371 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
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
Non-parabolicity of the conduction band of wurtzite GaN
Using cyclotron resonance, we measure the effective mass, *, of electrons
in AlGaN/GaN heterostructures with densities, cm. From our extensive data, we extrapolate a band edge
mass of . By comparing our * data with the results of a
multi-band \textbf{k.p} calculation we infer that the effect of remote bands is
essential in explaining the observed conduction band non-parabolicity (NP). Our
calculation of polaron mass corrections -- including finite width and screening
- suggests those to be negligible. It implies that the behavior of
can be understood solely in terms of NP. Finally, using our NP and polaron
corrections, we are able to reduce the large scatter in the published band edge
mass values
Infrared spectroscopy of Landau levels in graphene
We report infrared studies of the Landau level (LL) transitions in single
layer graphene. Our specimens are density tunable and show \textit{in situ}
half-integer quantum Hall plateaus. Infrared transmission is measured in
magnetic fields up to B=18 T at selected LL fillings. Resonances between hole
LLs and electron LLs, as well as resonances between hole and electron LLs are
resolved. Their transition energies are proportional to and the
deduced band velocity is m/s. The lack of
precise scaling between different LL transitions indicates considerable
contributions of many-particle effects to the infrared transition energies.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
Electron--Electron Scattering in Quantum Wires and it's Possible Suppression due to Spin Effects
A microscopic picture of electron-electron pair scattering in single mode
quantum wires is introduced which includes electron spin. A new source of
`excess' noise for hot carriers is presented. We show that zero magnetic field
`spin' splitting in quantum wires can lead to a dramatic `spin'-subband
dependence of electron--electron scattering, including the possibility of
strong suppression. As a consequence extremely long electron coherence lengths
and new spin-related phenomena are predicted. Since electron bands in III-V
semiconductor quantum wires are in general spin-split in zero applied magnetic
field, these new transport effects are of general importance.Comment: 11 pages, LaTeX and APS-RevteX 2, Rep.No. GF66,Figures from author,
Physical Review Letters, scheduled for 7 June 199
Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves
Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478
(1997)], we employ semiclassical composite-fermion theory to study the effect
of a periodic density modulation on a quantum Hall system near Landau level
filling factor nu=1/2. We show that even a weak density modulation leads to
dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an
explanation for several key features of the experimental observations. We
predict that properly arranged dc transport measurements would show a structure
similar to that seen in SAW measurements.Comment: Version published in Phys. Rev. Lett. Figures changed to show SAW
velocity shift. LaTeX, 5 pages, two included postscript figure
Evidence for an Anisotropic State of Two-Dimensional Electrons in High Landau Levels
Magneto-transport experiments on high mobility two-dimensional electron gases
in GaAs/AlGaAs heterostructures have revealed striking anomalies near
half-filling of several spin-resolved, yet highly excited, Landau levels. These
anomalies include strong anisotropies and non-linearities of the longitudinal
resistivity rho_xx which commence only below about 150mK. These phenomena are
not seen in the ground or first excited Landau level but begin abruptly in the
third level. Although their origin remains unclear, we speculate that they
reflect the spontaneous development of a generic anisotropic many-electron
state.Comment: 4 pages with 5 figures, to be published in Phys. Rev. Letters, new
figure added (Fig. 4
Explanation for the Resistivity Law in Quantum Hall System
We consider a 2D electron system in a strong magnetic field, where the local
Hall resistivity is a function of position and
is small compared to . Particularly if the
correlations fall off slowly with distance, or if fluctuations exist on several
length scales, one finds that the macroscopic longitudinal resistivity
is only weakly dependent on and is approximately proportional to
the magnitude of fluctuations in . This may provide an explanation
of the empirical law where is
the Hall resistance, and is the magnetic field.Comment: 11 pages (REVTeX 3.0). Revised Version. Complete postscript file for
this paper is available on the World Wide Web at
http://cmtw.harvard.edu/~simon/ ; Preprint number HU-CMT-94S0
Chiral Surface States in the Bulk Quantum Hall Effect
In layered samples which exhibit a bulk quantum Hall effect (QHE), a
two-dimensional (2d) surface ``sheath" of gapless excitations is expected.
These excitations comprise a novel 2d chiral quantum liquid which should
dominate the low temperature transport along the field (z-axis). For the
integer QHE, we show that localization effects are completely absent in the
``sheath", giving a metallic z-axis conductivity. For fractional filling , the ``sheath" is a 2d non-Fermi liquid, with incoherent z-axis transport
and . Experimental implications for the Bechgaard salts
are discussed.Comment: 4 pages, RevTeX 3.0, with two encapsulated postscript figures, which
can be automatically included in-text if desired. The complete postscript
file is available on the WWW at http://www.itp.ucsb.edu/~balents/sheath.p
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