2,929 research outputs found
The Proca-field in Loop Quantum Gravity
In this paper we investigate the Proca-field in the framework of Loop Quantum
Gravity. It turns out that the methods developed there can be applied to the
symplectically embedded Proca-field, giving a rigorous, consistent,
non-perturbative quantization of the theory. This can be achieved by
introducing a scalar field, which has completely different properties than the
one used in spontaneous symmetry breaking. The analysis of the kernel of the
Hamiltonian suggests that the mass term in the quantum theory has a different
role than in the classical theory.Comment: 15 pages. v2: 19 pages, amended sections 2 and 6, references added
v3: 20 pages, amended section 6 and minor correction
Polymer quantization of the free scalar field and its classical limit
Building on prior work, a generally covariant reformulation of free scalar
field theory on the flat Lorentzian cylinder is quantized using Loop Quantum
Gravity (LQG) type `polymer' representations. This quantization of the {\em
continuum} classical theory yields a quantum theory which lives on a discrete
spacetime lattice. We explicitly construct a state in the polymer Hilbert space
which reproduces the standard Fock vacuum- two point functions for long
wavelength modes of the scalar field. Our construction indicates that the
continuum classical theory emerges under coarse graining. All our
considerations are free of the "triangulation" ambiguities which plague
attempts to define quantum dynamics in LQG. Our work constitutes the first
complete LQG type quantization of a generally covariant field theory together
with a semi-classical analysis of the true degrees of freedom and thus provides
a perfect infinite dimensional toy model to study open issues in LQG,
particularly those pertaining to the definition of quantum dynamics.Comment: 58 page
Towards new background independent representations for Loop Quantum Gravity
Recently, uniqueness theorems were constructed for the representation used in
Loop Quantum Gravity. We explore the existence of alternate representations by
weakening the assumptions of the so called LOST uniqueness theorem. The
weakened assumptions seem physically reasonable and retain the key requirement
of explicit background independence. For simplicity, we restrict attention to
the case of gauge group U(1).Comment: 22 pages, minor change
High-Resolution Photoemission Study of MgB2
We have performed high-resolution photoemission spectroscopy on MgB2 and
observed opening of a superconducting gap with a narrow coherent peak. We found
that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K.
The temperature dependence (15 - 40 K) of gap value follows well the BCS form,
suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is
observed in the normal state. The present results strongly suggest that MgB2 is
categorized into a phonon-mediated BCS superconductor in the weak-coupling
regime.Comment: 3 pages, 3 figures, accepted in Physical Review Letter
4216 Application of artificial neural networks for image analysis of organ culture preserved donor corneas: A pilot study
Gastrointestinal (GI) cancers account for 1.5 million deaths worldwide. Endoscopic Submucosal Dissection (ESD) is an advanced therapeutic endoscopy technique with superior clinical outcome due to the minimally invasive and en bloc removal of tumours. In the western world, ESD is seldom carried out, due to its complex and challenging nature. Various surgical systems are being developed to make this therapy accessible, however, these solutions have shown limited operational workspace, dexterity, or low force exertion capabilities. The current paper shows the ESD CYCLOPS system, a bimanual surgical robotic attachment that can be mounted at the end of any flexible endoscope. The system is able to achieve forces of up to 46N, and showed a mean error of 0.217mm during an elliptical tracing task. The workspace and instrument dexterity is shown by pre-clinical ex vivo trials, in which ESD is successfully performed by a GI surgeon. The system is currently undergoing pre-clinical in vivo validation
Spontaneous symmetry breaking in Loop Quantum Gravity
In this paper we investigate the question how spontaneous symmetry breaking
works in the framework of Loop Quantum Gravity and we compare it to the results
obtained in the case of the Proca field, where we were able to quantise the
theory in Loop Quantum Gravity without introducing a Higgs field. We obtained
that the Hamiltonian of the two systems are very similar, the only difference
is an extra scalar field in the case of spontaneous symmetry breaking. This
field can be identified as the field that carries the mass of the vector field.
In the quantum regime this becomes a well defined operator, which turns out to
be a self adjoint operator with continuous spectrum. To calculate the spectrum
we used a new representation in the case of the scalar fields, which in
addition enabled us to rewrite the constraint equations to a finite system of
linear partial differential equations. This made it possible to solve part of
the constraints explicitly.Comment: 24 pages, two appendix. v2 modified abstract, amended each section,
28 pages, two appendi
Absolute properties of the main-sequence eclipsing binary FM Leo
First spectroscopic and new photometric observations of the eclipsing binary
FM Leo are presented. The main aims were to determine orbital and stellar
parameters of two components and their evolutionary stage. First spectroscopic
observations of the system were obtained with DDO and PST spectrographs. The
results of the orbital solution from radial velocity curves are combined with
those derived from the light-curve analysis (ASAS-3 photometry and
supplementary observations of eclipses with 1 m and 0.35 m telescopes) to
derive orbital and stellar parameters. JKTEBOP, Wilson-Devinney binary
modelling codes and a two-dimensional cross-correlation (TODCOR) method were
applied for the analysis. We find the masses to be M_1 = 1.318 0.007 and
M_2 = 1.287 0.007 M_sun, the radii to be R_1 = 1.648 0.043 and R_2
= 1.511 0.049 R_sun for primary and secondary stars, respectively. The
evolutionary stage of the system is briefly discussed by comparing physical
parameters with current stellar evolution models. We find the components are
located at the main sequence, with an age of about 3 Gyr.Comment: 5 pages, 4 figures, to appear in MNRA
Temperature dependence of the collective mode and its influence on the band splitting in bilayer cuprates
The recently observed bilayer splitting in high-T cuprates is analyzed
within a model where the charge carriers are coupled to a phenomenological
bosonic spectrum which interpolates between the marginal Fermi liquid structure
and collective mode type behavior as a function of temperature. We argue that
the origin of the collective mode is probably associated with dynamic
incommensurate charge density waves. Moreover it is shown that the resulting
temperature dependence of the self-energy is in good agreement with
as extracted from angle-resolved photoemission data.Comment: 6 pages, 4 figures, accepted for PR
The Fermi surface of Bi2Sr2CaCu2O8
We study the Fermi surface of Bi2Sr2CaCu2O8 (Bi2212) using angle resolved
photoemission (ARPES) with a momentum resolution of ~ 0.01 of the Brillouin
zone. We show that, contrary to recent suggestions, the Fermi surface is a
large hole barrel centered at (pi,pi), independent of the incident photon
energy.Comment: 4 pages (revtex), 4 figures (eps, 2 color
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