7,656 research outputs found
Relaxation of hole spins in quantum dots via two-phonon processes
We investigate theoretically spin relaxation in heavy hole quantum dots in
low external magnetic fields. We demonstrate that two-phonon processes and
spin-orbit interaction are experimentally relevant and provide an explanation
for the recently observed saturation of the spin relaxation rate in heavy hole
quantum dots with vanishing magnetic fields. We propose further experiments to
identify the relevant spin relaxation mechanisms in low magnetic fields.Comment: 5 pages, 2 figure
How Geometry Controls the Tearing of Adhesive Thin Films on Curved Surfaces
Flaps can be detached from a thin film glued on a solid substrate by tearing
and peeling. For flat substrates, it has been shown that these flaps
spontaneously narrow and collapse in pointy triangular shapes. Here we show
that various shapes, triangular, elliptic, acuminate or spatulate, can be
observed for the tears by adjusting the curvature of the substrate. From
combined experiments and theoretical models, we show that the flap morphology
is governed by simple geometric rules.Comment: 6 pages, 5 figure
Observation of Droplet Size Oscillations in a Two-Phase Fluid under Shear Flow
Experimental observations of droplet size sustained oscillations are reported
in a two-phase flow between a lamellar and a sponge phase. Under shear flow,
this system presents two different steady states made of monodisperse
multilamellar droplets, separated by a shear-thinning transition. At low and
high shear rates, the droplet size results from a balance between surface
tension and viscous stress whereas for intermediate shear rates, it becomes a
periodic function of time. A possible mechanism for such kind of oscillations
is discussed
Violation of action--reaction and self-forces induced by nonequilibrium fluctuations
We show that the extension of Casimir-like forces to fluctuating fluids
driven out of equilibrium can exhibit two interrelated phenomena forbidden at
equilibrium: self-forces can be induced on single asymmetric objects and the
action--reaction principle between two objects can be violated. These effects
originate in asymmetric restrictions imposed by the objects' boundaries on the
fluid's fluctuations. They are not ruled out by the second law of
thermodynamics since the fluid is in a nonequilibrium state. Considering a
simple reaction--diffusion model for the fluid, we explicitly calculate the
self-force induced on a deformed circle. We also show that the action--reaction
principle does not apply for the internal Casimir forces exerting between a
circle and a plate. Their sum, instead of vanishing, provides the self-force on
the circle-plate assembly.Comment: 4 pages, 1 figure. V2: New title; Abstract partially rewritten;
Largely enhanced introductory and concluding remarks (incl. new Refs.
Thermal quantum electrodynamics of nonrelativistic charged fluids
The theory relevant to the study of matter in equilibrium with the radiation
field is thermal quantum electrodynamics (TQED). We present a formulation of
the theory, suitable for non relativistic fluids, based on a joint functional
integral representation of matter and field variables. In this formalism
cluster expansion techniques of classical statistical mechanics become
operative. They provide an alternative to the usual Feynman diagrammatics in
many-body problems which is not perturbative with respect to the coupling
constant. As an application we show that the effective Coulomb interaction
between quantum charges is partially screened by thermalized photons at large
distances. More precisely one observes an exact cancellation of the dipolar
electric part of the interaction, so that the asymptotic particle density
correlation is now determined by relativistic effects. It has still the
decay typical for quantum charges, but with an amplitude strongly
reduced by a relativistic factor.Comment: 32 pages, 0 figures. 2nd versio
Equilibrium correlations in charged fluids coupled to the radiation field
We provide an exact microscopic statistical treatment of particle and field
correlations in a system of quantum charges in equilibrium with a classical
radiation field. Using the Feynman-Kac-Ito representation of the Gibbs weight,
the system of particles is mapped onto a collection of random charged wires.
The field degrees of freedom can be integrated out, providing an effective
pairwise magnetic potential. We then calculate the contribution of the
transverse field coupling to the large-distance particle correlations. The
asymptotics of the field correlations in the plasma are also exactly
determined.Comment: 31 pages, 0 figures. PACS 05.30.-d, 05.40.-a, 11.10.Wx. Changes:
Improved comparison with existing literature on field correlations. Added
Concluding Remarks. References update
The mixed analog/digital shaper of the LHCb preshower
The LHCb preshower signals show so many fluctuations at low energy that a classical shaping is not usable at all. Thanks to the fact that the fraction of the collected energy during a whole LHC beam crossing time is 85%, we studied the special solution we presented at Snowmass 1999 workshop. This solution consists of 2 interleaved fast integrators, one being in integrate mode when the other is digitally reset. Two track-and-hold systems and an analog multiplexer are used to give at the output 85% of the signal plus 15% of the previous one. These 15% are digitally computed from the previous sample, and subtracted. A completely new design of this solution had to be made. This new design is described, including new methods to decrease the supply voltage and the noise, as well as to increase the quality of the reset and the linearity. An output stage, consisting of an AB class push-pull using only NPN transistors is also described. Laboratory and beam test results are given. (5 refs)
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