces. We also study the nature of the induced superconductivity in such junctions, where we observe (i) an anomalous low-temperature enhancement of IC, (ii) Aharonov-Bohm oscillations of the normal-state resistance and IC in TINR-based Josephson junctions, and (iii) highly skewed (non-sinusoidal) current-phase relation in TI-based junctions, revealing the induced superconductivity is carried by ballistic topological surface states (TSS) of the TI/TINR. Such TSS in TI-based junctions are predicted to support topological superconductivity and host Majorana fermions, particles that are their own anti-particles and hence are of paramount importance in topological quantum computing applications.
Chen, Purdue University.
Condensed matter physics|Nanotechnology
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