Coulomb drag to thermopower response in dimensionally mismatched InAs nanowire-Graphene heterostructures and anisotropic noise in ReS2

Abstract

Coulomb drag technique is a notable tool for probing electron-electron interaction in double-layer systems, where the momentum and energy transfer from one layer to the other layer as a result of Coulomb interaction. In such a scenario, allowing current through a layer induces an open-circuit voltage in the nearby layer known as the 'Drag voltage'. In two-dimensional bilayer heterostructures, correlated charge inhomogeneity breaks the electron-hole symmetry and gives rise to a nonzero drag signal at the charge neutrality point. Many theories have been proposed to explain the drag signal with no consensus among them. In the first part of the thesis, we address this question by studying the Coulomb drag in novel drag systems consisting of a two-dimensional (2D) graphene and a one-dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostructures. For monolayer graphene (MLG)-NW heterostructures, we observe an unconventional drag resistance peak near the Dirac point due to the correlated interlayer charge puddles. The drag signal decreases monotonically with temperature T^(-2) and with the carrier density of NW n^(-4), but increases rapidly with magnetic field B^2. These anomalous responses, together with the mismatched thermal conductivities of graphene and NWs, establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag resistance reverses sign across the Dirac point, and the magnitude of the drag signal decreases with the carrier density of the NW n^(-1.5), consistent with the momentum drag but remains almost constant with magnetic field and temperature. This deviation from the expected T^2 arises due to the shift of the drag maximum on graphene carrier density. In the second part of the thesis, we present the thermoelectric properties of graphene in the NW-graphene heterostructure using the nanowire as a local nano-heater. Contrary to conventional thermoelectric measurement, where a heater is placed at one side, here we have used the InAs NW with a diameter of ~ 50 nm as a local heater at the middle of the heterostructure and measure the thermoelectric response across the graphene as a function of temperature (1.5K - 50K) and carrier concentration. The thermoelectric voltage in the NW-bilayer graphene (BLG) heterostructure follows Mott's response with a sign change around the Dirac point. In contrast, the thermoelectric voltage in NW-monolayer graphene (MLG) heterostructure show anomalous oscillations around the Dirac point, which is completely missing in the Mott's response extracted from the resistance data. This discrepancy is understood by the modification of the local density of states (DOS) in MLG due to a cavity formed by the 1D electrostatic potential of the NW. Moreover, using Fourier's law, we theoretically estimate the temperature profile for the heterostructure, which shows that the gradient of temperature is dominant around the graphene part underneath of the NW, and thus sensitive to detect the local DOS. In the last part of the thesis, we demonstrate the 1/f noise study in ReS2 transition metal dichalcogenide to characterize its in-plane anisotropy. The intrinsic structural deformity in ReS2 gives rise the in-plane anisotropy which is reflected in conductance and other measurement data. Our study shows that in a few-layer ReS2, 1/f noise is also anisotropic. By comparing the anisotropies, we found noise spectroscopy to be more sensitive than the conductance measurement.