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E-E interaction and Negative Compressibility

Resonant impurities - Negative compressibility - "e-e" Interaction 

→The electronic compressibility K,  being intimately related to the density of states (DOS) and electron-electron (e-e) interactions, is an important property of low dimensional electron systems.

We introduced hydrogenous impurities into monolayer graphene using a precise dosage of electron beam (e-beam) irradiation. We performed global capacitance measurements of the compressibility of the e-beam-irradiated graphene devices using top-gate geometry under different magnetic fields. Importantly, the resonant scattering enhancement of negative compressibility was observed under certain magnetic fields.

Resonant states located in the energy region around the charge neutrality point were probed in e-beam-irradiated graphene capacitors. Theoretical results based on tight-binding and Lifshitz models agreed well with experimental observations of graphene containing a low concentration of resonant impurities. The interaction between resonant states and Landau levels was detected by varying the applied magnetic field. The interaction mechanisms and enhancement of the negative compressibility in disordered graphene are discussed. See our recent paper published in APPLIED PHYSICS LETTERS 102, 203103 (2013).

→ Theoretical calculations have suggested that e–e interactions in graphene depend on both carrier density n and a coupling constant α. α is defined as the ratio of the Coulomb interaction to the kinetic energy of electrons and is used to describe e–e interaction strengths.

The inverse electronic compressibility, defined as the derivative of chemical potential, is intimately related to e–e interactions. One alternative method for retrieving the compressibility is quantum capacitance measurement of a graphene device in the top gate geometry. The top-gate geometry of the capacitance devices is schematically shown in Fig. 1(a). We mechanically exfoliated graphene on p-Si substrates coated with a 300 nm thermal silicon oxide. A thin layer of yttrium was deposited on graphene as the dielectric layers.



Figure 1 (a) Schematic illustration (upper part) and optical image (lower part) of the metal–Y2O3–graphene capacitance device. (b) Measured total capacitance Ctg at different temperatures. The charge-neutrality point is moved to 0 V. (c) HRTEM image of Y2O3 insulator. The thickness of yttrium oxide is 6 nm. (d) The dielectric constant of Y2O3 as a function of temperature measured from the metal–Y2O3–metal structure. The insert shows a schematic view and optical image of the metal–Y2O3–metal structure.

We demonstrate the effects of electron–electron (e–e) interactions in monolayer graphene quantum capacitors. Ultrathin yttrium oxide showed excellent performance as the dielectric layer in top-gate device geometry. The structure and dielectric constant of the yttrium oxide layers have been carefully studied. The inverse compressibility retrieved from the quantum capacitance agreed fairly well with the theoretical predictions for the e–e interactions in monolayer graphene at different temperatures. We found that electron–hole puddles played a significant role in the low-density carrier region in graphene. By considering the temperature-dependent charge fluctuation, we established a model to explain the round-off effect originating from the e–e interactions in monolayer graphene near the Dirac point. See details in:




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Copyright © 2013  Dr. Ning WANG. Department of Physics, The Hong Kong University of Science and Technology. All rights reserved.