942 research outputs found
Screening in gated bilayer graphene via variational calculus
We analyze the response of bilayer graphene to an external transverse
electric field using a variational method. A previous attempt to do so in a
recent paper by Falkovsky [Phys. Rev. B 80, 113413 (2009)] is shown to be
flawed. Our calculation reaffirms the original results obtained by one of us
[E. McCann, Phys. Rev. B 74, 161403(R) (2006)] by a different method. Finally,
we generalize these original results to describe a dual-gated bilayer graphene
device.Comment: 4 pages, 1 figur
Comment on ``Analytic Structure of One-Dimensional Localization Theory: Re-Examining Mott's Law''
The low-frequency conductivity of a disordered Fermi gas in one spatial
dimension is governed by the Mott-Berezinskii law . In a recent Letter [Phys. Rev. Lett. 84, 1760 (2000)]
A. O. Gogolin claimed that this law is invalid, challenging our basic
understanding of disordered systems and a massive amount of previous
theoretical work. We point out two calculational errors in Gogolin's paper.
Once we correct them, the Mott-Berezinskii formula is fully recovered. We also
present numerical results supporting the Mott-Berezinskii formula but ruling
out that of Gogolin.Comment: 1 page, 1 figure, RevTeX
Dynamics of disordered quantum Hall crystals
Charge density waves are thought to be common in two-dimensional electron
systems in quantizing magnetic fields. Such phases are formed by the
quasiparticles of the topmost occupied Landau level when it is partially
filled. One class of charge density wave phases can be described as electron
solids. In weak magnetic fields (at high Landau levels) solids with many
particles per unit cell - bubble phases - predominate. In strong magnetic
fields (at the lowest Landau level) only crystals with one particle per unit
cell - Wigner crystals - can form. Experimental identification of these phases
is facilitated by the fact that even a weak disorder influences their dc and ac
magnetotransport in a very specific way. In the ac domain, a range of
frequencies appears where the electromagnetic response is dominated by
magnetophonon collective modes. The effect of disorder is to localize the
collective modes and to create an inhomogeneously broadened absorption line,
the pinning mode. In recent microwave experiments pinning modes have been
discovered both at the lowest and at high Landau levels. We present the theory
of the pinning mode for a classical two-dimensional electron crystal
collectively pinned by weak impurities. We show that long-range Coulomb
interaction causes a dramatic line narrowing, in qualitative agreement with the
experiments.Comment: 6 pages, 3 figures. To be presented at EP2DS-15, Nara, Japan. One
typo correcte
The Absence of the Fractional Quantum Hall Effect at High Landau Levels
We compare the energies of the Laughlin liquid and a charge density wave in a
weak magnetic field for the upper Landau level filling factors
and . The charge density wave period has been optimized and was found to
be , where is the cyclotron radius. We conclude that the
optimal charge density wave is more energetically preferable than the Laughlin
liquid for the Landau level numbers at and for at . This implies that the fractional quantum Hall effect
cannot be observed for , in agreement with the experiment.Comment: 12 pages, revtex, 2 PostScript figures are applied. Revised and
corrected version. Also available at http://www.mnhep.umn.edu/~mfogler
Neutrality point of graphene with coplanar charged impurities
The ground-state and the transport properties of graphene subject to the
potential of in-plane charged impurities are studied. The screening of the
impurity potential is shown to be nonlinear, producing a fractal structure of
electron and hole puddles. Statistical properties of this density distribution
as well as the charge compressibility of the system are calculated in the
leading-log approximation. The conductivity depends logarithmically on
, the dimensionless strength of the Coulomb interaction. The theory is
asymptotically exact when is small, which is the case for graphene on
a substrate with a high dielectric constant.Comment: (v3) 4 pages main paper, 2 pages supplementary info, no figure
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