3,597 research outputs found
Composite fermions in the Fractional Quantum Hall Effect: Transport at finite wavevector
We consider the conductivity tensor for composite fermions in a close to
half-filled Landau band in the temperature regime where the scattering off the
potential and the trapped gauge field of random impurities dominates. The
Boltzmann equation approach is employed to calculate the quasiclassical
transport properties at finite effective magnetic field, wavevector and
frequency. We present an exact solution of the kinetic equation for all
parameter regimes. Our results allow a consistent description of recently
observed surface acoustic wave resonances and other findings.Comment: REVTEX, 4 pages, 1 figur
Effective mass of composite fermion: a phenomenological fit in with anomalous propagation of surface acoustic wave
We calculate the conductivity associated with the anomalous propagation of a
surface acoustic wave above a two-dimensional electron gas at .
Murthy-Shankar's middle representation is adopted and a contribution to the
response functions beyond the random phase approximation has been taken into
account. We give a phenomenological fit for the effective mass of composite
fermion in with the experimental data of the anomalous propagation of surface
acoustic wave at and find the phenomenological value of the effective
mass is several times larger than the theoretical value
derived from the Hartree-Fock approximation. We
compare our phenomenologically fitting composite fermion effective mass with
those appeared in the measurements of the activation energy and the
Shubnikov-de Haas effect and find that our result is fairly reasonable.Comment: 8 pages, 5 figures, the longer version of cond-mat/9801131 with
crucial corrections, accepted for publication by PR
Experimental Demonstration of Fermi Surface Effects at Filling Factor 5/2
Using small wavelength surface acoustic waves (SAW) on ultra-high mobility
heterostructures, Fermi surface properties are detected at 5/2 filling factor
at temperatures higher than those at which the quantum Hall state forms. An
enhanced conductivity is observed at 5/2 by employing sub 0.5 micron wavelength
SAW, indicating a quasiparticle mean-free-path substantially smaller than that
in the lowest Landau level. These findings are consistent with the presence of
a filled Fermi sea of composite fermions, which may pair at lower temperatures
to form the 5/2 ground state.Comment: 11 pages, 4 figure
Weiss Oscillations in Surface Acoustic Wave Propagation
The interaction of a surface acoustic wave (SAW) with a a two-dimensional
electron gas in a periodic electric potential and a classical magnetic field is
considered. We calculate the attenuation of the SAW and its velocity change and
show that these quantities exhibit Weiss oscillations.Comment: 4 pages REVTEX, 2 figures included as eps file
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
Stability and effective masses of composite-fermions in the first and second Landau Level
We propose a measure of the stability of composite fermions (CF's) at
even-denominator Landau-level filling fractions. Assuming Landau-level mixing
effects are not strong, we show that the CF liquid at in the
Landau level cannot exist and relate this to the absence of a hierarchy of
incompressible states for filling fractions . We find that
a polarized CF liquid should exist at . We also show that, for CF
states, the variation with system size of the ground state energy of
interacting electrons follows that for non-interacting particles in zero
magnetic field. We use this to estimate the CF effective masses.Comment: 9 pages, Revtex, PSIZ-TP-940
Supernova Ejecta in the Youngest Galactic Supernova Remnant G1.9+0.3
G1.9+0.3 is the youngest known Galactic supernova remnant (SNR), with an
estimated supernova (SN) explosion date of about 1900, and most likely located
near the Galactic Center. Only the outermost ejecta layers with free-expansion
velocities larger than about 18,000 km/s have been shocked so far in this
dynamically young, likely Type Ia SNR. A long (980 ks) Chandra observation in
2011 allowed spatially-resolved spectroscopy of heavy-element ejecta. We
denoised Chandra data with the spatio-spectral method of Krishnamurthy et al.,
and used a wavelet-based technique to spatially localize thermal emission
produced by intermediate-mass elements (IMEs: Si and S) and iron. The spatial
distribution of both IMEs and Fe is extremely asymmetric, with the strongest
ejecta emission in the northern rim. Fe Kalpha emission is particularly
prominent there, and fits with thermal models indicate strongly oversolar Fe
