6,833 research outputs found
Thermal budget of superconducting digital circuits at sub-kelvin temperatures
Superconducting single-flux-quantum (SFQ) circuits have so far been developed
and optimized for operation at or above helium temperatures. The SFQ approach,
however, should also provide potentially viable and scalable control and
read-out circuits for Josephson-junction qubits and other applications with
much lower, milli-kelvin, operating temperatures. This paper analyzes the
overheating problem which becomes important in this new temperature range. We
suggest a thermal model of the SFQ circuits at sub-kelvin temperatures and
present experimental results on overheating of electrons and silicon substrate
which support this model. The model establishes quantitative limitations on the
dissipated power both for "local" electron overheating in resistors and
"global" overheating due to ballistic phonon propagation along the substrate.
Possible changes in the thermal design of SFQ circuits in view of the
overheating problem are also discussed.Comment: 10 pages, 8 figures, submitted to J. Appl. Phy
Mixed Quantum/Classical Approach for Description of Molecular Collisions in Astrophysical Environments
An efficient and accurate mixed quantum/classical theory approach for computational treatment of inelastic scattering is extended to describe collision of an atom with a general asymmetric-top rotor polyatomic molecule. Quantum mechanics, employed to describe transitions between the internal states of the molecule, and classical mechanics, employed for description of scattering of the atom, are used in a self-consistent manner. Such calculations for rotational excitation of HCOOCH3 in collisions with He produce accurate results at scattering energies above 15 cm–1, although resonances near threshold, below 5 cm–1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cm–1), which enables calculations for larger molecules and at higher collision energies than was possible previously with the standard full-quantum approach. Theoretical prediction of inelastic cross sections for a number of complex organic molecules observed in space becomes feasible using this new computational tool
Persistent current noise and electron-electron interactions
We analyze fluctuations of persistent current (PC) produced by a charged
quantum particle moving in a ring and interacting with a dissipative
environment formed by diffusive electron gas. We demonstrate that in the
presence of interactions such PC fluctuations persist down to zero temperature.
In the case of weak interactions and/or sufficiently small values of the ring
radius PC noise remains coherent and can be tuned by external magnetic flux
piercing the ring. In the opposite limit of strong interactions and/or
large values of fluctuations in the electronic bath strongly suppress
quantum coherence of the particle down to and induce incoherent
-independent current noise in the ring which persists even at
when the average PC is absent.Comment: 12 pages, 8 figure
Magnetic reconnection during collisionless, stressed, X-point collapse using Particle-in-Cell simulation
Two cases of weakly and strongly stressed X-point collapse were considered.
Here descriptors weakly and strongly refer to 20 % and 124 % unidirectional
spatial compression of the X-point, respectively. In the weakly stressed case,
the reconnection rate, defined as the out-of-plane electric field in the
X-point (the magnetic null) normalised by the product of external magnetic
field and Alfv\'en speeds, peaks at 0.11, with its average over 1.25 Alfv\'en
times being 0.04. Electron energy distribution in the current sheet, at the
high energy end of the spectrum, shows a power law distribution with the index
varying in time, attaining a maximal value of -4.1 at the final simulation time
step (1.25 Alfv\'en times). In the strongly stressed case, magnetic
reconnection peak occurs 3.4 times faster and is more efficient. The peak
reconnection rate now attains value 2.5, with the average reconnection rate
over 1.25 Alfv\'en times being 0.5. The power law energy spectrum for the
electrons in the current sheet attains now a steeper index of -5.5, a value
close to the ones observed in the vicinity of X-type region in the Earth's
magneto-tail. Within about one Alfv\'en time, 2% and 20% of the initial
magnteic energy is converted into heat and accelerated particle energy in the
case of weak and strong stress, respectively. In the both cases, during the
peak of the reconnection, the quadruple out-of-plane magnetic field is
generated, hinting possibly to the Hall regime of the reconnection. These
results strongly suggest the importance of the collionless, stressed X-point
collapse as a possible contributing factor to the solution of the solar coronal
heating problem or more generally, as an efficient mechanism of converting
magnetic energy into heat and super-thermal particle energy.Comment: Final Accepted Version (Physics of Plasmas in Press 2007
Bootstrap and the physical values of resonance parameters
This is the 6th paper in the series developing the formalism to manage the
effective scattering theory of strong interactions. Relying on the theoretical
scheme suggested in our previous publications we concentrate here on the
practical aspect and apply our technique to the elastic pion-nucleon scattering
amplitude. We test numerically the pion-nucleon spectrum sum rules that follow
from the tree level bootstrap constraints. We show how these constraints can be
used to estimate the tensor and vector coupling constants. At last, we
demonstrate that the tree-level low energy expansion coefficients computed in
the framework of our approach show nice agreement with known experimental data.
These results allow us to claim that the extended perturbation scheme is quite
reasonable from the computational point of view.Comment: 41 pages, 7 figure
Fast Switching Ferroelectric Materials for Accelerator Applications
Fast switching (< 10 nsec) measurement results on the recently developed
BST(M) (barium strontium titanium oxide composition with magnesium-based
additions) ferroelectric materials are presented. These materials can be used
as the basis for new advanced technology components suitable for high-gradient
accelerators. A ferroelectric ceramic has an electric field-dependent
dielectric permittivity that can be altered by applying a bias voltage.
Ferroelectric materials offer significant benefits for linear collider
applications, in particular, for switching and control elements where a very
short response time of <10 nsec is required. The measurement results presented
here show that the new BST(M) ceramic exhibits a high tunability factor: a bias
field of 40-50 kV/cm reduces the permittivity by a factor of 1.3-1.5. The
recently developed technology of gold biasing contact deposition on large
diameter (110 cm) thin wall ferroelectric rings allowed ~few nsec switching
times in witness sample experiments. The ferroelectric rings can be used at
high pulsed power (tens of megawatts) for X-band components as well as at high
average power in the range of a few kilowatts for the L-band phase-shifter,
under development for optimization of the ILC rf coupling. Accelerator
applications include fast active X-band and Ka-band high-power ferroelectric
switches, high-power X-band and L-band phase shifters, and tunable
dielectric-loaded accelerating structures.Comment: 7 pages, 6 figures, submitted to Proceedings of 2006 Advanced
Accelerator Concepts Worksho
Detection of quantum light in the presence of noise
Detection of quantum light in the presence of dark counts and background
radiation noise is considered. The corresponding positive operator-valued
measure is obtained and photocounts statistics of quantum light in the presence
of noise is studied.Comment: 4 pages, 1 figure; misprints correcte
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