Executive Summary : | This project concerns the quantum spin Hall effect (QsHE) in two-dimensional topological insulators. The two-dimensional topological insulators exhibit quantized conductance of 2e² or h due to counterpropagating helical edge channels while the bulk is insulating. The effect is observed at the interface of two materials with mutually inverted band structures. After the first observation of the effect in 2007, several other experiments such as the nonlocal effect, spin polarization of the edge channels, induced superconductivity, and influence of magnetic impurities have been investigated in appropriate transport experiments. The topological edge channels are protected against backscattering under time-reversal symmetry. However, the reports on the conductance quantization have shown reproducible fluctuations in the quantum spin Hall regime in contrast to constant quantized values of conductance, implying the presence of some inelastic scattering mechanisms for the edge channels. Additionally, the thermal properties of such edge channels are relatively less explored. Though the 2D topological insulators have been investigated theoretically in detail, the experimental research in the QsHE is rather limited. This is because only a few model systems (HgTe quantum wells, monolayer WTe2, and graphene proximitized with high spin-orbit material) have shown quantized conductance due to QsHE. Additionally, only a few research groups have reported the conductance quantization due to QsHE (University of Wuerzburg, MIT-UsA, Novosibirsk state University, IIsc). In this proposal, we plan to explore the electrical and thermal properties of quantum spin Hall insulators. Theoretical reports suggest that 1T` phase of monolayer transition metal dichalcogenides (TMDCs) are candidates for the quantum spin Hall effect. The band inversion in such materials is due to a structural distortion and can be tuned with an applied electric field. We plan to use exfoliated monolayers of TMDCs (MX_2, where M can be tungsten or molybdenum and X can be tellurium, selenium, or sulfur). To ensure the high quality of the exfoliated layers, we plan to encapsulate the material in h-BN layers and use metal ohmics to contact the edge of the material. such edge-contacted two-dimensional materials have been shown to have excellent device quality. The fabricated devices will be investigated for their electrical and thermal properties. In terms of electrical properties, we will investigate the influence of magnetic fields on quantized conductance. We will also measure the low-frequency noise in the quantum spin Hall regime to get insights into the type of disorder as well as disorder dynamics for two-dimensional topological insulators. Finally, we will build a high-thermometry Johnson's noise thermometry setup to measure the heat transport due to the quantum spin Hall edge channels and check if the thermal conductance is quantized in terms of quanta of heat conductance. |