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 $v(x)=c(x)$, with $v(x),c(x)$ 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 $v_{cr}$. When the interspecies coupling is small the critical value approaches the value $v_{cr}=c_1+c_2$, 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