2,799 research outputs found
Magnetoroton scattering by phonons in the fractional quantum Hall regime
Motivated by recent phonon spectroscopy experiments in the fractional quantum
Hall regime we consider processes in which thermally excited magnetoroton
excitations are scattered by low energy phonons. We show that such scattering
processes can never give rise to dissociation of magnetorotons into unbound
charged quasiparticles as had been proposed previously. In addition we show
that scattering of magnetorotons to longer wavelengths by phonon absorption is
possible because of the shape of the magnetoroton dispersion curve and it is
shown that there is a characteristic cross-over temperature above which the
rate of energy transfer to the electron gas changes from an exponential
(activated) to a power law dependence on the effective phonon temperature.Comment: LaTex document, 3 eps figures. submitted to Phys Rev
Detectability of dissipative motion in quantum vacuum via superradiance
We propose an experiment for generating and detecting vacuum-induced
dissipative motion. A high frequency mechanical resonator driven in resonance
is expected to dissipate energy in quantum vacuum via photon emission. The
photons are stored in a high quality electromagnetic cavity and detected
through their interaction with ultracold alkali-metal atoms prepared in an
inverted population of hyperfine states. Superradiant amplification of the
generated photons results in a detectable radio-frequency signal temporally
distinguishable from the expected background.Comment: 4 pages, 2 figure
Collective states in highly symmetric atomic configurations, and single-photon traps
Abbreviated Abstract: We study correlated states in a circular and
linear-chain configuration of identical two-level atoms containing the energy
of a single quasi-resonant photon in the form of a collective excitation, where
the collective behaviour is mediated by exchange of transverse photons between
the atoms. For a circular configuration of atoms the effective Hamiltonian on
the radiationless subspace of the system can be diagonalized analytically. In
this case, the radiationless energy eigenstates carry a quantum
number which is analogous to the angular momentum quantum
number , carried by particles propagating in a central potential,
such as a hydrogen-like system. Just as the hydrogen s-states are the only
electronic wave functions which can occupy the central region of the Coulomb
potential, the quasi-particle corresponding to a collective excitation of the
circular atomic sample can occupy the central atom only for vanishing
quantum number . For large numbers of atoms in a maximally
subradiant state, a critical interatomic distance of emerges both
in the linear-chain and the circular configuration of atoms. The spontaneous
decay rate of the linear configuration exhibits a jump-like "critical"
behaviour for next-neighbour distances close to a half-wavelength. Furthermore,
both the linear-chain and the circular configuration exhibit exponential photon
trapping once the next-neighbour distance becomes less than a half-wavelength,
with the suppression of spontaneous decay being particularly pronounced in the
circular system. In this way, circular configurations containing sufficiently
many atoms may be natural candidates for {\it single-photon traps}.Comment: Invited contribution to "Xth International Conference on Quantum
Optics ICQO 2004" in Minsk, Belarus. To be published in Optics and
Spectroscop
Ab Initio Calculations of the Walls Shear Strength of Carbon Nanotubes
The dependence of the energy of interwall interaction in double-walled carbon
nanotubes (DWNT) on the relative position of walls has been calculated using
the density functional method. This dependence is used to evaluate forces that
are necessary for the relative telescopic motion of walls and to calculate the
shear strength of DWNT for the relative sliding of walls along the nanotube
axis and for their relative rotation about this axis. The possibility of
experimental verification of the obtained results is discussed.Comment: 4 pages, 1 figur
Exact results for interacting electrons in high Landau levels
We study a two-dimensional electron system in a magnetic field with a fermion
hardcore interaction and without disorder. Projecting the Hamiltonian onto the
n-th Landau level, we show that the Hartree-Fock theory is exact in the limit n
\rightarrow \infty, for the high temperature, uniform density phase of an
infinite system; for a finite-size system, it is exact at all temperatures. In
addition, we show that a charge-density wave arises below a transition
temperature T_t. Using Landau theory, we construct a phase diagram which
contains both unidirectional and triangular charge-density wave phases. We
discuss the unidirectional charge-density wave at zero temperature and argue
that quantum fluctuations are unimportant in the large-n limit. Finally, we
discuss the accuracy of the Hartree-Fock approximation for potentials with a
