4,304 research outputs found
Instability of the superfluid flow as black-hole lasing effect
We show that the instability leading to the decay of the one-dimensional
superfluid flow through a penetrable barrier are due to the black-hole lasing
effect. This dynamical instability is triggered by modes resonating in an
effective cavity formed by two horizons enclosing the barrier. The location of
the horizons is set by , with being the local fluid
velocity and sound speed, respectively. We compute the critical velocity
analytically and show that it is univocally determined by the horizons
configuration. In the limit of broad barriers, the continuous spectrum at the
origin of the Hawking-like radiation and of the Landau energetic instability is
recovered.Comment: 18 pages, 3 figure
The invisible reality: English teaching materials and the formation of gender and sexually oriented stereotypes (with a focus on primary education)
Gender stereotypes, understood as those structured sets of beliefs about personal attributes of women and men, have a great influence over self-perspective and the social interaction and organisation. However, their effects are sometimes invisible, and a great effort should be made to develop awareness of their influence in the population. Our main claim is that School has an essential role to teach gender and sexual equality through the curricula, using teaching materials that are free of these stereotypes. The main aim of this study is to examine the presence of gender and sexually oriented stereotypes in various English teaching materials within the context of the Spanish Primary School, focusing on its last stage. This research carries out a qualitative and quantitative analysis of three English textbooks together with the students' and teachers' perceptions of gender stereotypes. The analysis is done thanks to the review of key concepts, such as gender awareness and sexual identity, as well as the presentation of various examination tools that have allowed us to evaluate the sexist content in the textbooks selected and offer some guidelines to avoid them in the English classroom
Efficient low-power terahertz generation via on-chip triply-resonant nonlinear frequency mixing
Achieving efficient terahertz (THz) generation using compact turn-key sources
operating at room temperature and modest power levels represents one of the
critical challeges that must be overcome to realize truly practical
applications based on THz. Up to now, the most efficient approaches to THz
generation at room temperature -- relying mainly on optical rectification
schemes -- require intricate phase-matching set-ups and powerful lasers. Here
we show how the unique light-confining properties of triply-resonant photonic
resonators can be tailored to enable dramatic enhancements of the conversion
efficiency of THz generation via nonlinear frequency down-conversion processes.
We predict that this approach can be used to reduce up to three orders of
magnitude the pump powers required to reach quantum-limited conversion
efficiency of THz generation in nonlinear optical material systems.
Furthermore, we propose a realistic design readily accesible experimentally,
both for fabrication and demonstration of optimal THz conversion efficiency at
sub-W power levels
Counter-flow instability of a quantum mixture of two superfluids
We study the instability of a mixture of two interacting counter-flowing
superfluids. For a homogeneous system, we show that superfluid hydrodynamics
leads to the existence of a dynamical instability at a critical value of the
relative velocity . When the interspecies coupling is small the
critical value approaches the value , given by the sum of the
sound velocities of the two uncoupled superfluids, in agreement with the recent
prediction of [1] based on Landau's argument. The crucial dependence of the
critical velocity on the interspecies coupling is explicitly discussed. Our
results agree with previous predictions for weakly interacting Bose-Bose
mixtures and applies to Bose-Fermi superfluid mixtures as well. Results for the
stability of transversally trapped mixtures are also presented.Comment: 5 pages, 2 figue
Mn valence instability in La2/3Ca1/3MnO3 thin films
A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3
(001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge
and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a
Curie temperature well below that of the bulk material. Upon air exposure a
reduction of the saturation magnetization, MS, of the films is detected.
Simultaneously a Mn2+ spectral signature develops, in addition to the expected
Mn3+ and Mn4+ contributions, which increases with time. The similarity of the
spectral results obtained by total electron yield and fluorescence yield
spectroscopy indicates that the location of the Mn valence anomalies is not
confined to a narrow surface region of the film, but can extend throughout the
whole thickness of the sample. High temperature annealing at 1000 C in air,
immediately after growth, improves the magnetic and transport properties of
such films towards the bulk values and the Mn2+ signature in the spectra does
not appear. The Mn valence is then stable even to prolonged air exposure. We
propose a mechanism for the Mn2+ ions formation and discuss the importance of
these observations with respect to previous findings and production of thin
films devices.Comment: Double space, 21 pages, 6 figure
Integrability of a t-J model with impurities
A t-J model for correlated electrons with impurities is proposed. The
impurities are introduced in such a way that integrability of the model in one
dimension is not violated. The algebraic Bethe ansatz solution of the model is
also given and it is shown that the Bethe states are highest weight states with
respect to the supersymmetry algebra gl(2/1)Comment: 14 page
The Single State Dominance Hypothesis and the Two-Neutrino Double Beta Decay of Mo100
The hypothesis of the single state dominance (SSD) in the calculation of the
two-neutrino double beta decay of Mo100 is tested by exact consideration of the
energy denominators of the perturbation theory. Both transitions to the ground
state as well as to the 0+ and 2+ excited states of the final nucleus Ru100 are
considered. We demonstrate, that by experimental investigation of the single
electron energy distribution and the angular correlation of the outgoing
electrons, the SSD hypothesis can be confirmed or ruled out by a precise
two-neutrino double beta decay measurement (e.g. by NEMO III collaboration).Comment: 13 pages, RevTex, 1 figur
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