Gate-Tunable Rashba Valleys And Unconventional Quantum Hall States In Few-Layer Black Arsenic

Since the discovery of graphene in 2004,two-dimensional(2D)materials have been revo-lutionizing researches in condensed matter physics.Two-dimensional crystals have unique van der Waals(vdW)layered structures,which can be micromechanically exfoliated down to the 2D limit in the units of monolayer thickness.As a result of quantum confinement,interlayer vdW interactions and/or symmetry changes,the physical properties of few-layer 2D materials are dra-matically different from the bulk counterparts.2D materials in the 2D limit also show fascinat-ing layer-dependent physical properties,such as massless Dirac fermions in monolayer graphene,massive Dirac fermions in bilayer graphene,and the coexistence of massless and massive Dirac fermions in trilayer graphene.The gold rush in exploring novel 2D materials beyond graphene has led to the flourish of prototypical 2D systems,such as hexagonal boron nitride,transition metal dichalcogenides(TMDCs),black phosphorus(BP),InSe,MXene,2D main-group compounds,and 2D ferromagnetic materials.In 2013,Nobel laureate Prof.Andre Geim proposed the concept of van der Waals heterostructures,by the micromechanical exfoliations and‘LEGO'combinations of different 2D crystals.In combination with the state-of-art dry transfer techniques,magic an-gle vdW heterostructures with small stacking angles between atomically thin layers emerge as the new frontier,by exhibiting extraordinary optical,electronic and optoelectronic properties.In this thesis,we studied external modulations of the novel physical properties and unconventional quantum Hall effect of few-layer black arsenic(BAs),with emphases on gate-tunable spin-orbit coupling for discovering new physical phenomena and novel mechanisms in the 2D limitBAs shares the same centrosymmetric square lattice as the renowned black phosphorous However,BAs has considerable spin-orbit coupling effect,since As element is one period heavier than P.As a result of reduced s-p hybridization,BAs also shows unique multivalley band struc-ture near the valence band maximum.Using in-situ micromechanical exfoliation and dry transfer methods,we prepared hBN/BAs/hBN/graphite heterostructure,where the graphite layer serves as the back gate.The resulting BAs field-effect transistors host ambipolar and high mobility 2D elec-tronic systems(2DESs),i.e.2D electron gases(2DEGs)for positive gate voltages and 2D hole gases(2DHGs)for negative biases.We found that giant Stark effect and strong Rashba effect can be synergetically activated and continuously tuned by external electric fields,producing particle-hole asymmetric Rashba valley formations in BAs 2DESs.For 2DEGs,conventional ? Rashba bands are formed under external gating,while exotic spin-valley flavoured Rashba energy split-tings are discovered for 2DHGs,hallmarked by unconventional quantum Hall states with unique Landau level crossing behaviours.Our work unveils the great promise of gate-tunable spin-orbit coupling in heavy 2D electronic systems.