Electronic Transport In Twisted Bilayer Molybdenum Disulfide And Few Layer Molybdenum Disulfide

In 2004,A.K.Geim et al.firstly used adhesive tape to exfoliate a two-dimensional（2D）layered material called graphene.The emergence of graphene creates a new era in2D physics,materials science,and technological applications.Up to now,more than two thousand 2D materials with distinct structures and physical properties have been discovered,including semi-metallic graphene,insulating hexagonal boron nitride（h-BN）,and semiconducting molybdenum disulfide（MoS₂）.By utilizing weak interlayer van der Waals force,2D materials can be stacked together to form van der Waals heterostructures,breaking the limitation of traditional lattice matching and unlocking a wealth of electronic,photonic,and topological states.In 2018,Pablo et al.discovered correlated insulating states and superconductivity in“magic angle graphene”which has since promoted widespread interest in moirésuperlattices in 2D materials including transition metal chalcogenides（TMD）.The moirématerials have become a new platform for exploring strongly correlated electron states and nontrivial topological energy bands.However,due to the problem of the contact,it is still hard to perform low temperature transport measurement for TMD.For this dissertation,we focus on MoS₂,a typical TMD,and perform both room temperature and low temperature electrical measurements on twisted bilayer and four-layer MoS₂,revealing the abundant electronic states in the MoS₂ moirésuperlattice.Specific research contents are outlined as follows:1.Charge shielding in graphene-contacted MoS₂ transistors.The contact to semiconductors has always been the focus of scientific and industrial research.To study the low temperature transport behaviors of MoS₂,the contact problem must be solved.Graphene is a good contact material for MoS₂ at low temperatures.We conducted electrical and magnetic transport measurements on dual-gated bilayer MoS₂ devices contacted by graphene.At room temperature,the device shows good electrical performance with no hysteresis.The current’s average threshold swing is 83 m V/dec close to ideal threshold swing limit.The field-effect mobility is up to 106 cm²V^-1s^-1.The device also shows good electrical performance down to 3 K.Both the two-terminal and four-terminal resistances exhibit remarkable Shubnikov-de Haas（Sd H）oscillation in vertical magnetic field.The resistance oscillation amplitudes are up to megaohms.The critical magnetic field of the Sd H oscillation is found to be 1 T at3 K and the critical temperature is above 25 K.The landau levels show different behaviors by the top gate and the back gate which is dependent on the placement of graphene contact in the device.One gate has a fan structure and the other gate has a parallel structure.Thus,we confirm that the oscillation originates from the graphene contact.The parallel structure comes from charge shielding effect of MoS₂ on graphene.We introduce a quantum capacitance model to explain the oscillation behaviors observed in 2D insulators.This model also explains the rapid disappearance of oscillation signals in very insulating and metallic regions.2.The correlated insulating states in AB-stacked twisted bilayer MoS₂.Using“Cut and stack”method,we fabricated high-quality twisted bilayer MoS₂devices.Our findings reveal correlated insulating states at v=1,2,3 and a band insulator at v=4 between 57°-57.5°twisted angles.Interestingly,unlike the SU（2）Hubbard model in twisted bilayer WSe₂,the correlated insulating states at v=1,2,3indicate that the twisted bilayer MoS₂ simulates the SU（4）Hubbard model.The four insulating states are like the Lifshitz phase transition reported in graphene superlattice.We mainly investigated the transport behaviors of 57.15°sample and found that the correlated insulating states can be fine-tuned by the displacement field.For instance,the charge gap at v=2 is maximized near zero displacement field and gradually closes with an increase of displacement field,demonstrating the continuous tunability of the insulting state.The insulating states showed negative magnetoresistance under vertical magnetic field,which is likely due to Zeeman splitting in the flat band.The insulating states have no response to the parallel magnetic field,indicating that the spin has no in-plane component at the±K valleys of MoS₂.3.Room-temperature correlated insulating states and nonlinear Hall effects.We measured the evolution of correlated insulating states with varying temperatures in twisted bilayer MoS₂ and calculated the charge gap by thermal excitation model.Our experimental findings reveal that the system displays a huge charge gap of up to 126 me V at v=1,indicating strong electron correlation.The correlated insulating state at v=1 can survive up to room temperature.We offer a plausible explanation for this strong correlation by performing DFT calculations,which demonstrated that interlayer resonances can enhance electron correlation by reducing the bandwidth.Additionally,we measured second-order nonlinear Hall effects near the correlated insulating states.The second-order voltage exhibited a good quadratic relationship with first-order current.Notably,our findings indicate that the nonlinear generation efficiency at low temperatures can reach up to 360 V^-1,which is two orders of magnitude larger than bilayer and few layer WTe₂.Moreover,this response can reach more than 1 V^-1 near v=1 at room temperature.This enormous nonlinear Hall response at room temperature has the potential to promote the application of radio frequency rectification and frequency doubling technologies.4.Coupling of correlated insulating states and ferroelectricity in the twisted four-layer MoS₂moirésuperlattice.The recent study of moiréphysics mainly focuses on the single physical properties,while the coupling between different quantum phenomenon is rarely studied.In this section,we firstly reveal the stacking effect in twisted bilayer MoS₂:The AA stacking exhibits ferroelectricity but no flat bands in the conduction band,while the AB stacking offers flat bands but no ferroelectricity.Our conclusion is supported by the DFT calculations.Furthermore,we study the electrical transport behaviors of twisted four-layer MoS₂ with coexistence of AB stacking and AA stacking.The twisted angles between the two layers are controlled in sequence to be～57°,～0°,～-57°.We found a coexistence of correlated insulating states and ferroelectricity in this four-layer MoS₂ moirésuperlattice.What’s more,we found that ferroelectricity is greatly enhanced when meeting the correlated insulating states and disappears quickly when being away from integer fillings,hinting the coupling between these two states.