Transport Study On Twisted Double Bilayer Graphene

Recent years,the rapid development of twisted 2D materials provides new playground for condensed matter physics.Introducing moiré superlattice by twist angle and lattice mismatch in 2D material systems,researchers observed exotic states such as strong correlated insulator,superconductor,quantized anomalous Hall effect,and Wigner crystal.This thesis mainly focuses on transport phenomena in twisted double bilayer graphene,including correlated insulator states,electron-phonon interaction,critical behavior and magnetoresistance scaling.Firstly,we prepared high-quality twisted double bilayer graphene device and observed field-tunable half-filling correlated insulator states in moiré conduction band near 1.3°.The correlated insulator states emerge in an intermediate range of displacement field,surrounded by the halo structure.What’s more,the gap of the insulator state can be enhanced linearly by in-plane magnetic field,holding a g factor～2.This result demonstrates the possible spin polarization nature at half-filling.Our work shows twisted double bilayer graphene is a field-tunable strong correlated moirésystem to study quantum many-body states.Secondly,we studied electron-phonon interaction and critical behavior in twisted double bilayer graphene by T-linear resistivity.We found the T-linear resistivity is dominated by electron-phonon interaction at relatively larger twist angle in the absence of correlated states.And the electron-phonon interaction is displacement field tunable.Near the ‘optimal’ twist angle(～1.3°),the slope of T-linear resistivity maximizes at the phase transition point between correlated states and normal metal states.This suggests enhanced effective mass at the phase transition,in agreement with the divergent quadratic-in-T coefficient at low temperature and direct measurement of effective mass.We hold the opinion that the enhanced effective mass at phase transition may point to quantum criticality there.This work will inspire in-depth research into other moirésystems and help map out the nature of ground states in them.Lastly,we studied the magnetoresistance behavior in twisted double bilayer graphene.Quadratic magnetoresistance can be observed in both moiré valence band and conduction band.We observed validity of Kohler’s rule in moiré valence band,consistent with classical theory.While in moiré conduction band,the magnetoresistance shows strong violation of Kohler’s rule near strong correlated regime.We also proposed an alternative scaling rule by in-depth analysis about the positive linear dependence between quadratic-in-B coefficient and zero-field resistance.This alternative scaling shows correlation between the halo boundaries which implies enhanced effective mass near the phase transition plays a role.