14,790 research outputs found
Chiral corrections to baryon properties with composite pions
A calculational scheme is developed to evaluate chiral corrections to
properties of composite baryons with composite pions. The composite baryons and
pions are bound states derived from a microscopic chiral quark model. The model
is amenable to standard many-body techniques such as the BCS and RPA
formalisms. An effective chiral model involving only hadronic degrees of
freedom is derived from the macroscopic quark model by projection onto hadron
states. Chiral loops are calculated using the effective hadronic Hamiltonian. A
simple microscopic confining interaction is used to illustrate the derivation
of the pion-nucleon form factor and the calculation of pionic self-energy
corrections to the nucleon and Delta(1232) masses.Comment: 29 pages, Revtex, 4 ps figure
Magnetocaloric effect in integrable spin-s chains
We study the magnetocaloric effect for the integrable antiferromagnetic
high-spin chain. We present an exact computation of the Gr\"uneisen parameter,
which is closely related to the magnetocaloric effect, for the quantum spin-s
chain on the thermodynamical limit by means of Bethe ansatz techniques and the
quantum transfer matrix approach. We have also calculated the entropy S and the
isentropes in the (H,T) plane. We have been able to identify the quantum
critical points H_c^{(s)}=2/(s+1/2) looking at the isentropes and/or the
characteristic behaviour of the Gr\"uneisen parameter.Comment: 6 pages, 3 figure
Transport properties of a two impurity system: a theoretical approach
A system of two interacting cobalt atoms, at varying distances, was studied
in a recent scanning tunneling microscope experiment by Bork et. al.[Nature
Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all
experimentally analyzed interatomic distances, the physics observed in these
experiments. Our proposal is based on the two-impurity Anderson model, with the
inclusion of a two-path geometry for charge transport. This many-body system is
treated in the finite-U slave boson mean-field approximation and the
logarithmic-discretization embedded-cluster approximation. We physically
characterize the different charge transport regimes of this system at various
interatomic distances and show that, as in the experiments, the features
observed in the transport properties depend on the presence of two impurities
but also on the existence of two conducting channels for electron transport. We
interpret the splitting observed in the conductance as the result of the
hybridization of the two Kondo resonances associated with each impurity.Comment: 5 pages, 5 figure
Are the anti-charmed and bottomed pentaquarks molecular heptaquarks?
I study the charmed resonance D*p (3100) very recently
discovered by the H1 collaboration at Hera. An anticharmed resonance was
already predicted, in a recent publication mostly dedicated to the S=1
resonance Theta+(1540). To confirm these recent predictions, I apply the same
standard quark model with a quark-antiquark annihilation constrained by chiral
symmetry. I find that repulsion excludes the D*p (3100) as a
s-wave pentaquark. I explore the D*p (3100) as a heptaquark, equivalent to a
N-pi-D* linear molecule, with positive parity and total isospin I=0. I find
that the N-D repulsion is cancelled by the attraction existing in the N-pi and
pi-D channels. In our framework this state is harder to bind than the Theta+
described by a k-pi-N borromean bound-state, a lower binding energy is expected
in agreement with the H1 observation. Multiquark molecules N-pi-D, N-pi-B* and
N-pi-B are also predicted.Comment: 5 pages, 2 figures, RevTe
Coherent Adiabatic Spin Control in the Presence of Charge Noise Using Tailored Pulses
We study finite-time Landau-Zener transitions at a singlet-triplet level
crossing in a GaAs double quantum dot, both experimentally and theoretically.
Sweeps across the anticrossing in the high driving speed limit result in
oscillations with a small visibility. Here we demonstrate how to increase the
oscillation visibility while keeping sweep times shorter than T2* using a
tailored pulse with a detuning dependent level velocity. Our results show an
improvement of a factor ~2.9 for the oscillation visibility. In particular, we
were able to obtain a visibility of ~0.5 for St\"uckelberg oscillations, which
demonstrates the creation of an equally weighted superposition of the qubit
states.Comment: Related papers at http://pettagroup.princeton.ed
The Apparent Fractal Conjecture: Scaling Features in Standard Cosmologies
This paper presents an analysis of the smoothness problem in cosmology by
focussing on the ambiguities originated in the simplifying hypotheses aimed at
observationally verifying if the large-scale distribution of galaxies is
homogeneous, and conjecturing that this distribution should follow a fractal
pattern in perturbed standard cosmologies. This is due to a geometrical effect,
appearing when certain types of average densities are calculated along the past
light cone. The paper starts reviewing the argument concerning the possibility
that the galaxy distribution follows such a scaling pattern, and the premises
behind the assumption that the spatial homogeneity of standard cosmology can be
observable. Next, it is argued that to discuss observable homogeneity one needs
to make a clear distinction between local and average relativistic densities,
and showing how the different distance definitions strongly affect them,
leading the various average densities to display asymptotically opposite
behaviours. Then the paper revisits Ribeiro's (1995: astro-ph/9910145) results,
showing that in a fully relativistic treatment some observational average
densities of the flat Friedmann model are not well defined at z ~ 0.1, implying
that at this range average densities behave in a fundamentally different manner
as compared to the linearity of the Hubble law, well valid for z < 1. This
conclusion brings into question the widespread assumption that relativistic
corrections can always be neglected at low z. It is also shown how some key
features of fractal cosmologies can be found in the Friedmann models. In view
of those findings, it is suggested that the so-called contradiction between the
cosmological principle, and the galaxy distribution forming an unlimited
fractal structure, may not exist.Comment: 30 pages, 2 figures, LaTeX. This paper is a follow-up to
gr-qc/9909093. Accepted for publication in "General Relativity and
Gravitation
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