4,174 research outputs found
Loop Quantization of Maxwell Theory and Electric Charge Quantization
We consider the loop quantization of Maxwell theory. A quantization of this
type leads to a quantum theory in which the fundamental excitations are
loop-like rather than particle-like. Each such loop plays the role of a
quantized Faraday's line of electric flux. We find that the quantization
depends on an arbitrary choice of a parameter e that carries the dimension of
electric charge. For each value of e an electric charge that can be contained
inside a bounded spatial region is automatically quantized in units of
hbar/4*pi*e. The requirement of consistency with the quantization of electric
charge observed in our Universe fixes a value of the, so far arbitrary,
parameter e of the theory. Finally, we compare the ambiguity in the choice of
parameter e with the beta-ambiguity that, as pointed by Immirzi, arises in the
loop quantization of general relativity, and comment on a possible way this
ambiguity can be fixed.Comment: 7 pages, Revtex, no figures, typos corrected and one reference adde
A simple parameter-free one-center model potential for an effective one-electron description of molecular hydrogen
For the description of an H2 molecule an effective one-electron model
potential is proposed which is fully determined by the exact ionization
potential of the H2 molecule. In order to test the model potential and examine
its properties it is employed to determine excitation energies, transition
moments, and oscillator strengths in a range of the internuclear distances, 0.8
< R < 2.5 a.u. In addition, it is used as a description of an H2 target in
calculations of the cross sections for photoionization and for partial
excitation in collisions with singly-charged ions. The comparison of the
results obtained with the model potential with literature data for H2 molecules
yields a good agreement and encourages therefore an extended usage of the
potential in various other applications or in order to consider the importance
of two-electron and anisotropy effects.Comment: 8 pages, 6 figure
Effective potential at finite temperature in a constant magnetic field I: Ring diagrams in a scalar theory
We study symmetry restoration at finite temperature in the theory of a
charged scalar field interacting with a constant, external magnetic field. We
compute the finite temperature effective potential including the contribution
from ring diagrams. We show that in the weak field case, the presence of the
field produces a stronger first order phase transition and that the temperature
for the onset of the transition is lower, as compared to the case without
magnetic field.Comment: Expanded comments, 4 figures added. Conclusions unchanged. Version to
match published pape
Distribution of chirality in the quantum walk: Markov process and entanglement
The asymptotic behavior of the quantum walk on the line is investigated
focusing on the probability distribution of chirality independently of
position. The long-time limit of this distribution is shown to exist and to
depend on the initial conditions, and it also determines the asymptotic value
of the entanglement between the coin and the position. It is shown that for
given asymptotic values of both the entanglement and the chirality distribution
it is possible to find the corresponding initial conditions within a particular
class of spatially extended Gaussian distributions. Moreover it is shown that
the entanglement also measures the degree of Markovian randomness of the
distribution of chirality.Comment: 5 pages, 3 figures, It was accepted in Physcial Review
Effective potential at finite temperature in a constant hypermagnetic field: Ring diagrams in the Standard Model
We study the symmetry breaking phenomenon in the standard model during the
electroweak phase transition in the presence of a constant hypermagnetic field.
We compute the finite temperature effective potential up to the contribution of
ring diagrams in the weak field, high temperature limit and show that under
these conditions, the phase transition becomes stronger first order.Comment: 15 pages, 8 Postscript figure
Spectroscopy of the Lens Galaxy of Q0957+561A,B. Implications of a possible central massive dark object
We present new long-slit William Herschel Telescope spectroscopic
observations of the lens galaxy G1 associated with the double-imaged QSO
0957+561A,B. The obtained central stellar velocity dispersion, sigma_l = 310
+/- 20 km/s, is in reasonable agreement with other measurements of this
dynamical parameter. Using all updated measurements of the stellar velocity
dispersion in the internal region of the galaxy (at angular separations < 1".5)
and a simple isotropic model, we discuss the mass of a possible central massive
dark object. It is found that the data of Falco et al. (1997) suggest the
existence of an extremely massive object of (0.5-2.1) x 10E10/h M_\odot (80%
confidence level), whereas the inclusion of very recent data (Tonry & Franx
1998, and this paper) substantially changes the results: the compact central
mass must be 6 x10E9/h M_\odot at the 90% confidence level. We note that,
taking into account all the available dynamical data, a compact nucleus with a
mass of 10E9/h M_\odot (best fit) cannot be ruled out.Comment: 20 pages, 10 figures ApJ, in pres
Symmetry breaking as the origin of zero-differential resistance states of a 2DEG in strong magnetic fields
Zero resistance differential states have been observed in two-dimensional
electron gases (2DEG) subject to a magnetic field and a strong dc current. In a
recent work we presented a model to describe the nonlinear transport regime of
this phenomenon. From the analysis of the differential resistivity and the
longitudinal voltage we predicted the formation of negative differential
resistivity states, although these states are known to be unstable. Based on
our model, we derive an analytical approximated expression for the
Voltage-Current characteristics, that captures the main elements of the
problem. The result allow us to construct an energy functional for the system.
In the zero temperature limit, the system presents a quantum phase transition,
with the control parameter given by the magnetic field. It is noted that above
a threshold value (), the symmetry is spontaneously broken. At
sufficiently high magnetic field and low temperature the model predicts a phase
with a non-vanishing permanent current; this is a novel phase that has not been
observed so far.Comment: 6 pages, 2 figure
Multiple-event probability in general-relativistic quantum mechanics: a discrete model
We introduce a simple quantum mechanical model in which time and space are
discrete and periodic. These features avoid the complications related to
continuous-spectrum operators and infinite-norm states. The model provides a
tool for discussing the probabilistic interpretation of generally-covariant
quantum systems, without the confusion generated by spurious infinities. We use
the model to illustrate the formalism of general-relativistic quantum
mechanics, and to test the definition of multiple-event probability introduced
in a companion paper. We consider a version of the model with unitary
time-evolution and a version without unitary time-evolutio
Kubo formula for Floquet states and photoconductivity oscillations in a 2D electron gas
The recent discovery of the microwave induced vanishing resistance states in
a two dimensional electron system (2DES) is an unexpected and surprising
phenomena. In these experiments the magnetoresistance of a high mobility 2DES
under the influence of microwave radiation of frequency at moderate
values of the magnetic field, exhibits strong oscillations with zero-resistance
states (ZRS) governed by the ratio , where is the
cyclotron frequency. In this work we present a model for the photoconductivity
of a two dimensional electron system (2DES) subjected to a magnetic field. The
model includes the microwave and Landau contributions in a non-perturbative
exact way, impurity scattering effects are treated perturbatively. In our
model, the Landau-Floquet states act coherently with respect to the oscillating
field of the impurities, that in turn induces transitions between these levels.
Based on this formalism, we provide a Kubo-like formula that takes into account
the oscillatory Floquet structure of the problem. We study the effects of both
short-range and long-range disorder on the photoconductivity. Our calculation
yields a magnetoresistance oscillatory behavior with the correct period and
phase. It is found that, in agreement with experiment, negative dissipation can
only be induced in very high mobility samples. We analyze the dependence of the
results on the microwave power and polarization. For high-intensity radiation
multi-photon processes take place predicting new negative-resistance states
centered at , and .Comment: Final version, accepted for publication in Phys. Rev.
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