7,876 research outputs found
On the Groenewold-Van Hove problem for R^{2n}
We discuss the Groenewold-Van Hove problem for R^{2n}, and completely solve
it when n = 1. We rigorously show that there exists an obstruction to
quantizing the Poisson algebra of polynomials on R^{2n}, thereby filling a gap
in Groenewold's original proof without introducing extra hypotheses. Moreover,
when n = 1 we determine the largest Lie subalgebras of polynomials which can be
unambiguously quantized, and explicitly construct all their possible
quantizations.Comment: 15 pages, Latex. Error in the proof of Prop. 3 corrected; minor
rewritin
Boxfishes (Teleostei: Ostraciidae) as a model system for fishes swimming with many fins: kinematics
Swimming movements in boxfishes were much more
complex and varied than classical descriptions indicated.
At low to moderate rectilinear swimming speeds
(<5 TL s^(-1), where TL is total body length), they were
entirely median- and paired-fin swimmers, apparently
using their caudal fins for steering. The pectoral and
median paired fins generate both the thrust needed for
forward motion and the continuously varied, interacting
forces required for the maintenance of rectilinearity. It
was only at higher swimming speeds (above 5 TL s^(-1)), when
burst-and-coast swimming was used, that they became
primarily body and caudal-fin swimmers. Despite their
unwieldy appearance and often asynchronous fin beats,
boxfish swam in a stable manner. Swimming boxfish used
three gaits. Fin-beat asymmetry and a relatively nonlinear
swimming trajectory characterized the first gait
(0–1 TL s^(-1)). The beginning of the second gait (1–3 TL s^(-1))
was characterized by varying fin-beat frequencies and
amplitudes as well as synchrony in pectoral fin motions.
The remainder of the second gait (3–5 TL s^(-1)) was
characterized by constant fin-beat amplitudes, varying finbeat
frequencies and increasing pectoral fin-beat
asynchrony. The third gait (>5 TL s^(-1)) was characterized
by the use of a caudal burst-and-coast variant. Adduction
was always faster than abduction in the pectoral fins.
There were no measurable refractory periods between
successive phases of the fin movement cycles. Dorsal and
anal fin movements were synchronized at speeds greater
than 2.5 TL s^(-1), but were often out of phase with pectoral
fin movements
Early evidence of improved soil quality with conservation farming under smallholder farming conditions in Zimbabwe
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
A frequency-domain analysis of varistor current under distorted supply voltage
Abstract: In this paper, the impact of supply voltage harmonics on the third harmonic current-based condition assessment of varistor is experimentally verified. Therefore, time-domain current and voltage waveforms, measured from ten identical varistor samples, are decomposed in frequency-domain. The flattop window of the FFT technique is used to determine the rms values and subsequently the third harmonic amplitude of the varistor current, before and after injection of harmonics. The harmonic-generating load consists of a triac-based ac voltage controller driving a resistive load unit at fixed firing angle of 10 degrees. All varistor devices used in this work were subjected to rated ac operating voltage. However, the results obtained indicated that the operation of a harmonic source connected across the varistor arrestor has the effect of increasing the magnitude of the third harmonic component of the varistor current
The order of the metal to superconductor transition
We present results from large-scale Monte Carlo simulations on the full
Ginzburg-Landau (GL) model, including fluctuations in the amplitude and the
phase of the matter-field, as well as fluctuations of the non-compact
gauge-field of the theory. {}From this we obtain a precise critical value of
the GL parameter \kct separating a first order metal to superconductor
transition from a second order one, \kct = (0.76\pm 0.04)/\sqrt{2}. This
agrees surprisingly well with earlier analytical results based on a disorder
theory of the superconductor to metal transition, where the value
\kct=0.798/\sqrt{2} was obtained. To achieve this, we have done careful
infinite volume and continuum limit extrapolations. In addition we offer a
novel interpretation of \kct, namely that it is also the value separating
\typeI and \typeII behaviour.<Comment: Minor corrections, present version accepted for publication in PR
Emergence of clusters: Halos, Efimov states, and experimental signals
We investigate emergence of halos and Efimov states in nuclei by use of a
newly designed model which combines self-consistent mean-field and three-body
descriptions. Recent interest in neutron heavy calcium isotopes makes Ca
(Ca+n+n) an ideal realistic candidate on the neutron dripline, and we
use it as a representative example that illustrates our broadly applicable
conclusions. By smooth variation of the interactions we simulate the crossover
from well-bound systems to structures beyond the threshold of binding, and find
that halo-configurations emerge from the mean-field structure for three-body
binding energy less than keV. Strong evidence is provided that Efimov
states cannot exist in nuclei. The structure that bears the most resemblance to
an Efimov state is a giant halo extending beyond the neutron-core scattering
length. We show that the observable large-distance decay properties of the wave
function can differ substantially from the bulk part at short distances, and
that this evolution can be traced with our combination of few- and many-body
formalisms. This connection is vital for interpretation of measurements such as
those where an initial state is populated in a reaction or by a beta-decay.Comment: 5 pages, 5 figures, under revie
Combined few-body and mean-field model for nuclei
The challenging nuclear many-body problem is discussed along with
classifications and qualitative descriptions of existing methods and models. We
present detailed derivations of a new method where cluster correlations
co-exist with an underlying mean-field described core-structure. The variation
of an antisymmetrized product of cluster and core wave functions and a given
nuclear interaction, provide sets of self-consistent equations of motion.
After the applications on dripline nuclei we discuss perspectives with
improvements and applications. In the conclusion we summarize while emphasizing
the merits of consistently treating both short- and large-distance properties,
few- and many-body correlations, ordinary nuclear structure, and concepts of
halos and Efimov states
A combined mean-field and three-body model tested on the O-nucleus
We combine few- and many-body degrees of freedom in a model applicable to
both bound and continuum states and adaptable to different subfields of
physics. We formulate a self-consistent three-body model for a core-nucleus
surrounded by two valence nucleons. We treat the core in the mean-field
approximation and use the same effective Skyrme interaction between both core
and valence nucleons. We apply the model to O where we reproduce the
known experimental data as well as phenomenological models with more
parameters. The decay of the ground state is found to proceed directly into the
continuum without effect of the virtual sequential decay through the well
reproduced -resonance of O.Comment: 5 pages, 5 figures, under revie
Combining few-body cluster structures with many-body mean-field methods
Nuclear cluster physics implicitly assumes a distinction between groups of
degrees-of-freedom, that is the (frozen) intrinsic and (explicitly treated)
relative cluster motion. We formulate a realistic and practical method to
describe the coupled motion of these two sets of degrees-of-freedom. We derive
a coupled set of differential equations for the system using the
phenomenologically adjusted effective in-medium Skyrme type of nucleon-nucleon
interaction. We select a two-nucleon plus core system where the mean-field
approximation corresponding to the Skyrme interaction is used for the core. A
hyperspherical adiabatic expansion of the Faddeev equations is used for the
relative cluster motion. We shall specifically compare both the structure and
the decay mechanism found from the traditional three-body calculations with the
result using the new boundary condition provided by the full microscopic
structure at small distance. The extended Hilbert space guaranties an improved
wave function compared to both mean-field and three-body solutions. We shall
investigate the structures and decay mechanism of C (C+n+n). In
conclusion, we have developed a method combining nuclear few- and many-body
techniques without losing the descriptive power of each approximation at
medium-to-large distances and small distances respectively. The coupled set of
equations are solved self-consistently, and both structure and dynamic
evolution are studied.Comment: 4 pages, 3 figures, conference proceedings, publishe
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