Finite-temperature pseudospin torque effect in graphene bilayers

Gilbert, Matthew J.

doi:10.1103/physrevb.82.165408

Cited by 14 publications

(14 citation statements)

References 46 publications

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“…Top and bottom gates (V TO and V BG) manipulate individual concentrations in each layer, and are set to be VTG = -VBG = 0.4 V which results in a quasiparticle density of 1013 cm-2 in each layer. Contact biases are set in the drag-counterflow geometry to maximize condensate current driven through the system [1].…”

Section: Introductionmentioning

confidence: 99%

“…An excitonic superfluid is predicted to form in double layer graphene systems at room temperature if the two individual mono layers of graphene are separated by an oxide no more than a few nanometers thick [1]. Recent experiments have shown evidence of interaction enhanced transport in double layer graphene [2], but there is a significant discrepancy in the quality of the two graphene layers which may be occluding the phase transition.…”

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confidence: 99%

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Effect of disorder on superfluidity in double layer graphene

Dellabetta

Gilbert

2011

69th Device Research Conference

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Post-CMOS logic in bilayer graphene is very promising due to the possibility of observing room temperature collective states. An excitonic superfluid is predicted to form in double layer graphene systems at room temperature if the two individual mono layers of graphene are separated by an oxide no more than a few nanometers thick [1]. Recent experiments have shown evidence of interaction enhanced transport in double layer graphene [2], but there is a significant discrepancy in the quality of the two graphene layers which may be occluding the phase transition. We present and compare the performance characteristics of ideal and disordered double layer graphene systems at room temperature in the purported regime of superfluidity.We perform quantum transport calculations on double layer graphene using the Non-Equilibrium Green's Function (NEGF) formalism in an effort to elucidate the evolution of a BEC under non-equilibrium conditions in the presence of lattice defects. We find that lattice defects spread throughout the channel can degrade interlayer current by 30%, but disorder concentrated near the contacts causes a much more significant reduction of 80% in interlayer current. We also fmd that steady-state spontaneous coherence is lost for defect concentrations greater than 4%; a very clean system is therefore necessary for potential post-CMOS logic applications.

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Section: Introductionmentioning

confidence: 99%

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confidence: 99%

Effect of disorder on superfluidity in double layer graphene

Dellabetta

Gilbert

2011

69th Device Research Conference

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“…Recent experiments in doublelayer graphene 19 have shown tantalizing signs of interlayer coherence but no signs of a dipolar exciton condensate which would manifest itself through a number of observables such as the presence of an interlayer critical current. 12,20 In this Brief Report, we use a fermionic path-integral quantum Monte Carlo (PIMC) 21,22 framework to elucidate the role of screening in exciton condensates formed in symmetric electron-hole double-layer systems. We show that increased fermion flavor does increase the screening in exciton condensates formed in a symmetric double-layer system.…”

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confidence: 99%

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

Shumway¹,

Gilbert²

2012

Phys. Rev. B

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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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“…Beyond the interesting correlated physics these systems demonstrate, they continue to harbor tantalizing prospects for ultra-efficient, electrically-tunable information processing systems based on predictions of elevated Kosterlitz-Thouless transition temperatures (T c ) without the need for external magnetic fields to quench the kinetic energy 17 . These prospects may be directly traced to the realization of new Dirac material systems such as graphene [18][19][20] and time-reversal invariant topological insulators [21][22][23][24][25] . In particular, recent experimental work in monolayers of graphene separated by hexagonal boron nitride show signatures of correlated behavior well-above cryogenic temperatures 26 .…”

Section: Introductionmentioning

confidence: 99%

Voltage-induced dynamical quantum phase transitions in exciton condensates

2016

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We explore non-analytic quantum phase dynamics of dipolar exciton condensates formed in a system of 2D quantum layers subjected to voltage quenches. We map the exciton condensate physics on to the pseudospin ferromagnet model showing an additional oscillatory metastable phase beyond the well-known ferromagnetic phase by utilizing a time-dependent, non-perturbative theoretical model. We explain the coherent phase of the exciton condensate in quantum Hall bilayers, observed for currents equal to and slightly larger than the critical current, as a stable time-dependent phase characterized by persistent flow of charged order parameter defect in each of the individual layers with a characteristic AC Josephson frequency. As the magnitude of the voltage quench is further increased, we find that the time-dependent current oscillations associated with the charged order parameter defect flow decay, resulting in a transient pseudospin paramagnet phase characterized by partially coherent charge transfer between layers, before the state relaxes to incoherent charge transfer between the layers.

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Finite-temperature pseudospin torque effect in graphene bilayers

Cited by 14 publications

References 46 publications

Effect of disorder on superfluidity in double layer graphene

Effect of disorder on superfluidity in double layer graphene

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

Voltage-induced dynamical quantum phase transitions in exciton condensates

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