141,788 research outputs found
Instantons And Baryon Mass Splittings in the MIT Bag Model
The contribution of instanton-induced effective inter-quark interactions to
the baryon mass splittings was considered in the bag model. It is found that
results are different from those obtained in the constituent quark model where
the instanton effects are like those from one-gluon exchange. This is because
in the context of the bag model calculation the one-body instanton-induced
interaction has to be included.Comment: 23 pages, report ZTF-93/10 (to appear in Phys.Rev. D
Acoustic collective excitations and static dielectric response in incommensurate crystals with real order parameter
Starting from the basic Landau model for the incommensurate-commensurate
materials of the class II, we derive the spectrum of collective modes for all
(meta)stable states from the corresponding phase diagram. It is shown that all
incommensurate states posses Goldstone modes with acoustic dispersions. The
representation in terms of collective modes is also used in the calculation and
discussion of static dielectric response for systems with the commensurate wave
number in the center of the Brillouin zone.Comment: 7 pages, 4 figures, REVTe
Electronically Induced Anomaly in LO Phonon Dispersion of High - Tc Superconductors
The strong, electronically induced anomaly in the spectrum of the
longitudinal optical (LO) phonons propagating along the main axes of the
CuO plane is tentatively attributed to the oxygen-oxygen charge transfer
between the two oxygens in the plane. It is argued that this charge transfer
can be large and that it is strongly coupled to the zone boundary LO phonons.
The corresponding negative contribution to the free energy is quartic in the LO
phonon amplitude, making the LO phonon unstable through the first order phase
transition, with a concomitant domain structure.Comment: 3 pages, 2 figure
Configuration mixing of angular-momentum projected triaxial relativistic mean-field wave functions
The framework of relativistic energy density functionals is extended to
include correlations related to the restoration of broken symmetries and to
fluctuations of collective variables. The generator coordinate method is used
to perform configuration mixing of angular-momentum projected wave functions,
generated by constrained self-consistent relativistic mean-field calculations
for triaxial shapes. The effects of triaxial deformation and of -mixing is
illustrated in a study of spectroscopic properties of low-spin states in
Mg.Comment: 15 pages, 11 figures, 4 tables, accepted for publication in Phys.
Rev.
Energy Density Functional analysis of shape evolution in N=28 isotones
The structure of low-energy collective states in proton-deficient N=28
isotones is analyzed using structure models based on the relativistic energy
density functional DD-PC1. The relativistic Hartree-Bogoliubov model for
triaxial nuclei is used to calculate binding energy maps in the
- plane. The evolution of neutron and proton single-particle
levels with quadrupole deformation, and the occurrence of gaps around the Fermi
surface, provide a simple microscopic interpretation of the onset of
deformation and shape coexistence. Starting from self-consistent constrained
energy surfaces calculated with the functional DD-PC1, a collective Hamiltonian
for quadrupole vibrations and rotations is employed in the analysis of
excitation spectra and transition rates of Ar, S, and Si.
The results are compared to available data, and previous studies based either
on the mean-field approach or large-scale shell-model calculations. The present
study is particularly focused on S, for which data have recently been
reported that indicate pronounced shape coexistence.Comment: 31 pages, 11 figures. arXiv admin note: text overlap with
arXiv:1102.419
Screening effect on the optical absorption in graphene and metallic monolayers
Screening is one of the fundamental concepts in solid state physics. It has a
great impact on the electronic properties of graphene where huge mobilities
were observed in spite of the large concentration of charged impurities. While
static screening has successfully explained DC mobilities, screening properties
can be significantly changed at infrared or optical frequencies. In this paper
we discuss the influence of dynamical screening on the optical absorption of
graphene and other 2D electron systems like metallic monolayers. This research
is motivated by recent experimental results which pointed out that graphene
plasmon linewidths and optical scattering rates can be much larger than
scattering rates determined by DC mobilities. Specifically we discuss a process
where a photon incident on a graphene plane can excite a plasmon by scattering
from an impurity, or surface optical phonon of the substrate.Comment: 19 pages, 2 figure
Recovering the chiral critical end-point via delocalization of quark interactions
We show that for the lower branch of the quark condensate and values higher
than approximately the chiral critical end-point in
the Nambu--Jona-Lasinio model does not occur in the phase diagram. By using
lattice motivated non-local quark interactions, we demonstrate that the
critical end-point can be recovered. We study this behavior for a range of
condensate values and find that the variation in the position of the critical
end-point is more pronounced as the condensate is increased.Comment: title changed, minor changes in text, version to match the one
published in PR
Axial gravity, massless fermions and trace anomalies
This article deals with two main topics. One is odd parity trace anomalies in
Weyl fermion theories in a 4d curved background, the second is the introduction
of axial gravity. The motivation for reconsidering the former is to clarify the
theoretical background underlying the approach and complete the calculation of
the anomaly. The reference is in particular to the difference between Weyl and
massless Majorana fermions and to the possible contributions from tadpole and
seagull terms in the Feynman diagram approach. A first, basic, result of this
paper is that a more thorough treatment, taking account of such additional
terms { and using dimensional regularization}, confirms the earlier result. The
introduction of an axial symmetric tensor besides the usual gravitational
metric is instrumental to a different derivation of the same result using Dirac
fermions, which are coupled not only to the usual metric but also to the
additional axial tensor. The action of Majorana and Weyl fermions can be
obtained in two different limits of such a general configuration. The results
obtained in this way confirm the previously obtained ones.Comment: 55 pages, comments added in section 2 and 5. Sections 6.4, 6.6, 7,
7.1, 7.2 and Appendices 5.3, 5.5 partially modifie
The Quantum Hall Effect with Wilczek's charged magnetic flux tubes instead of electrons
Composites formed from charged particles and magnetic flux tubes, proposed by
Wilczek, are one model for anyons - particles obeying fractional statistics.
Here we propose a scheme for realizing charged flux tubes, in which a charged
object with an intrinsic magnetic dipole moment is placed between two
semi-infinite blocks of a high permeability () material, and the images
of the magnetic moment create an effective flux tube. We show that the scheme
can lead to a realization of Wilczek's anyons, when a two-dimensional electron
system, which exhibits the integer quantum Hall effect (IQHE), is sandwiched
between two blocks of the high- material with a temporally fast response
(in the cyclotron and Larmor frequency range). The signature of Wilczek's
anyons is a slight shift of the resistivity at the plateau of the IQHE. Thus,
the quest for high- materials at high frequencies, which is underway in
the field of metamaterials, and the quest for anyons, are here found to be on
the same avenue.Comment: are welcom
UV active plasmons in alkali and alkaline earth intercalated graphene
The interband pi and pi+sigma plasmons in pristine graphene and the Dirac
plasmon in doped graphene are not applicable, since they are broad or weak, and
weakly couple to an external longitudinal or electromagnetic probe. Therefore,
the ab initio Density Function Theory is used to demonstrate that the chemical
doping of the graphene by the alkali or alkaline earth atoms dramatically
changes the poor graphene excitation spectrum in the ultra-violet frequency
range (4 - 10 eV). Four prominent modes are detected. Two of them are the
intra-layer plasmons with the square-root dispersion, characteristic for the
two-dimensional modes. The remaining two are the inter-layer plasmons, very
strong in the long-wavelength limit but damped for larger wave-vectors. The
optical absorption calculations show that the inter-layer plasmons are both
optically active, which makes these materials suitable for small organic
molecule sensing. This is particularly intriguing because the optically active
two-dimensional plasmons have not been detected in other materials
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