|dc.description.abstract||Graphene have grown to be a major experimental platform for studying two-dimensional (2D) physics due to its high quality, versatility and tunablity. Created by stacking graphene with other 2D materials, 2D heterostructures expand the possibility for exciting physics beyond graphene itself. This dissertation reports new phenomena observed in graphene double-layer heterostructures. In our devices, two graphene layers are placed in close proximity but separated by a thin hexagonal Boron Nitride (hBN) insulator, such that the two layers interact through Coulomb interaction while direct tunneling is prohibited.
We used Coulomb drag effect to characterize the interactions between layers as well as probe interlayer correlated phases. Experimental effort in graphene double-layer device fabrication and Coulomb drag measurements are presented. In the high temperature and weak coupling regime, we studied frictional drag effect due to interlayer Coulomb scattering. By correlating the drag signal with transport behavior of each individual layer, we furthered our understanding of magneto- and Hall-drag effect in the quantum Hall regime.
The most exciting possibility enabled by a double-layer electronic system is the exciton condensation (EC). Through Coulomb attraction, electrons in one layer and holes in the other layer bind into interlayer excitons, which are capable of establishing Bose-Einstein condensate under low temperatures. Graphene double-layer heterostructure, being highly tunable and strongly interacting, is a perfect system to realize this exotic superfluid state. EC of quasi-electrons and quasi-holes is first achieved between two partially filled Landau levels (LLs) in the same band in bilayer graphene double-layers. By studying the EC phase transition induced by perpendicular electric field, the conditions for LLs to establish EC is acquired.
In monolayer graphene double-layers, a novel EC phase can be established between an electron-doped graphene and a hole-doped graphene with equal carrier densities. We call this state exciton insulator, as each layer is found to be insulating when the other layer is open-circuit. However, they become conducting if current is allowed to flow in the drag layer. A perfect drag current arises at the same time. This exciton insulator phase is similar to EC under zero magnetic field.
Superfluids caused by condensation of fermion pairs, such as the exciton condensate, have two distinct types: Bose-Einstein condensation (BEC) and Bardeen–Cooper–Schrieffer (BCS) condensation. Tuning interlayer coupling strength, we observed BEC-BCS crossover of the exciton condensate. In the BEC limit, tightly bound excitons pairs first form exciton gas and subsequently condense at a much lower temperatures. Oppositely, when the coupling is weak (BCS), excitons, similar to Cooper pairs in a BCS superconductor, pair and condense simultaneously. This observation provides useful insight in studying other superfluids, such as high-Tc superconductors.
With improved sample quality, more intricate interlayer correlated states at fractional filling factors arise. Some of them can be described by the integer quantum Hall effect of composite fermions (CFs), with intralayer and interlayer Chern-Simons field coupling. The others corresponding to half integer CF fillings, are regarded as exciton condensation of CFs. Away from equal density, semi-quantized fractional Hall state, where a full CF LL couples to a continuously varying partially filled LL, can be understood by pairing of integer or fractional charged quasi-particles.||