nonzero range such as the Coulomb interaction.Comment: RevTex, 12 pages with figures included in same file; to appear in
Physical Review
A study of minority carrier lifetime versus doping concentration in n‐type GaAs grown by metalorganic chemical vapor deposition
Time‐resolved photoluminescence decay measurements are used to explore minority carrier recombination in n‐type GaAs grown by metalorganic chemical vapor deposition, and doped with selenium to produce electron concentrations from 1.3×1017 cm−3 to 3.8×1018 cm−3. For electron densities n0\u3c1018 cm−3, the lifetime is found to be controlled by radiative recombination and photon recycling with no evidence of Shockley–Read–Hall recombination. For higher electron densities, samples show evidence of Shockley–Read–Hall recombination as reflected in the intensity dependence of the photoluminescence decay. Still, we find that radiative recombination and photon recycling are important for all electron concentrations studied, and no evidence for Auger recombination was observed
Localized states in strong magnetic field: resonant scattering and the Dicke effect
We study the energy spectrum of a system of localized states coupled to a 2D
electron gas in strong magnetic field. If the energy levels of localized states
are close to the electron energy in the plane, the system exhibits a kind of
collective behavior analogous to the Dicke effect in optics. The latter
manifests itself in ``trapping'' of electronic states by localized states. At
the same time, the electronic density of states develops a gap near the
resonance. The gap and the trapping of states appear to be complementary and
reflect an intimate relation between the resonant scattering and the Dicke
effect. We reveal this relation by presenting the exact solution of the problem
for the lowest Landau level. In particular, we show that in the absence of
disorder the system undergoes a phase transition at some critical concentration
of localized states.Comment: 28 pages + 9 fig
Deep learning methods for screening patients' S-ICD implantation eligibility
Subcutaneous Implantable Cardioverter-Defibrillators (S-ICDs) are used for
prevention of sudden cardiac death triggered by ventricular arrhythmias. T Wave
Over Sensing (TWOS) is an inherent risk with S-ICDs which can lead to
inappropriate shocks. A major predictor of TWOS is a high T:R ratio (the ratio
between the amplitudes of the T and R waves). Currently patients'
Electrocardiograms (ECGs) are screened over 10 seconds to measure the T:R
ratio, determining the patients' eligibility for S-ICD implantation. Due to
temporal variations in the T:R ratio, 10 seconds is not long enough to reliably
determine the normal values of a patient's T:R ratio. In this paper, we develop
a convolutional neural network (CNN) based model utilising phase space
reconstruction matrices to predict T:R ratios from 10-second ECG segments
without explicitly locating the R or T waves, thus avoiding the issue of TWOS.
This tool can be used to automatically screen patients over a much longer
period and provide an in-depth description of the behaviour of the T:R ratio
over that period. The tool can also enable much more reliable and descriptive
screenings to better assess patients' eligibility for S-ICD implantation
A Spitzer/IRAC Search for Substellar Companions of the Debris Disk Star epsilon Eridani
We have used the InfraRed Array Camera (IRAC) onboard the Spitzer Space
telescope to search for low mass companions of the nearby debris disk star
epsilon Eridani. The star was observed in two epochs 39 days apart, with
different focal plane rotation to allow the subtraction of the instrumental
Point Spread Function, achieving a maximum sensitivity of 0.01 MJy/sr at 3.6
and 4.5 um, and 0.05 MJy/sr at 5.8 and 8.0 um. This sensitivity is not
sufficient to directly detect scattered or thermal radiation from the epsilon
Eridani debris disk. It is however sufficient to allow the detection of Jovian
planets with mass as low as 1 MJ in the IRAC 4.5 um band. In this band, we
detected over 460 sources within the 5.70 arcmin field of view of our images.
To test if any of these sources could be a low mass companion to epsilon
Eridani, we have compared their colors and magnitudes with models and
photometry of low mass objects. Of the sources detected in at least two IRAC
bands, none fall into the range of mid-IR color and luminosity expected for
cool, 1 Gyr substellar and planetary mass companions of epsilon Eridani, as
determined by both models and observations of field M, L and T dwarf. We
identify three new sources which have detections at 4.5 um only, the lower
limit placed on their [3.6]-[4.5] color consistent with models of planetary
mass objects. Their nature cannot be established with the currently available
data and a new observation at a later epoch will be needed to measure their
proper motion, in order to determine if they are physically associated to
epsilon Eridani.Comment: 36 pages, to be published on The Astrophysical Journal, vol. 647,
August 200
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