5,795 research outputs found
Onsager's Wien Effect on a Lattice
The Second Wien Effect describes the non-linear, non-equilibrium response of
a weak electrolyte in moderate to high electric fields. Onsager's 1934
electrodiffusion theory along with various extensions has been invoked for
systems and phenomena as diverse as solar cells, surfactant solutions, water
splitting reactions, dielectric liquids, electrohydrodynamic flow, water and
ice physics, electrical double layers, non-Ohmic conduction in semiconductors
and oxide glasses, biochemical nerve response and magnetic monopoles in spin
ice. In view of this technological importance and the experimental ubiquity of
such phenomena, it is surprising that Onsager's Wien effect has never been
studied by numerical simulation. Here we present simulations of a lattice
Coulomb gas, treating the widely applicable case of a double equilibrium for
free charge generation. We obtain detailed characterisation of the Wien effect
and confirm the accuracy of the analytical theories as regards the field
evolution of the free charge density and correlations. We also demonstrate that
simulations can uncover further corrections, such as how the field-dependent
conductivity may be influenced by details of microscopic dynamics. We conclude
that lattice simulation offers a powerful means by which to investigate
system-specific corrections to the Onsager theory, and thus constitutes a
valuable tool for detailed theoretical studies of the numerous practical
applications of the Second Wien Effect.Comment: Main: 12 pages, 4 figures. Supplementary Information: 7 page
Path integral Monte Carlo calculations of helium and hydrogen-helium plasma thermodynamics and of the deuterium shock Hugoniot
In this work we calculate the thermodynamic properties of hydrogen-helium
plasmas with different mass fractions of helium by the direct path integral
Monte Carlo method. To avoid unphysical approximations we use the path integral
representation of the density matrix. We pay special attention to the region of
weak coupling and degeneracy and compare the results of simulation with a model
based on the chemical picture. Further with the help of calculated deuterium
isochors we compute the shock Hugoniot of deuterium. We analyze our results in
comparison with recent experimental and calculated data on the deuterium
Hugoniot.Comment: 7 pages, 5 Postscript figures, accepted for publication in J. Phys.
A: Math. Ge
High fidelity quantum memory via dynamical decoupling: theory and experiment
Quantum information processing requires overcoming decoherence---the loss of
"quantumness" due to the inevitable interaction between the quantum system and
its environment. One approach towards a solution is quantum dynamical
decoupling---a method employing strong and frequent pulses applied to the
qubits. Here we report on the first experimental test of the concatenated
dynamical decoupling (CDD) scheme, which invokes recursively constructed pulse
sequences. Using nuclear magnetic resonance, we demonstrate a near order of
magnitude improvement in the decay time of stored quantum states. In
conjunction with recent results on high fidelity quantum gates using CDD, our
results suggest that quantum dynamical decoupling should be used as a first
layer of defense against decoherence in quantum information processing
implementations, and can be a stand-alone solution in the right parameter
regime.Comment: 6 pages, 3 figures. Published version. This paper was initially
entitled "Quantum gates via concatenated dynamical decoupling: theory and
experiment", by Jacob R. West, Daniel A. Lidar, Bryan H. Fong, Mark F. Gyure,
Xinhua Peng, and Dieter Suter. That original version split into two papers:
http://arxiv.org/abs/1012.3433 (theory only) and the current pape
Effects of electrojet turbulence on a magnetosphere-ionosphere simulation of a geomagnetic storm
Ionospheric conductance plays an important role in regulating the response of the magnetosphereâionosphere system to solar wind driving. Typically, models of magnetosphereâionosphere coupling include changes to ionospheric conductance driven by extreme ultraviolet ionization and electron precipitation. This paper shows that effects driven by the FarleyâBuneman instability can also create significant enhancements in the ionospheric conductance, with substantial impacts on geospace. We have implemented a method of including electrojet turbulence (ET) effects into the ionospheric conductance model utilized within geospace simulations. Our particular implementation is tested with simulations of the LyonâFedderâMobarry global magnetosphere model coupled with the Rice Convection Model of the inner magnetosphere. We examine the impact of including ETâmodified conductances in a case study of the geomagnetic storm