61,639 research outputs found
Rabi oscillations of a qubit coupled to a two-level system
The problem of Rabi oscillations in a qubit coupled to a fluctuator and in
contact with a heath bath is considered. A scheme is developed for taking into
account both phase and energy relaxation in a phenomenological way, while
taking full account of the quantum dynamics of the four-level system subject to
a driving AC field. Significant suppression of the Rabi oscillations is found
when the qubit and fluctuator are close to resonance. The effect of the
fluctuator state on the read-out signal is discussed. This effect is shown to
modify the observed signal significantly. This may be relevant to recent
experiments by Simmonds et al. [Phys. Rev. Lett. 93, 077003 (2004)].Comment: 4 pages, 4 figure
Magnetic susceptibility of ultra-small superconductor grains
For assemblies of superconductor nanograins, the magnetic response is
analyzed as a function of both temperature and magnetic field. In order to
describe the interaction energy of electron pairs for a huge number of
many-particle states, involved in calculations, we develop a simple
approximation, based on the Richardson solution for the reduced BCS Hamiltonian
and applicable over a wide range of the grain sizes and interaction strengths
at arbitrary distributions of single-electron energy levels in a grain. Our
study is focused upon ultra-small grains, where both the mean value of the
nearest-neighbor spacing of single-electron energy levels in a grain and
variations of this spacing from grain to grain significantly exceed the
superconducting gap in bulk samples of the same material. For these ultra-small
superconductor grains, the overall profiles of the magnetic susceptibility as a
function of magnetic field and temperature are demonstrated to be qualitatively
different from those for normal grains. We show that the analyzed signatures of
pairing correlations are sufficiently stable with respect to variations of the
average value of the grain size and its dispersion over an assembly of
nanograins. The presence of these signatures does not depend on a particular
choice of statistics, obeyed by single-electron energy levels in grains.Comment: 40 pages, 12 figures, submitted to Phys. Rev. B, E-mail addresses:
[email protected], [email protected], [email protected]
Acoustic Probing of the Jamming Transition in an Unconsolidated Granular Medium
Experiments with acoustic waves guided along the mechanically free surface of
an unconsolidated granular packed structure provide information on the
elasticity of granular media at very low pressures that are naturally
controlled by the gravitational acceleration and the depth beneath the surface.
Comparison of the determined dispersion relations for guided surface acoustic
modes with a theoretical model reveals the dependencies of the elastic moduli
of the granular medium on pressure. The experiments confirm recent theoretical
predictions that relaxation of the disordered granular packing through
non-affine motion leads to a peculiar scaling of shear rigidity with pressure
near the jamming transition corresponding to zero pressure. Unexpectedly, and
in disagreement with the most of the available theories, the bulk modulus
depends on pressure in a very similar way to the shear modulus
Electron spin resonance investigation of Mn^{2+} ions and their dynamics in manganese doped SrTiO_3
Using electron spin resonance, lattice position and dynamic properties of
Mn2+ ions were studied in 0.5 and 2 % manganese doped SrTiO3 ceramics prepared
by conventional mixed oxide method. The measurements showed that Mn2+ ions
substitute preferably up to 97 % for Sr if the ceramics is prepared with a
deficit of Sr ions. Motional narrowing of the Mn2+ ESR spectrum was observed
when temperature increases from 120 K to 240-250 K that was explained as a
manifestation of off-center position of this ion at the Sr site. From the
analysis of the ESR spectra the activation energy Ea = 86 mV and frequency
factor 1/?0 ? (2-10)x10^(-14) 1/s for jumping of the impurity between
symmetrical off-center positions were determined. Both values are in agreement
with those derived previously from dielectric relaxation. This proves the
origin of dielectric anomalies in SrTiO3:Mn as those produced by the
reorientation dynamics of Mn2+ dipoles.Comment: 16 pages, 6 figure
Analytic results for particles with interaction in two dimensions and an external magnetic field
The -dimensional quantum problem of particles (e.g. electrons) with
interaction in a two-dimensional parabolic potential
(e.g. quantum dot) and magnetic field , reduces exactly to solving a
-dimensional problem which is independent of and . An
exact, infinite set of relative mode excitations are obtained for any . The
problem reduces to that of a ficticious particle in a two-dimensional,
non-linear potential of strength , subject to a ficticious magnetic
field , the relative angular momentum.Comment: To appear in Physical Review Letters (in press). RevTeX file. Two
figures available from [email protected] or
[email protected]
Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings
Granular packings slowly driven towards their instability threshold are
studied using a digital imaging technique as well as a nonlinear acoustic
method. The former method allows us to study grain rearrangements on the
surface during the tilting and the latter enables to selectively probe the
modifications of the weak-contact fraction in the material bulk. Gradual ageing
of both the surface activity and the weak-contact reconfigurations is observed
as a result of repeated tilt cycles up to a given angle smaller than the angle
of avalanche. For an aged configuration reached after several consecutive tilt
cycles, abrupt resumption of the on-surface activity and of the weak-contact
rearrangements occurs when the packing is subsequently inclined beyond the
previous maximal tilting angle. This behavior is compared with literature
results from numerical simulations of inclined 2D packings. It is also found
that the aged weak-contact configurations exhibit spontaneous restoration
towards the initial state if the packing remains at rest for tens of minutes.
