286 research outputs found
A European Drought Reference Database: Design and Online Implementation
This report presents the structure and status of the online European Drought Reference (EDR) database. This website provides detailed historical information regarding major historical European drought events. Each drought event is summarized using climatological drought indices, hydrological drought indices, and user-generated drought impacts. The database currently highlights 11 drought events, from 1959 to 2007. In addition, an online tool is provided to query and view climatological drought indices for any day in the available historical record. The EDR database is tool designed to compile drought statistics in a usable manner and will continually improve as more data becomes available. It is our hope that the EDR database can become a standard reference tool which improve public awareness of drought issues and stimulate data collection, sharing, and analysis
Electron Exchange Coupling for Single Donor Solid-State Qubits
Inter-valley interference between degenerate conduction band minima has been
shown to lead to oscillations in the exchange energy between neighbouring
phosphorus donor electron states in silicon \cite{Koiller02,Koiller02A}. These
same effects lead to an extreme sensitivity of the exchange energy on the
relative orientation of the donor atoms, an issue of crucial importance in the
construction silicon-based spin quantum computers. In this article we calculate
the donor electron exchange coupling as a function of donor position
incorporating the full Bloch structure of the Kohn-Luttinger electron
wavefunctions. It is found that due to the rapidly oscillating nature of the
terms they produce, the periodic part of the Bloch functions can be safely
ignored in the Heitler-London integrals as was done by Koiller et. al. [Phys.
Rev. Lett. 88,027903(2002),Phys. Rev. B. 66,115201(2002)], significantly
reducing the complexity of calculations.
We address issues of fabrication and calculate the expected exchange coupling
between neighbouring donors that have been implanted into the silicon substrate
using an 15keV ion beam in the so-called 'top down' fabrication scheme for a
Kane solid-state quantum computer. In addition we calculate the exchange
coupling as a function of the voltage bias on control gates used to manipulate
the electron wavefunctions and implement quantum logic operations in the Kane
proposal, and find that these gate biases can be used to both increase and
decrease the magnitude of the exchange coupling between neighbouring donor
electrons. The zero-bias results reconfirm those previously obtained by
Koiller.Comment: 10 Pages, 8 Figures. To appear in Physical Review
Optimization of inhomogeneous electron correlation factors in periodic solids
A method is presented for the optimization of one-body and inhomogeneous
two-body terms in correlated electronic wave functions of Jastrow-Slater type.
The most general form of inhomogeneous correlation term which is compatible
with crystal symmetry is used and the energy is minimized with respect to all
parameters using a rapidly convergent iterative approach, based on Monte Carlo
sampling of the energy and fitting energy fluctuations. The energy minimization
is performed exactly within statistical sampling error for the energy
derivatives and the resulting one- and two-body terms of the wave function are
found to be well-determined. The largest calculations performed require the
optimization of over 3000 parameters. The inhomogeneous two-electron
correlation terms are calculated for diamond and rhombohedral graphite. The
optimal terms in diamond are found to be approximately homogeneous and
isotropic over all ranges of electron separation, but exhibit some
inhomogeneity at short- and intermediate-range, whereas those in graphite are
found to be homogeneous at short-range, but inhomogeneous and anisotropic at
intermediate- and long-range electron separation.Comment: 23 pages, 15 figures, 1 table, REVTeX4, submitted to PR
First- principle calculations of magnetic interactions in correlated systems
We present a novel approach to calculate the effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals in local approach with the frequency dependent self energy. The analog
of ``local force theorem'' in the density functional theory is proven for
highly correlated systems. The expressions for effective exchange parameters,
Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The
first-principle calculations of magnetic excitation spectrum for ferromagnetic
iron, with the local correlation effects from the numerically exact QMC-scheme
is presented.Comment: 17 pages, 3 Postscript figure
Metal Surface Energy: Persistent Cancellation of Short-Range Correlation Effects beyond the Random-Phase Approximation
The role that non-local short-range correlation plays at metal surfaces is
investigated by analyzing the correlation surface energy into contributions
from dynamical density fluctuations of various two-dimensional wave vectors.
Although short-range correlation is known to yield considerable correction to
the ground-state energy of both uniform and non-uniform systems, short-range
correlation effects on intermediate and short-wavelength contributions to the
surface formation energy are found to compensate one another. As a result, our
calculated surface energies, which are based on a non-local
exchange-correlation kernel that provides accurate total energies of a uniform
electron gas, are found to be very close to those obtained in the random-phase
approximation and support the conclusion that the error introduced by the
local-density approximation is small.Comment: 5 pages, 1 figure, to appear in Phys. Rev.
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
Metallo-dielectric diamond and zinc-blende photonic crystals
It is shown that small inclusions of a low absorbing metal can have a
dramatic effect on the photonic band structure. In the case of diamond and
zinc-blende photonic crystals, several complete photonic band gaps (CPBG's) can
open in the spectrum, between the 2nd-3rd, 5th-6th, and 8th-9th bands. Unlike
in the purely dielectric case, in the presence of small inclusions of a low
absorbing metal the largest CPBG for a moderate dielectric constant
(epsilon<=10) turns out to be the 2nd-3rd CPBG. The 2nd-3rd CPBG is the most
important CPBG, because it is the most stable against disorder. For a diamond
and zinc-blende structure of nonoverlapping dielectric and metallo-dielectric
spheres, a CPBG begins to decrease with an increasing dielectric contrast
roughly at the point where another CPBG starts to open--a kind of gap
competition. A CPBG can even shrink to zero when the dielectric contrast
increases further. Metal inclusions have the biggest effect for the dielectric
constant 2<=epsilon<=12, which is a typical dielectric constant at near
infrared and in the visible for many materials, including semiconductors and
polymers. It is shown that one can create a sizeable and robust 2nd-3rd CPBG at
near infrared and visible wavelengths even for a photonic crystal which is
composed of more than 97% low refractive index materials (n<=1.45, i.e., that
of silica glass or a polymer). These findings open the door for any
semiconductor and polymer material to be used as genuine building blocks for
the creation of photonic crystals with a CPBG and significantly increase the
possibilities for experimentalists to realize a sizeable and robust CPBG in the
near infrared and in the visible. One possibility is a construction method
using optical tweezers, which is analyzed here.Comment: 25 pp, 23 figs, RevTex, to appear in Phys Rev B. For more information
look at
http://www.amolf.nl/research/photonic_materials_theory/moroz/moroz.htm
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
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