Electronic transport experiments in 2D materials with spin-orbit coupling

Abstract

This thesis comprises two distinct investigations in the field of two-dimensional materials with spin-orbit coupling (SOC). The first part of this thesis focuses on investigating the electronic properties of transition metal dichalcogenides (TMDs), a class of emerging two-dimensional materials with the chemical composition MX2, where M is a transition metal and X is a chalcogen atom. Specifically, we concentrate on the semiconducting molybdenum disulfide (MoS2), renowned for its direct band gap in the optical range in the monolayer limit (from 1 eV to 2 eV), strong intrinsic spin-orbit coupling (SOC), relative stability in ambient conditions, and decent electron mobilities. Our investigation employs magnetotransport experiments to probe the conduction band of MoS2, with a particular emphasis on its layer dependence. Building on prior work by R. Pisoni on monolayer and bilayer MoS2, we extend our study to three-layer and four-layer samples. We fabricate dual-gated few-layer MoS2 devices, enabling control of the electron density via voltages applied between MoS2 and both top and bottom gate electrodes. Through careful analysis of Shubnikov-de Haas oscillations (SdHO) originating from different bands, we determine the corresponding carrier densities in the various layers. Additionally, the Landau level degeneracy provides insights into the band structure, particularly its location in the first Brillouin zone. In three-layer MoS2, we observe a twofold Landau level degeneracy for each band, suggesting that the conduction band minima reside at the corners of the hexagonal Brillouin zone (i.e., the K points), contrary to theoretical predictions. Moreover, the large electron mass results in strong inter-layer screening, influencing the distribution of electrons among the layers, which preferentially occupy the MoS2 layers adjacent to the positively biased gate electrode. When the density of the two outermost layers is equal, the bands become energetically resonant, leading to electron scattering between the outermost layers, thereby increasing resistance and inducing magneto-interband oscillations at finite magnetic fields. Despite the anticipated band hybridization due to inter-layer tunnel coupling, each layer of three-layer MoS2 exhibits electrical properties similar to an individual monolayer. We hypothesize about the impact of the contact potential between the dielectric material and the outermost MoS2 layers, suggesting that it may potentially hinder the transition in the band structure within three-layer MoS2. Preliminary results from our investigation in four-layer MoS2 suggest electronic behavior reminiscent of a bilayer sandwiched between two monolayers, supporting our hypothesis. The second project delves into transport experiments conducted on bilayer graphene (BLG) supported by a MoS2 substrate. BLG, being a zero-band gap semiconductor with a highly adjustable band structure, offers versatility in tuning its properties, including the introduction of a band gap through an out-of-plane electric field. Moreover, BLG offers the advantage of ambipolar charge carrier transport and high charge carrier mobilities. The integration of a MoS2 substrate aims to introduce significant SOC to BLG, which is typically devoid of such effects, through a phenomenon known as the proximity effect. The motivation for introducing SOC stems from the desire to control spin states using time-dependent electric fields. However, the specific type of SOC introduced remains uncertain. Our study aims to elucidate this by analyzing SdHO. We identify two types of SOC: an Ising SOC, similar to the intrinsic SOC of MoS2, which locks spins out-of-plane, and a Rashba SOC, inducing in-plane spin polarization. Despite their comparable strengths, the Ising SOC primarily governs the splitting of low-energy bands at the K points, resulting in spin-polarized bands where the spin and valley degrees of freedom are locked. Additionally, we observe a non-monotonic conductivity response to an applied displacement field when BLG is charge neutral. By comparing experimental findings with tight-binding calculations, we attribute conductivity maxima at a critical displacement field Dc to the closure of SOC-induced gaps in spin-polarized bands. At this critical displacement field, the application of an external magnetic field rapidly suppresses the conductivity, challenging existing theoretical models and suggesting the involvement of many-body interactions. The investigations presented in this thesis shed light on the intricate electronic properties of two-dimensional materials with SOC and their potential for future applications in nanoelectronics and spintronics. The comprehensive analysis of MoS2 unraveled valuable insights into the layer-dependent behavior and interlayer interactions, paving the way for optimized device designs and tailored functionalities. Furthermore, understanding the effects of SOC in BLG on MoS2 substrates holds promise for manipulating spin states and enhancing spin-related phenomena, which are essential for advancing next-generation spin-based technologies. By addressing key uncertainties regarding SOC types and their influence on electronic transport, this work contributes to the fundamental understanding of SOC in two-dimensional materials.