When the packing is titled forth and back between zero and near-critical
angles, instead of ageing, the weak-contact configuration exhibits "internal
weak-contact avalanches" in the vicinity of both the near-critical and zero
angles. By contrast, the stronger-contact skeleton remains stable
Preparing and probing atomic number states with an atom interferometer
We describe the controlled loading and measurement of number-squeezed states
and Poisson states of atoms in individual sites of a double well optical
lattice. These states are input to an atom interferometer that is realized by
symmetrically splitting individual lattice sites into double wells, allowing
atoms in individual sites to evolve independently. The two paths then
interfere, creating a matter-wave double-slit diffraction pattern. The time
evolution of the double-slit diffraction pattern is used to measure the number
statistics of the input state. The flexibility of our double well lattice
provides a means to detect the presence of empty lattice sites, an important
and so far unmeasured factor in determining the purity of a Mott state
Optics with Quantum Hall Skyrmions
A novel type of charged excitation, known as a Skyrmion, has recently been
discovered in quantum Hall systems with filling factor near \nu = 1. A Skyrmion
-- which can be thought of as a topological twist in the spin density of the
electron gas -- has the same charge as an electron, but a much larger spin. In
this review we present a detailed theoretical investigation of the optical
properties of Skyrmions. Our results provide means for the optical detection of
Skyrmions using photoluminescence (PL) spectroscopy. We first consider the
optical properties of Skyrmions in disordered systems. A calculation of the
luminescence energy reveals a special optical signature which allows us to
distinguish between Skyrmions and ordinary electrons. Two experiments to
measure the optical signature are proposed. We then turn to the optical
properties of Skyrmions in pure systems. We show that, just like an ordinary
electron, a Skyrmion may bind with a hole to form a Skyrmionic exciton. The
Skyrmionic exciton can have a lower energy than the ordinary magnetoexciton.
The optical signature of Skyrmions is found to be a robust feature of the PL
spectrum in both disordered and pure systems.Comment: 31 pages, LaTex, 11 eps figures. ijmpb style file included. Review
article submitted to Int. J. Mod. Phys.
Mathematical Models for Estimating the Risk of vCJD Transmission
We present two different simple models for vCJD transmission by blood transfusion. Both models indicate that transfusions alone are unlikely to cause more than a few infections, unless the number of primary cases increases.
To improve our models, future work should pursue data collection, empirical estimation of the model parameters, and examination of the underlying assumptions of our frameworks.
Further improvements could also include examining susceptibility to vCJD infection by age group and iatrogenic infections introduced through surgical instruments. Regarding the latter, it may be worthwhile to conduct experiments to quantify the transmission of prions from an infected surgical instrument after repeated sterilization procedures
Studies of sidewall boundary layer in the Langley 0.3 meter transonic cryogenic tunnel with and without suction
Boundary layer measurements on the sidewalls of the Langley 0.3 Meter Transonic Cryogenic Tunnel were made to determine the effectiveness of the passive boundary layer bleed system over a Reynolds number range from 20 to 200 x 10 to the sixth power per meter at Mach numbers from 0.30 to 0.76. The tunnel sidewall boundary layer displacement thickness was about 2 percent of the width of the test section without the boundary layer bleed. Measured velocity profiles correlated well with the defect law of Hama. With the boundary layer bleed equivalent to about 2 percent of the test section mass flow, the boundary layer displacement thickness reduced to about 1 percent of the test section width, which is generally considered acceptable for testing airfoils. It was also noticed that effectiveness of the bleed was nearly independent of the Mach number and Reynolds number over the range of conditions tested. A comparison of the measured suction effectiveness of the bleed with the finite difference and integral methods of boundary layer calculation showed good agreement
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