518 research outputs found
The Nature of Quantum Hall States near the Charge Neutral Dirac Point in Graphene
We investigate the quantum Hall (QH) states near the charge neutral Dirac
point of a high mobility graphene sample in high magnetic fields. We find that
the QH states at filling factors depend only on the perpendicular
component of the field with respect to the graphene plane, indicating them to
be not spin-related. A non-linear magnetic field dependence of the activation
energy gap at filling factor suggests a many-body origin. We therefore
propose that the and states arise from the lifting of the spin
and sub-lattice degeneracy of the LL, respectively.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Energy Spectra for Fractional Quantum Hall States
Fractional quantum Hall states (FQHS) with the filling factor nu = p/q of q <
21 are examined and their energies are calculated. The classical Coulomb energy
is evaluated among many electrons; that energy is linearly dependent on 1/nu.
The residual binding energies are also evaluated. The electron pair in nearest
Landau-orbitals is more affected via Coulomb transition than an electron pair
in non-nearest orbitals. Each nearest electron pair can transfer to some empty
orbital pair, but it cannot transfer to the other empty orbital pair because of
conservation of momentum. Counting the numbers of the allowed and forbidden
transitions, the binding energies are evaluated for filling factors of 126
fraction numbers. Gathering the classical Coulomb energy and the pair
transition energy, we obtain the spectrum of energy versus nu. This energy
spectrum elucidates the precise confinement of Hall resistance at specific
fractional filling factors.Comment: 5 pages, 3 figure
Electron Scattering in AlGaN/GaN Structures
We present data on mobility lifetime, , quantum lifetime, ,
and cyclotron resonance lifetime, , of a sequence of high-mobility
two-dimensional electron gases in the AlGaN/GaN system, covering a density
range of cm. We observe a large discrepancy
between and (/6) and explain it as
the result of density fluctuations of only a few percent. Therefore, only
--and not -- is a reliable measure of the time between
electron scattering events in these specimens. The ratio
increases with increasing density in this series of samples, but scattering
over this density range remains predominantly in the large-angle scattering
regime
Electron Depletion Due to Bias of a T-Shaped Field-Effect Transistor
A T-shaped field-effect transistor, made out of a pair of two-dimensional
electron gases, is modeled and studied. A simple numerical model is developed
to study the electron distribution vs. applied gate voltage for different gate
lengths. The model is then improved to account for depletion and the width of
the two-dimensional electron gases. The results are then compared to the
experimental ones and to some approximate analytical calculations and are found
to be in good agreement with them.Comment: 16 pages, LaTex (RevTex), 8 fig
Limit to 2D mobility in modulation-doped GaAs quantum structures: How to achieve a mobility of 100 millions
Considering scattering by unintentional background charged impurities and by
charged dopants in the modulation doping layer as well as by GaAs acoustic
phonons, we theoretically consider the practical intrinsic (phonons) and
extrinsic (background and dopants) limits to carrier mobility in modulation
doped AlGaAs-GaAs 2D semiconductor structures. We find that reducing background
impurity density to cm along with a modulation doping
separation of 1000 \AA or above will achieve a mobility of
cm/Vs at a carrier density of cm for T=1K. At T=4
(10)K, however, the hard limit to the 2D mobility would be set by acoustic
phonon scattering with the maximum intrinsic mobility being no higher than 22
cm/Vs. Detailed numerical results are presented as a
function of carrier density, modulation doping distance, and temperature to
provide a quantitative guide to experimental efforts for achieving ultra-high
2D mobilities.Comment: 6 pages, 6 figure
Formation of a high quality two-dimensional electron gas on cleaved GaAs
We have succeeded in fabricating a two-dimensional electron gas (2DEG) on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE). A (100) wafer previously prepared by MBE growth is reinstalled in the MBE chamber so that an in situ cleave exposes a fresh (110) GaAs edge for further MBE overgrowth. A sequence of Si-doped AlGaAs layers completes the modulation-doped structure at the cleaved edge. Mobilities as high as 6.1×10^5 cm^2/V s are measured in the 2DEG at the cleaved interface
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