3,350 research outputs found
Observing the spin of a free electron
Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions
Magnetic order in the Ising model with parallel dynamics
It is discussed how the equilibrium properties of the Ising model are
described by an Hamiltonian with an antiferromagnetic low temperature behavior
if only an heat bath dynamics, with the characteristics of a Probabilistic
Cellular Automaton, is assumed to determine the temporal evolution of the
system.Comment: 9 pages, 3 figure
Multi-interaction mean-field renormalization group
We present an extension of the previously proposed mean-field renormalization
method to model Hamiltonians which are characterized by more than just one type
of interaction. The method rests on scaling assumptions about the magnetization
of different sublattices of the given lattice and it generates as many flow
equations as coupling constants without arbitrary truncations on the
renormalized Hamiltonian. We obtain good results for the test case of Ising
systems with an additional second-neighbor coupling in two and three
dimensions. An application of the method is also done to a morphological model
of interacting surfaces introduced recenlty by Likos, Mecke and Wagner [J.
Chem. Phys. {\bf{102}}, 9350 (1995)].
PACS: 64.60.Ak, 64.60.Fr, 05.70.JkComment: Tex file and three macros appended at the end. Five figures available
upon request to: [email protected], Fax: [+]39-40-224-60
Classical Evolution of Quantum Elliptic States
The hydrogen atom in weak external fields is a very accurate model for the
multiphoton excitation of ultrastable high angular momentum Rydberg states, a
process which classical mechanics describes with astonishing precision. In this
paper we show that the simplest treatment of the intramanifold dynamics of a
hydrogenic electron in external fields is based on the elliptic states of the
hydrogen atom, i.e., the coherent states of SO(4), which is the dynamical
symmetry group of the Kepler problem. Moreover, we also show that classical
perturbation theory yields the {\it exact} evolution in time of these quantum
states, and so we explain the surprising match between purely classical
perturbative calculations and experiments. Finally, as a first application, we
propose a fast method for the excitation of circular states; these are
ultrastable hydrogenic eigenstates which have maximum total angular momentum
and also maximum projection of the angular momentum along a fixed direction. %Comment: 8 Pages, 2 Figures. Accepted for publication in Phys. Rev.
Run Scenarios for the Linear Collider
Scenarios are developed for runs at a Linear Collider, in the case that there
is a rich program of new physics.Comment: 12 pages, 10 tables, Latex; Snowmass 2001 plenary repor
Quantum Particle-Trajectories and Geometric Phase
"Particle"-trajectories are defined as integrable paths
in projective space.
Quantum states evolving on such trajectories, open or closed, do not
delocalise in projection, the phase associated with the trajectories
being related to the geometric (Berry) phase and the Classical Mechanics
action. High Energy Physics properties of states evolving on
"particle"-trajectories are discussed.Comment: 4 page
Renormalization Group Theory And Variational Calculations For Propagating Fronts
We study the propagation of uniformly translating fronts into a linearly
unstable state, both analytically and numerically. We introduce a perturbative
renormalization group (RG) approach to compute the change in the propagation
speed when the fronts are perturbed by structural modification of their
governing equations. This approach is successful when the fronts are
structurally stable, and allows us to select uniquely the (numerical)
experimentally observable propagation speed. For convenience and completeness,
the structural stability argument is also briefly described. We point out that
the solvability condition widely used in studying dynamics of nonequilibrium
systems is equivalent to the assumption of physical renormalizability. We also
implement a variational principle, due to Hadeler and Rothe, which provides a
very good upper bound and, in some cases, even exact results on the propagation
speeds, and which identifies the transition from ` linear'- to `
nonlinear-marginal-stability' as parameters in the governing equation are
varied.Comment: 34 pages, plain tex with uiucmac.tex. Also available by anonymous ftp
to gijoe.mrl.uiuc.edu (128.174.119.153), file /pub/front_RG.tex (or .ps.Z
Optical identification of X-ray source 1RXS J180431.1-273932 as a magnetic cataclysmic variable
The X-ray source 1RXS J180431.1-273932 has been proposed as a new member of
the symbiotic X-ray binary (SyXB) class of systems, which are composed of a
late-type giant that loses matter to an extremely compact object, most likely a
neutron star. In this paper, we present an optical campaign of imaging plus
spectroscopy on selected candidate counterparts of this object. We also
reanalyzed the available archival X-ray data collected with XMM-Newton. We find
that the brightest optical source inside the 90% X-ray positional error circle
is spectroscopically identified as a magnetic cataclysmic variable (CV), most
likely of intermediate polar type, through the detection of prominent Balmer,
He I, He II, and Bowen blend emissions. On either spectroscopic or statistical
grounds, we discard as counterparts of the X-ray source the other optical
objects in the XMM-Newton error circle. A red giant star of spectral type M5
III is found lying just outside the X-ray position: we consider this latter
object as a fore-/background one and likewise rule it out as a counterpart of
1RXS J180431.1-273932. The description of the X-ray spectrum of the source
using a bremsstrahlung plus black-body model gives temperatures of around 40
keV and around 0.1 keV for these two components, respectively. We estimate a
distance of about 450 pc and a 0.2-10 keV X-ray luminosity of about 1.7e32
erg/s for this system and, using the information obtained from the X-ray
spectral analysis, a mass of about 0.8 solar masses for the accreting white
dwarf (WD). We also confirm an X-ray periodicity of 494 s for this source,
which we interpret as the spin period of the WD. In summary, 1RXS
J180431.1-273932 is identified as a magnetic CV and its SyXB nature is
excluded.Comment: 9 pages, 7 figures, 3 tables, accepted for publication on Astronomy &
Astrophysics, main journal. Version 2 includes the A&A Language Editor's
correction
Geometry, thermodynamics, and finite-size corrections in the critical Potts model
We establish an intriguing connection between geometry and thermodynamics in
the critical q-state Potts model on two-dimensional lattices, using the q-state
bond-correlated percolation model (QBCPM) representation. We find that the
number of clusters of the QBCPM has an energy-like singularity for q different
from 1, which is reached and supported by exact results, numerical simulation,
and scaling arguments. We also establish that the finite-size correction to the
number of bonds, has no constant term and explains the divergence of related
quantities as q --> 4, the multicritical point. Similar analyses are applicable
to a variety of other systems.Comment: 12 pages, 6 figure
Physical Consequences of Complex Dimensions of Fractals
It has recently been realized that fractals may be characterized by complex
dimensions, arising from complex poles of the corresponding zeta function, and
we show here that these lead to oscillatory behavior in various physical
quantities. We identify the physical origin of these complex poles as the
exponentially large degeneracy of the iterated eigenvalues of the Laplacian,
and discuss applications in quantum mesoscopic systems such as oscillations in
the fluctuation of the number of levels, as a correction to
results obtained in Random Matrix Theory. We present explicit expressions for
these oscillations for families of diamond fractals, also studied as
hierarchical lattices.Comment: 4 pages, 3 figures; v2: references added, as published in Europhysics
Letter
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