874 research outputs found
Multistage Zeeman deceleration of metastable neon
A supersonic beam of metastable neon atoms has been decelerated by exploiting
the interaction between the magnetic moment of the atoms and time-dependent
inhomogeneous magnetic fields in a multistage Zeeman decelerator. Using 91
deceleration solenoids, the atoms were decelerated from an initial velocity of
580m/s to final velocities as low as 105m/s, corresponding to a removal of more
than 95% of their initial kinetic energy. The phase-space distribution of the
cold, decelerated atoms was characterized by time-of-flight and imaging
measurements, from which a temperature of 10mK was obtained in the moving frame
of the decelerated sample. In combination with particle-trajectory simulations,
these measurements allowed the phase-space acceptance of the decelerator to be
quantified. The degree of isotope separation that can be achieved by multistage
Zeeman deceleration was also studied by performing experiments with pulse
sequences generated for Ne and Ne.Comment: 16 pages, 15 figure
Defining an age cut-off for older offenders: a systematic review of literature
PurposeIn the literature, 65 years is commonly used as the age to designate an older person in the community. When studying older prisoners, there is much variation. The purpose of this paper is to investigate how researchers define older offenders and for what reasons.Design/methodology/approachThe authors reviewed articles on health and well-being of older offenders to assess terminology used to describe this age group, the chosen age cut-offs distinguishing younger offenders from older offenders, the arguments provided to support this choice as well as the empirical base cited in this context.FindingsThe findings show that the age cut-off of 50 years and the term "older" were most frequently used by researchers in the field. The authors find eight main arguments given to underscore the use of specific age cut-offs delineating older offenders. They outline the reasoning provided for each argument and evaluate it for its use to define older offenders.Originality/valueWith this review, it is hoped to stimulate the much-needed discussion advancing towards a uniform definition of the older offender. Such a uniform definition would make future research more comparable and ensure that there is no ambiguity when researchers state that the study population is "older offenders"
Dissociation energy of the hydrogen molecule at 10 accuracy
The ionization energy of ortho-H has been determined to be
cm
from measurements of the GK(1,1)--X(0,1) interval by Doppler-free two-photon
spectroscopy using a narrow band 179-nm laser source and the ionization energy
of the GK(1,1) state by continuous-wave near-infrared laser spectroscopy.
(H) was used to derive the dissociation energy of
H, (H), at cm with a
precision that is more than one order of magnitude better than all previous
results. The new result challenges calculations of this quantity and represents
a benchmark value for future relativistic and QED calculations of molecular
energies.Comment: 6 pages, 5 figure
Localization in non-chiral network models for two-dimensional disordered wave mechanical systems
Scattering theoretical network models for general coherent wave mechanical
systems with quenched disorder are investigated. We focus on universality
classes for two dimensional systems with no preferred orientation: Systems of
spinless waves undergoing scattering events with broken or unbroken time
reversal symmetry and systems of spin 1/2 waves with time reversal symmetric
scattering. The phase diagram in the parameter space of scattering strengths is
determined. The model breaking time reversal symmetry contains the critical
point of quantum Hall systems but, like the model with unbroken time reversal
symmetry, only one attractive fixed point, namely that of strong localization.
Multifractal exponents and quasi-one-dimensional localization lengths are
calculated numerically and found to be related by conformal invariance.
Furthermore, they agree quantitatively with theoretical predictions. For
non-vanishing spin scattering strength the spin 1/2 systems show
localization-delocalization transitions.Comment: 4 pages, REVTeX, 4 figures (postscript
Proposal for a standard problem for micromagnetic simulations including spin-transfer torque
The spin-transfer torque between itinerant electrons and the magnetization in a ferromagnet is of fundamental interest for the applied physics community. To investigate the spin-transfer torque, powerful simulation tools are mandatory. We propose a micromagnetic standard problem includingthe spin-transfer torque that can be used for the validation and falsication of micromagnetic simulation tools. The work is based on the micromagnetic model extended by the spin-transfer torque in continuously varying magnetizations as proposed by Zhang and Li. The standard problem geometry is a permalloy cuboid of 100 nm edge length and 10 nm thickness, which contains a Landau pattern with a vortex in the center of the structure. A spin-polarized dc current density of 1012 A/m2 flows laterally through the cuboid and moves the vortex core to a new steady-state position. We show that the new vortex-core position is a sensitive measure for the correctness of micromagnetic simulatorsthat include the spin-transfer torque. The suitability of the proposed problem as a standard problem is tested by numerical results from four different finite-difference and finite-element-based simulation tools
Coulombically Interacting Electrons in a One-dimensional Quantum Dot
The spectral properties of up to four interacting electrons confined within a
quasi one--dimensional system of finite length are determined by numerical
diagonalization including the spin degree of freedom. The ground state energy
is investigated as a function of the electron number and of the system length.
The limitations of a description in terms of a capacitance are demonstrated.
The energetically lowest lying excitations are physically explained as
vibrational and tunneling modes. The limits of a dilute, Wigner-type
arrangement of the electrons, and a dense, more homogeneous charge distribution
are discussed.Comment: 10 pages (excl. Figures), Figures added in POSTSCRIPT, LaTe
Two ground-state modifications of quantum-dot beryllium
Exact electronic properties of a system of four Coulomb-interacting
two-dimensional electrons in a parabolic confinement are reported. We show that
degenerate ground states of this system are characterized by qualitatively
different internal electron-electron correlations, and that the formation of
Wigner molecule in the strong-interaction regime is going on in essentially
different ways in these ground states.Comment: 5 pages, incl 5 Figures and 2 Table
Effective charge-spin models for quantum dots
It is shown that at low densities, quantum dots with few electrons may be
mapped onto effective charge-spin models for the low-energy eigenstates. This
is justified by defining a lattice model based on a many-electron pocket-state
basis in which electrons are localised near their classical ground-state
positions. The equivalence to a single-band Hubbard model is then established
leading to a charge-spin () model which for most geometries reduces to a
spin (Heisenberg) model. The method is refined to include processes which
involve cyclic rotations of a ``ring'' of neighboring electrons. This is
achieved by introducing intermediate lattice points and the importance of ring
processes relative to pair-exchange processes is investigated using high-order
degenerate perturbation theory and the WKB approximation. The energy spectra
are computed from the effective models for specific cases and compared with
exact results and other approximation methods.Comment: RevTex, 24 pages, 7 figures submitted as compressed and PostScript
file
Determination of the Interval between the Ground States of Para- and Ortho- H2
ISSN:0031-9007ISSN:1079-711
Crossover from Fermi liquid to Wigner molecule behavior in quantum dots
The crossover from weak to strong correlations in parabolic quantum dots at
zero magnetic field is studied by numerically exact path-integral Monte Carlo
simulations for up to eight electrons. By the use of a multilevel blocking
algorithm, the simulations are carried out free of the fermion sign problem. We
obtain a universal crossover only governed by the density parameter . For
, the data are consistent with a Wigner molecule description, while
for , Fermi liquid behavior is recovered. The crossover value is surprisingly small.Comment: 4 pages RevTeX, 3 figures, corrected Tabl
- …