Electronic Transport In Two-dimensional Transition Metal Dichalcogenides And Graphene

The family of two-dimensional（2D）materials has grown by leaps and bounds since the first exfoliation of graphene by A.K.Geim and his colleagues in 2004.Two-dimensional materials include elementary substances such as graphene,black phospho-rus and silicene,and compounds such as transition metal dichalcogenides,hexagonal boron nitride.These 2D materials have various energy band structures and exhibit abundant electric properties such as superconductors,metals,semiconductors,and insu-lators.The electronic transport measurement of the 2D materials at cryogenic temper-ature offers great opportunities to discover novel phases of matter in condensed matter physics.This thesis focuses on the low temperature electronic transport studies on three different types of 2D materials,including the 2D superconductor 1T’phase WS₂,the2D semimetal 1T phase Pt Se₂,and monolayer（bilayer）graphene.The specific studies are as follows:1.Enhanced critical field and anomalous metallic state in 2D centrosymmetric 1T’phase WS₂The nature of superconductivity has been a major concern in condensed matter physics.In the early of 1980s,2D superconductivity was found in amorphous thin film,such as Mo Ge.Until very recently,with the development of various 2D materials and the fabrication techniques,2D superconductivity were observed in 2D transition metal dichalcogenides.Here,we investigate the electrical transport study of 2D centrosym-metric superconductor 1T’-WS₂.For a typical few layers sample,the 2D superconduc-tivity is revealed in Berezinskii Kosterlitz Thouless（BKT）transition with characteristic temperature_BKT～5 K.The superconductivity is observed with strong resist against the in-plane magnetic field.We find that the critical in-plane magnetic field_c2^∥is more than 3 times the Pauli paramagnetic limit_p,with effective Zeeman-type spin-orbital coupling（SOC）?_SO～2.8 me V.In addition,we find the magneto-resistance trend to saturate at low temperature,suggesting anomalous quantum metallic（AQM）state.2.Strain-induced periodic resistance peaks in 2D semimetal 1T phase Pt Se₂2D transition metal dichalcogenides are of great interest due to their diverse lattice structures（1H phase,1T phase,1T’phase）,various electric properties（semiconduc-tors,semimetals,metals,insulators）,and abundant physical properties at low tempera-ture（superconductivity,magnetism,charge density waves）.The 1T-Pt Se₂,a transition metal dichalcogenides discovered in recent years,has many advantages including high theoretical mobility,layer-dependent electrical properties,and magnetism at low tem-peratures.Here,we fabricated a 3.5 nm（6 layers）1T-Pt Se₂dual-gate device by dry transfer technique,with a high electrical mobility of～205 cm²V^-1s^-1at room tem-perature.In addition,we find the periodic resistance peaks near the band energy gap at=1.7 K.The periodic resistance peaks exhibit insulating behaviors,while the resis-tance between these peaks exhibit metallic behaviors.We attribute these observations to the localization effects due to inhomogeneous strain.3.phase transition in graphene induced by current annealingAt the field of materials science and engineering,the most important topic is to achieve the functional devices by manipulating the electronic properties.Graphene is an ideal material to achieve the electronic manipulating,due to its special two-dimensional properties and linear-dependent band.Here,we fabricated monolayer（bilayer）graphene encapsulated by hexagonal boron nitride.An important and interesting feature is the emergence ofphase transition in Landau level at finite magnetic field after the current annealing.Thephase transition might come from the variation of doping locally,showing the Fabry-Perot like resonant features.One possible reason lies in the changed graphene doping near the contact areas by current annealing.Another reason is the activations of impurity states in h BN due to current annealing,which might produce localized charge puddles in h BN and then effectively lead to inhomogeneous carrier doping in graphene device.