30,058 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 $T_c$ 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
$dynamic$ $Hubbard$ $models$ 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.

### A general parametrization for the long-range part of neutrinoless double beta decay

Double beta decay has been proven to be a powerful tool to constrain $B-L$
violating physics beyond the standard model. We present a representation for
the long-range part of the general $0\nu\beta\beta$ decay rate allowed by
Lorentz-invariance. Combined with the short range part this general
parametrization in terms of effective $B-L$ violating couplings will provide
the $0\nu\beta\beta$ limits on arbitrary lepton number violating theories.Comment: Talk presented by H. P\"as at the Erice School on Nuclear Physics,
19th course "Neutrinos in Astro, Particle and Nuclear Physics", Erice, Italy,
16-24 September 1997, 2 pages, Latex, requires worldsci.st

### A superformula for neutrinoless double beta decay II: The short range part

A general Lorentz-invariant parameterization for the short-range part of the
0vBB decay rate is derived. Combined with the long range part already published
this general parameterization in terms of effective B-L violating couplings
allows one to extract the 0vBB limits on arbitrary lepton number violating
theories.Comment: 8 pages, LaTeX, 2 figure

### Meissner effect, Spin Meissner effect and charge expulsion in superconductors

The Meissner effect and the Spin Meissner effect are the spontaneous
generation of charge and spin current respectively near the surface of a metal
making a transition to the superconducting state. The Meissner effect is well
known but, I argue, not explained by the conventional theory, the Spin Meissner
effect has yet to be detected. I propose that both effects take place in all
superconductors, the first one in the presence of an applied magnetostatic
field, the second one even in the absence of applied external fields. Both
effects can be understood under the assumption that electrons expand their
orbits and thereby lower their quantum kinetic energy in the transition to
superconductivity. Associated with this process, the metal expels negative
charge from the interior to the surface and an electric field is generated in
the interior. The resulting charge current can be understood as arising from
the magnetic Lorentz force on radially outgoing electrons, and the resulting
spin current can be understood as arising from a spin Hall effect originating
in the Rashba-like coupling of the electron magnetic moment to the internal
electric field. The associated electrodynamics is qualitatively different from
London electrodynamics, yet can be described by a small modification of the
conventional London equations. The stability of the superconducting state and
its macroscopic phase coherence hinge on the fact that the orbital angular
momentum of the carriers of the spin current is found to be exactly $\hbar/2$,
indicating a topological origin. The simplicity and universality of our theory
argue for its validity, and the occurrence of superconductivity in many classes
of materials can be understood within our theory.Comment: Submitted to SLAFES XX Proceeding

### R-parity violation: Hide & Seek

We point out that, if R-parity is broken spontaneously, the neutralino can
decay to the final state majoron plus neutrino, which from the experimental
point of view is indistinguishable from the standard missing momentum signal of
supersymmetry. We identify the regions of parameter space where this decay mode
is dominant and show that they are independent of R-parity conserving SUSY
parameters. Thus, (a) only very weak limits on R-parity violating couplings can
be derived from the observation of missing momentum events and (b) at future
collider experiments huge statistics might be necessary to establish that
R-parity indeed is broken. Parameter combinations which give calculated relic
neutralino density larger than the measured dark matter density in case of
conserved R-parity are valid points in this scenario and their phenomenology at
the LHC deserves to be studied.Comment: 8 pages, 2 figures; ref. added; matches published version (title
changed in the published version

### Predicted electric field near small superconducting ellipsoids

We predict the existence of large electric fields near the surface of
superconducting bodies of ellipsoidal shape of dimensions comparable to the
penetration depth. The electric field is quadrupolar in nature with significant
corrections from higher order multipoles. Prolate (oblate) superconducting
ellipsoids are predicted to exhibit fields consistent with negative (positive)
quadrupole moments, reflecting the fundamental charge asymmetry of matter.Comment: To be published in Phys.Rev.Let

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