Effects of fermion flavor on exciton condensation in double-layer systems

Abstract

We use a fermionic path-integral quantum Monte Carlo framework to study the effects of fermion flavor on the physical properties of dipolar exciton condensates in double-layer systems. We find that by including spin in the system the effective interlayer interaction strength is weakened, yet this has very little effect on the Kosterlitz-Thouless transition temperature. We further find that, to obtain the correct description of screening, it is necessary to account for correlation in both the interlayer and intralayer interactions. We show that while the excitonic binding cannot completely suppress screening by additional fermion flavors, their screening effectiveness is reduced by intralayer correlations leading to much higher transition temperatures than predicted with large-N analysis. PACS number(s): 71.35.Lk, 71.10.−w Dipolar fermionic condensates have been a topic of great interest in a diverse assortment of physical systems for many years. Recently, double-layer systems-two spatially segregated quantum systems-have provided a fruitful playground in which to study dipolar superfluidity in microcavities, 1,2 cold-atom systems, 3-7 and semiconductor quantum wells. [8][9][10][11][12][13] Interest in dipolar superfluids has received increased attention due in large part to the prediction of dipolar superfluid behavior at or above room temperature in double-layer graphene. 14-16 This is uniquely possible in graphene due to the symmetric linear band structure and ability to sustain large carrier concentrations in two closely spaced layers. Yet, this prediction is not without significant controversy. As superfluidity is predicted to occur in the double-layer graphene system outside of the quantum Hall regime, additional fermion flavors, or degrees of freedom, beyond the top-or bottom-layer freedom may participate in the phase transition. Theoretical disagreements over the Kosterlitz-Thouless transition temperature (T KT ) arise from differing assumptions about the importance of these extra flavors for screening in dipolar exciton condensates. In the works predicting a high transition temperature of T KT ≈ 0.1 T F (where T F is the system Fermi temperature) the fermionic degrees of freedom in the system were taken to be constrained by the large energy gap formed when the interacting electrons and holes bind. The quasiparticles lose their individual fermionic character and cannot screen interlayer interactions, which does not result in a significantly lower T KT . Other works on the same double-layer graphene system predict a low transition temperature of T KT ≈ 10 −7 T F by assuming that screening from additional degrees of freedom add independently to effectively screen out the interlayer interaction. 17,18 This assumption leads to the conclusion that screening in condensate is as strong as in the normal phase and results in the small value for T F . While experiment will be the ultimate arbiter of the value of T KT , many-body theoretical approaches will play a significant role in understanding the nature of the p...

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