abundances. In a localized, outlying region in the northern rim, IMEs are less
abundant than Fe, indicating that undiluted Fe-group elements (including 56Ni)
with velocities larger than 18,000 km/s were ejected by this SN. But in the
inner west rim, we find Si- and S-rich ejecta without any traces of Fe, so
high-velocity products of O-burning were also ejected. G1.9+0.3 appears similar
to energetic Type Ia SNe such as SN 2010jn where iron-group elements at such
high free-expansion velocities have been recently detected. The pronounced
asymmetry in the ejecta distribution and abundance inhomogeneities are best
explained by a strongly asymmetric SN explosion, similar to those produced in
some recent 3D delayed-detonation Type Ia models.Comment: 6 pages, 3 figures, submitted to ApJ Letter
Nonuniform Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3
We report measurements of X-ray expansion of the youngest Galactic supernova
remnant, G1.9+0.3, using Chandra observations in 2007, 2009, and 2011. The
measured rates strongly deviate from uniform expansion, decreasing radially by
about 60% along the X-ray bright SE-NW axis from 0.84% +/- 0.06% per yr to
0.52% +/- 0.03% per yr. This corresponds to undecelerated ages of 120-190 yr,
confirming the young age of G1.9+0.3, and implying a significant deceleration
of the blast wave. The synchrotron-dominated X-ray emission brightens at a rate
of 1.9% +/- 0.4% per yr. We identify bright outer and inner rims with the blast
wave and reverse shock, respectively. Sharp density gradients in either ejecta
or ambient medium are required to produce the sudden deceleration of the
reverse shock or the blast wave implied by the large spread in expansion ages.
The blast wave could have been decelerated recently by an encounter with a
modest density discontinuity in the ambient medium, such as found at a wind
termination shock, requiring strong mass loss in the progenitor. Alternatively,
the reverse shock might have encountered an order-of-magnitude density
discontinuity within the ejecta, such as found in pulsating delayed-detonation
Type Ia models. We demonstrate that the blast wave is much more decelerated
than the reverse shock in these models for remnants at ages similar to
G1.9+0.3. Similar effects may also be produced by dense shells possibly
associated with high-velocity features in Type Ia spectra. Accounting for the
asymmetry of G1.9+0.3 will require more realistic 3D Type Ia models.Comment: 6 pages, 4 figures, accepted for publication in ApJ Letters, minor
revision
A Fermi Fluid Description of the Half-Filled Landau Level
We present a many-body approach to calculate the ground state properties of a
system of electrons in a half-filled Landau level. Our starting point is a
simplified version of the recently proposed trial wave function where one
includes the antisymmetrization operator to the bosonic Laughlin state. Using
the classical plasma analogy, we calculate the pair-correlation function, the
static structure function and the ground state energy in the thermodynamic
limit. These results are in good agreement with the expected behavior at
.Comment: 4 pages, REVTEX, and 4 .ps file
Refining Chandra/ACIS Subpixel Event Repositioning Using a Backside Illuminated CCD Model
Subpixel event repositioning (SER) techniques have been demonstrated to
significantly improve the already unprecedented spatial resolution of Chandra
X-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCD
SER techniques are based on the premise that the impact position of events can
be refined, based on the distribution of charge among affected CCD pixels. ACIS
SER models proposed thus far are restricted to corner split (3- and 4-pixel)
events, and assume that such events take place at the split pixel corners. To
improve the event counting statistics, we modified the ACIS SER algorithms to
include 2-pixel split events and single pixel events, using refined estimates
for photon impact locations. Furthermore, simulations that make use of a
high-fidelity backside illuminated (BI) CCD model demonstrate that mean photon
impact positions for split events are energy dependent leading to further
modification of subpixel event locations according to event type and energy,
for BI ACIS devices. Testing on Chandra CCD X-ray observations of the Orion
Nebula Cluster indicates that these modified SER algorithms further improve the
spatial resolution of Chandra/ACIS, to the extent that the spreading in the
spatial distribution of photons is dominated by the High Resolution Mirror
Assembly, rather than by ACIS pixelization.Comment: 23 pages, 8 figures, 2nd version, submitted to Ap
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