15,834 research outputs found
Antiferromagnetism in NiO Observed by Transmission Electron Diffraction
Neutron diffraction has been used to investigate antiferromagnetism since
1949. Here we show that antiferromagnetic reflections can also be seen in
transmission electron diffraction patterns from NiO. The diffraction patterns
taken here came from regions as small as 10.5 nm and such patterns could be
used to form an image of the antiferromagnetic structure with a nanometre
resolution.Comment: 10 pages, 7 figures. Typos corrected. To appear in Physical Review
Letter
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.
Invisible Higgs Boson Decays in Spontaneously Broken R-Parity
The Higgs boson may decay mainly to an invisible mode characterized by
missing energy, instead of the Standard Model channels. This is a generic
feature of many models where neutrino masses arise from the spontaneous
breaking of ungauged lepton number at relatively low scales, such as
spontaneously broken R-parity models. Taking these models as framework, we
reanalyze this striking suggestion in view of the recent data on neutrino
oscillations that indicate non-zero neutrino masses. We show that, despite the
smallness of neutrino masses, the Higgs boson can decay mainly to the invisible
Goldstone boson associated to the spontaneous breaking of lepton number. This
requires a gauge singlet superfield coupling to the electroweak doublet
Higgses, as in the Next to Minimal Supersymmetric Standard Model (NMSSM)
scenario for solving the -problem. The search for invisibly decaying Higgs
bosons should be taken into account in the planning of future accelerators,
such as the Large Hadron Collider and the Next Linear Collider.Comment: 24 pages, 10 figures; typos corrected, published versio
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
Coupling Between Thermal Oscillations in the Surface of a Micro-Cylinder and Vortex Shedding
his article studies the coupling between prescribed thermal oscillations in the surface of a micro-cylinder and vortex shedding. We deal with the unsteady, laminar, compressible flow regime where the aerodynamics forces have a periodic behavior. It is shown that appropriate spatial and time-dependent temperature oscillations on the surface of the micro-cylinder create a resonance that controls the amplitude and frequency of both lift and drag coefficients. In practice, what we study is a mechanism to modulate the amplitude and frequency of mechanical loads of aerodynamics origin in a micro-structure by using surface temperature fluctuations as the control parameter
Condensation energy in Eliashberg theory -- from weak to strong coupling
We consider two issues related to the condensation energy in superconductors
described by the Eliashberg theory for various forms of the pairing
interaction, associated either with phonon or electronic mechanisms of
superconductivity. First, we derive a leading correction to the BCS formula for
the condensation energy to first order in the coupling . Second, we
show that at a given , the value of the condensation energy strongly
depends on the functional form of the effective pairing interaction .Comment: 6 pages, 5 figures, published in PRB; missing reference has been
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Dynamics of Coherent States in Regular and Chaotic Regimes of the Non-integrable Dicke Model
The quantum dynamics of initial coherent states is studied in the Dicke model
and correlated with the dynamics, regular or chaotic, of their classical limit.
Analytical expressions for the survival probability, i.e. the probability of
finding the system in its initial state at time , are provided in the
regular regions of the model. The results for regular regimes are compared with
those of the chaotic ones. It is found that initial coherent states in regular
regions have a much longer equilibration time than those located in chaotic
regions. The properties of the distributions for the initial coherent states in
the Hamiltonian eigenbasis are also studied. It is found that for regular
states the components with no negligible contribution are organized in
sequences of energy levels distributed according to Gaussian functions. In the
case of chaotic coherent states, the energy components do not have a simple
structure and the number of participating energy levels is larger than in the
regular cases.Comment: Contribution to the proceedings of the Escuela Latinoamericana de
F\'isica (ELAF) Marcos Moshinsky 2017. (9 pages, 4 figures
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