3,583 research outputs found
Diffractoid grating configuration for X-ray and ultraviolet focusing
An aspheric grating is described which is operable to image local or distant point sources sharply in a designated wavelength, i.e., produce a perfectly stigmatic image in the given wavelength at grazing angles of incidence. The grating surface comprises a surface of revolution defined by a curve which does not have a constant radius of curvature but is defined by a nonlinear differential equation
Are Dark Energy and Dark Matter Different Aspects of the Same Physical Process?
It is suggested that the apparently disparate cosmological phenomena
attributed to so-called 'dark matter' and 'dark energy' arise from the same
fundamental physical process: the emergence, from the quantum level, of
spacetime itself. This creation of spacetime results in metric expansion around
mass points in addition to the usual curvature due to stress-energy sources of
the gravitational field. A recent modification of Einstein's theory of general
relativity by Chadwick, Hodgkinson, and McDonald incorporating spacetime
expansion around mass points, which accounts well for the observed galactic
rotation curves, is adduced in support of the proposal. Recent observational
evidence corroborates a prediction of the model that the apparent amount of
'dark matter' increases with the age of the universe. In addition, the proposal
leads to the same result for the small but nonvanishing cosmological constant,
related to 'dark energy, as that of the causet model of Sorkin et al.Comment: Some typos corrected. Comments welcome, pro or co
Equilibration in long-range quantum spin systems from a BBGKY perspective
The time evolution of -spin reduced density operators is studied for a
class of Heisenberg-type quantum spin models with long-range interactions. In
the framework of the quantum Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY)
hierarchy, we introduce an unconventional representation, different from the
usual cluster expansion, which casts the hierarchy into the form of a
second-order recursion. This structure suggests a scaling of the expansion
coefficients and the corresponding time scales in powers of with the
system size , implying a separation of time scales in the large system
limit. For special parameter values and initial conditions, we can show
analytically that closing the BBGKY hierarchy by neglecting -spin
correlations does never lead to equilibration, but gives rise to quasi-periodic
time evolution with at most independent frequencies. Moreover, for the
same special parameter values and in the large- limit, we solve the complete
recursion relation (the full BBGKY hierarchy), observing a superexponential
decay to equilibrium in rescaled time .Comment: 3 figure
Stationary point approach to the phase transition of the classical XY chain with power-law interactions
The stationary points of the Hamiltonian H of the classical XY chain with
power-law pair interactions (i.e., decaying like r^{-{\alpha}} with the
distance) are analyzed. For a class of "spinwave-type" stationary points, the
asymptotic behavior of the Hessian determinant of H is computed analytically in
the limit of large system size. The computation is based on the Toeplitz
property of the Hessian and makes use of a Szeg\"o-type theorem. The results
serve to illustrate a recently discovered relation between phase transitions
and the properties of stationary points of classical many-body Hamiltonian
functions. In agreement with this relation, the exact phase transition energy
of the model can be read off from the behavior of the Hessian determinant for
exponents {\alpha} between zero and one. For {\alpha} between one and two, the
phase transition is not manifest in the behavior of the determinant, and it
might be necessary to consider larger classes of stationary points.Comment: 9 pages, 6 figure
Nonlinear dielectric response at the excess wing of glass-forming liquids
We present nonlinear dielectric measurements of glass-forming glycerol and
propylene carbonate applying electrical fields up to 671 kV/cm. The
measurements extend to sufficiently high frequencies to allow for the
investigation of the nonlinear behavior in the regime of the so-far mysterious
excess wing, showing up in the loss spectra of many glass formers as a second
power law at high frequencies. Surprisingly, we find a complete lack of
nonlinear behavior in the excess wing, in marked contrast to the
alpha-relaxation where, in agreement with previous reports, a strong increase
of dielectric constant and loss is found.Comment: 8 pages (including 3 pages Supplementary Information), 4 + 1 figures.
Revised according to suggestions of referee
Energy thresholds for discrete breathers
Discrete breathers are time-periodic, spatially localized solutions of the
equations of motion for a system of classical degrees of freedom interacting on
a lattice. An important issue, not only from a theoretical point of view but
also for their experimental detection, are their energy properties. We
considerably enlarge the scenario of possible energy properties presented by
Flach, Kladko, and MacKay [Phys. Rev. Lett. 78, 1207 (1997)]. Breather energies
have a positive lower bound if the lattice dimension is greater than or equal
to a certain critical value d_c. We show that d_c can generically be greater
than two for a large class of Hamiltonian systems. Furthermore, examples are
provided for systems where discrete breathers exist but do not emerge from the
bifurcation of a band edge plane wave. Some of these systems support breathers
of arbitrarily low energy in any spatial dimension.Comment: 4 pages, 4 figure
On three-dimensional reconstruction of optically thin solar emission sources
Calculations are given for constructing the three dimensional distribution of optically thin EUV emission sources associated with solar active regions, from two dimensional observations (projections) recorded by the spectroheliograph on the OSO 7 satellite. The relation of the method to other image reconstruction methods is briefly discussed as well as the special requirements imposed in the solar case such as a knowledge of the true solar rotation function. A useful correlation criterion for establishing the physical validity of solutions is given
Modelling chemical reactions using semiconductor quantum dots
We propose using semiconductor quantum dots for a simulation of chemical
reactions as electrons are redistributed among such artificial atoms. We show
that it is possible to achieve various reaction regimes and obtain different
reaction products by varying the speed of voltage changes applied to the gates
forming quantum dots. Considering the simplest possible reaction, , we show how the necessary initial state can be obtained and what
voltage pulses should be applied to achieve a desirable final product. Our
calculations have been performed using the Pechukas gas approach, which can be
extended for more complicated reactions
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