of 17 March 2013. Simulations with ET show a 13% reduction in the cross polar cap potential at the beginning of the storm and up to 20% increases in the Pedersen and Hall conductance. These simulation results show better agreement with Defense Meteorological Satellite Program observations, including capturing features of subauroral polarization streams. The fieldâaligned current (FAC) patterns show little differences during the peak of storm and agree well with Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) reconstructions. Typically, the simulated FAC densities are stronger and at slightly higher latitudes than shown by AMPERE. The inner magnetospheric pressures derived from TsyganenkoâSitnov empirical magnetic field model show that the inclusion of the ET effects increases the peak pressure and brings the results into better agreement with the empirical model.This material is based upon work supported by NASA grants NNX14AI13G, NNX13AF92G, and NNX16AB80G. The National Center for Atmospheric Research is sponsored by the National Science Foundation. This work used the XSEDE and TACC computational facilities, supported by National Science Foundation grant ACI-1053575. We would like to acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation. We thank the AMPERE team and the AMPERE Science Center for providing the Iridium derived data products. All model output, simulation codes, and analysis routines are being preserved on the NCAR High-Performance Storage System and will be made available upon written request to the lead author of this publication. (NNX14AI13G - NASA; NNX13AF92G - NASA; NNX16AB80G - NASA; National Science Foundation; ACI-1053575 - National Science Foundation
Aperiodic dynamical decoupling sequences in presence of pulse errors
Dynamical decoupling (DD) is a promising tool for preserving the quantum
states of qubits. However, small imperfections in the control pulses can
seriously affect the fidelity of decoupling, and qualitatively change the
evolution of the controlled system at long times. Using both analytical and
numerical tools, we theoretically investigate the effect of the pulse errors
accumulation for two aperiodic DD sequences, the Uhrig's DD UDD) protocol [G.
S. Uhrig, Phys. Rev. Lett. {\bf 98}, 100504 (2007)], and the Quadratic DD (QDD)
protocol [J. R. West, B. H. Fong and D. A. Lidar, Phys. Rev. Lett {\bf 104},
130501 (2010)]. We consider the implementation of these sequences using the
electron spins of phosphorus donors in silicon, where DD sequences are applied
to suppress dephasing of the donor spins. The dependence of the decoupling
fidelity on different initial states of the spins is the focus of our study. We
investigate in detail the initial drop in the DD fidelity, and its long-term
saturation. We also demonstrate that by applying the control pulses along
different directions, the performance of QDD protocols can be noticeably
improved, and explain the reason of such an improvement. Our results can be
useful for future implementations of the aperiodic decoupling protocols, and
for better understanding of the impact of errors on quantum control of spins.Comment: updated reference
Valley splitting of Si/SiGe heterostructures in tilted magnetic fields
We have investigated the valley splitting of two-dimensional electrons in
high quality Si/SiGe heterostructures under tilted magnetic fields.
For all the samples in our study, the valley splitting at filling factor
() is significantly different before and after the
coincidence angle, at which energy levels cross at the Fermi level. On both
sides of the coincidence, a linear density dependence of on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. We argue that screening of the Coulomb
interaction from the low-lying filled levels, which also explains the observed
spin-dependent resistivity, is responsible for the large difference of
before and after the coincidence.Comment: REVTEX 4 pages, 4 figure
Host isotope mass effects on the hyperfine interaction of group-V donors in silicon
The effects of host isotope mass on the hyperfine interaction of group-V
donors in silicon are revealed by pulsed electron nuclear double resonance
(ENDOR) spectroscopy of isotopically engineered Si single crystals. Each of the
hyperfine-split P-31, As-75, Sb-121, Sb-123, and Bi-209 ENDOR lines splits
further into multiple components, whose relative intensities accurately match
the statistical likelihood of the nine possible average Si masses in the four
nearest-neighbor sites due to random occupation by the three stable isotopes
Si-28, Si-29, and Si-30. Further investigation with P-31 donors shows that the
resolved ENDOR components shift linearly with the bulk-averaged Si mass.Comment: 5 pages, 4 figures, 1 tabl
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