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
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
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: