316 research outputs found
PSpectRe: A Pseudo-Spectral Code for (P)reheating
PSpectRe is a C++ program that uses Fourier-space pseudo-spectral methods to
evolve interacting scalar fields in an expanding universe. PSpectRe is
optimized for the analysis of parametric resonance in the post-inflationary
universe, and provides an alternative to finite differencing codes, such as
Defrost and LatticeEasy. PSpectRe has both second- (Velocity-Verlet) and
fourth-order (Runge-Kutta) time integrators. Given the same number of spatial
points and/or momentum modes, PSpectRe is not significantly slower than finite
differencing codes, despite the need for multiple Fourier transforms at each
timestep, and exhibits excellent energy conservation. Further, by computing the
post-resonance equation of state, we show that in some circumstances PSpectRe
obtains reliable results while using substantially fewer points than a finite
differencing code. PSpectRe is designed to be easily extended to other problems
in early-universe cosmology, including the generation of gravitational waves
during phase transitions and pre-inflationary bubble collisions. Specific
applications of this code will be pursued in future work.Comment: 22 pages; source code for PSpectRe available:
http://easther.physics.yale.edu v2 Typos fixed, minor improvements to
wording; v3 updated as per referee comment
MHSP in reversed-biased operation mode for ion blocking in gas-avalanche multipliers
We present recent results on the operation of gas-avalanche detectors
comprising a cascade of gas electron multipliers (GEMs) and Micro-Hole and
Strip Plates (MHSPs) multiplier operated in reversed-bias (R-MHSP) mode. The
operation mechanism of the R-MHSP is explained and its potential contribution
to ion-backflow (IBF) reduction is demonstrated. IBF values of 4E-3 were
obtained in cascaded R-MHSP and GEM multipliers at gains of about 1E+4, though
at the expense of reduced effective gain in the first R- MHSP multiplier in the
cascade.Comment: 23 pages, 8 figure
Popular matchings in the marriage and roommates problems
Popular matchings have recently been a subject of study in the context of the so-called House Allocation Problem, where the objective is to match applicants to houses over which the applicants have preferences. A matching M is called popular if there is no other matching MâČ with the property that more applicants prefer their allocation in MâČ to their allocation in M. In this paper we study popular matchings in the context of the Roommates Problem, including its special (bipartite) case, the Marriage Problem. We investigate the relationship between popularity and stability, and describe efficient algorithms to test a matching for popularity in these settings. We also show that, when ties are permitted in the preferences, it is NP-hard to determine whether a popular matching exists in both the Roommates and Marriage cases
Effect of the Surface on the Electron Quantum Size Levels and Electron g-Factor in Spherical Semiconductor Nanocrystals
The structure of the electron quantum size levels in spherical nanocrystals
is studied in the framework of an eight--band effective mass model at zero and
weak magnetic fields. The effect of the nanocrystal surface is modeled through
the boundary condition imposed on the envelope wave function at the surface. We
show that the spin--orbit splitting of the valence band leads to the
surface--induced spin--orbit splitting of the excited conduction band states
and to the additional surface--induced magnetic moment for electrons in bare
nanocrystals. This additional magnetic moment manifests itself in a nonzero
surface contribution to the linear Zeeman splitting of all quantum size energy
levels including the ground 1S electron state. The fitting of the size
dependence of the ground state electron g factor in CdSe nanocrystals has
allowed us to determine the appropriate surface parameter of the boundary
conditions. The structure of the excited electron states is considered in the
limits of weak and strong magnetic fields.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
Theory of nuclear induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots
We propose a model for spectral diffusion of localized spins in
semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each
nuclear spin flip-flop is assumed to be independent, the rate for this process
being calculated by a method of moments. Our calculated spin decoherence time
ms for donor electron spins in Si:P is a factor of two longer than
spin echo decay measurements. For P nuclear spins we show that spectral
diffusion is well into the motional narrowing regime. The calculation for GaAs
quantum dots gives s depending on the quantum dot size. Our
theory indicates that nuclear induced spectral diffusion should not be a
serious problem in developing spin-based semiconductor quantum computer
architectures.Comment: 15 pages, 9 figures. Accepted for publication in Phys. Rev.
On the Potential of the Imaging Atmospheric Cherenkov Technique for Study of the Mass Composition of Primary Cosmic Radiation in the Energy Region above 30 TeV
We suggest a new approach to study the cosmis ray (CR) mass composition in
the energy region from 30 TeV/nucleus up to the "knee" region, i.e. up to a few
PeV/nucleus, using an array of imaging atmospheric Cherenkov telescopes (IACTs)
of a special architecture. This array consists of telescopes with a relatively
small mirror size (~10 square meters) separated from each other by large
distances (~500 meters) and equipped by multichannel cameras with a modest
pixel size (0.3-0.5 degree) and a sufficiently large viewing angle (6-7
degree).
Compared to traditional IACT systems (like HEGRA, HESS or VERITAS) the IACT
array considered here could provide a very large detection area (several square
kilometers or more). At the same time, it allows an accurate measurement of the
energy of CR induced air showers (the energy resolution ranges within 25-35%)
and an effective separation of air showers created by different nuclei.
Particularly, it is possible to enrich air showers belonging to the nucleus
group assigned for selection up to ~90% purity at a detection efficiency of
15-20% of such showers.Comment: 28 pages, 12 figures, accepted for publication in Nucl. Instr. Met
Radiation-induced oscillatory magnetoresistance as a sensitive probe of the zero-field spin splitting in high mobility GaAs/AlGaAs devices
We suggest an approach for characterizing the zero-field spin splitting of
high mobility two-dimensional electron systems, when beats are not readily
observable in the Shubnikov-de Haas effect. The zero-field spin splitting and
the effective magnetic field seen in the reference frame of the electron is
evaluated from a quantitative study of beats observed in radiation-induced
magnetoresistance oscillations.Comment: 4 pages, 4 color figure
FIESTA 2: parallelizeable multiloop numerical calculations
The program FIESTA has been completely rewritten. Now it can be used not only
as a tool to evaluate Feynman integrals numerically, but also to expand Feynman
integrals automatically in limits of momenta and masses with the use of sector
decompositions and Mellin-Barnes representations. Other important improvements
to the code are complete parallelization (even to multiple computers),
high-precision arithmetics (allowing to calculate integrals which were undoable
before), new integrators and Speer sectors as a strategy, the possibility to
evaluate more general parametric integrals.Comment: 31 pages, 5 figure
Spin-Lattice Relaxation in Si Quantum Dots
We consider spin-lattice relaxation processes for electrons trapped in
lateral Si quantum dots in a inversion layer. Such dots are
characterized by strong confinement in the direction perpendicular to the
surface and much weaker confinement in the lateral direction. The spin
relaxation is assumed to be due to the modulation of electron -factor by the
phonon-induced strain, as was shown previously for the shallow donors. The
results clearly indicate that the specific valley structure of the ground
electron state in Si quantum dots causes strong anisotropy for both the
one-phonon and two-phonon spin relaxation rates. In addition, it gives rise to
a partial suppression of the two-phonon relaxation in comparison to the spin
relaxation of donor electrons.Comment: RevTex file, 3 PS figure
Electron spin as a spectrometer of nuclear spin noise and other fluctuations
This chapter describes the relationship between low frequency noise and
coherence decay of localized spins in semiconductors. Section 2 establishes a
direct relationship between an arbitrary noise spectral function and spin
coherence as measured by a number of pulse spin resonance sequences. Section 3
describes the electron-nuclear spin Hamiltonian, including isotropic and
anisotropic hyperfine interactions, inter-nuclear dipolar interactions, and the
effective Hamiltonian for nuclear-nuclear coupling mediated by the electron
spin hyperfine interaction. Section 4 describes a microscopic calculation of
the nuclear spin noise spectrum arising due to nuclear spin dipolar flip-flops
with quasiparticle broadening included. Section 5 compares our explicit
numerical results to electron spin echo decay experiments for phosphorus doped
silicon in natural and nuclear spin enriched samples.Comment: Book chapter in "Electron spin resonance and related phenomena in low
dimensional structures", edited by Marco Fanciulli. To be published by
Springer-Verlag in the TAP series. 35 pages, 9 figure
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