20,039 research outputs found
Evolving Lucene search queries for text classification
We describe a method for generating accurate, compact, human
understandable text classifiers. Text datasets are indexed using Apache Lucene and Genetic Programs are used to construct
Lucene search queries. Genetic programs acquire fitness by
producing queries that are effective binary classifiers for a
particular category when evaluated against a set of training
documents. We describe a set of functions and terminals and
provide results from classification tasks
Correcting 100 years of misunderstanding: electric fields in superconductors, hole superconductivity, and the Meissner effect
From the outset of superconductivity research it was assumed that no
electrostatic fields could exist inside superconductors, and this assumption
was incorporated into conventional London electrodynamics. Yet the London
brothers themselves initially (in 1935) had proposed an electrodynamic theory
of superconductors that allowed for static electric fields in their interior,
which they unfortunately discarded a year later. I argue that the Meissner
effect in superconductors necessitates the existence of an electrostatic field
in their interior, originating in the expulsion of negative charge from the
interior to the surface when a metal becomes superconducting. The theory of
hole superconductivity predicts this physics, and associated with it a
macroscopic spin current in the ground state of superconductors ("Spin Meissner
effect"), qualitatively different from what is predicted by conventional
BCS-London theory. A new London-like electrodynamic description of
superconductors is proposed to describe this physics. Within this theory
superconductivity is driven by lowering of quantum kinetic energy, the fact
that the Coulomb repulsion strongly depends on the character of the charge
carriers, namely whether electron- or hole-like, and the spin-orbit
interaction. The electron-phonon interaction does not play a significant role,
yet the existence of an isotope effect in many superconductors is easily
understood. In the strong coupling regime the theory appears to favor local
charge inhomogeneity. The theory is proposed to apply to all superconducting
materials, from the elements to the high cuprates and pnictides, is
highly falsifiable, and explains a wide variety of experimental observations.Comment: Proceedings of the conference "Quantum phenomena in complex matter
2011 - Stripes 2011", Rome, 10 July -16 July 2011, to be published in J.
Supercond. Nov. Mag
Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model
A one-dimensional model of electrons locally coupled to spin-1/2 degrees of
freedom is studied by numerical techniques. The model is one in the class of
that describe the relaxation of an atomic orbital
upon double electron occupancy due to electron-electron interactions. We study
the parameter regime where pairing occurs in this model by exact
diagonalization of small clusters. World line quantum Monte Carlo simulations
support the results of exact diagonalization for larger systems and show that
kinetic energy is lowered when pairing occurs. The qualitative physics of this
model and others in its class, obtained through approximate analytic
calculations, is that superconductivity occurs through hole undressing even in
parameter regimes where the effective on-site interaction is strongly
repulsive. Our numerical results confirm the expected qualitative behavior, and
show that pairing will occur in a substantially larger parameter regime than
predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published
in Phys.Rev.
Lepton number violation phenomenology of d=7 neutrino mass models
We study the phenomenology of d=7 1-loop neutrino mass models. All models in
this particular class require the existence of several new
multiplets, both scalar and fermionic, and thus predict a rich phenomenology at
the LHC. The observed neutrino masses and mixings can easily be fitted in these
models. Interestingly, despite the smallness of the observed neutrino masses,
some particular lepton number violating (LNV) final states can arise with
observable branching ratios. These LNV final states consists of leptons and
gauge bosons with high multiplicities, such as 4l+4W, 6l+2W, etc. We study
current constraints on these models from upper bounds on charged lepton flavour
violating decays, existing lepton number conserving searches at the LHC and
discuss possible future LNV searches
Electromotive forces and the Meissner effect puzzle
In a voltaic cell, positive (negative) ions flow from the low (high)
potential electrode to the high (low) potential electrode, driven by an
`electromotive force' which points in opposite direction and overcomes the
electric force. Similarly in a superconductor charge flows in direction
opposite to that dictated by the Faraday electric field as the magnetic field
is expelled in the Meissner effect. The puzzle is the same in both cases: what
drives electric charges against electromagnetic forces? I propose that the
answer is also the same in both cases: kinetic energy lowering, or `quantum
pressure'
R-parity Conserving Supersymmetry, Neutrino Mass and Neutrinoless Double Beta Decay
We consider contributions of R-parity conserving softly broken supersymmetry
(SUSY) to neutrinoless double beta (\znbb) decay via the (B-L)-violating
sneutrino mass term. The latter is a generic ingredient of any weak-scale SUSY
model with a Majorana neutrino mass. The new R-parity conserving SUSY
contributions to \znbb are realized at the level of box diagrams. We derive
the effective Lagrangian describing the SUSY-box mechanism of \znbb-decay and
the corresponding nuclear matrix elements. The 1-loop sneutrino contribution to
the Majorana neutrino mass is also derived.
Given the data on the \znbb-decay half-life of Ge and the neutrino
mass we obtain constraints on the (B-L)-violating sneutrino mass. These
constraints leave room for accelerator searches for certain manifestations of
the 2nd and 3rd generation (B-L)-violating sneutrino mass term, but are most
probably too tight for first generation (B-L)-violating sneutrino masses to be
searched for directly.Comment: LATEX, 29 pages + 4 (uuencoded) figures appende
Determining R-parity violating parameters from neutrino and LHC data
In supersymmetric models neutrino data can be explained by R-parity violating
operators which violate lepton number by one unit. The so called bilinear model
can account for the observed neutrino data and predicts at the same time
several decay properties of the lightest supersymmetric particle. In this paper
we discuss the expected precision to determine these parameters by combining
neutrino and LHC data and discuss the most important observables. We show that
one can expect a rather accurate determination of the underlying R-parity
parameters assuming mSUGRA relations between the R-parity conserving ones and
discuss briefly also the general MSSM as well as the expected accuracies in
case of a prospective e+ e- linear collider. An important observation is that
several parameters can only be determined up to relative signs or more
generally relative phases.Comment: 13 pages, 13 figure
Spherical agglomeration of superconducting and normal microparticles with and without applied electric field
It was reported by R. Tao and coworkers that in the presence of a strong
electric field superconducting microparticles assemble into balls of
macroscopic dimensions. Such a finding has potentially important implications
for the understanding of the fundamental physics of superconductors. However,
we report here the results of experimental studies showing that (i) ball
formation also occurs in the absence of an applied electric field, (ii) the
phenomenon also occurs at temperatures above the superconducting transition
temperature, and (iii) it can also occur for non-superconducting materials.
Possible origins of the phenomenon are discussed.Comment: Small changes in response to referee's comments. To be published in
Phys. Rev.
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