Structural Invariance and the Energy Spectrum

We extend the application of the concept of structural invariance to bounded time independent systems. This concept, previously introduced by two of us to argue that the connection between random matrix theory and quantum systems with a chaotic classical counterpart is in fact largely exact in the semiclassical limit, is extended to the energy spectra of bounded time independent systems. We proceed by showing that the results obtained previously for the quasi-energies and eigenphases of the S-matrix can be extended to the eigenphases of the quantum Poincare map which is unitary in the semiclassical limit. We then show that its eigenphases in the chaotic case move rather stiffly around the unit circle and thus their local statistical fluctuations transfer to the energy spectrum via Bogomolny's prescription. We verify our results by studying numerically the properties of the eigenphases of the quantum Poincare map for billiards by using the boundary integral method.Comment: 10 pages, 5 figure

Intrinsic Curie temperature bistability in ferromagnetic semiconductor resonant tunneling diodes

We predict bistability in the Curie temperature-voltage characteristic of double barrier resonant-tunneling structures with dilute ferromagnetic semiconductor quantum wells. Our conclusions are based on simulations of electrostatics and ballistic quantum transport combined with a mean-field theory description of ferromagnetism in dilute magnetic semiconductors.Comment: 10 pages, 3 figures, submitted to Phys. Rev. B; typo removed in revised version - spurious eq.12 immediately after eq.1

Symmetric and Asymmetric Coalescence of Drops on a Substrate

The coalescence of viscous drops on a substrate is studied experimentally and theoretically. We consider cases where the drops can have different contact angles, leading to a very asymmetric coalescence process. Side view experiments reveal that the "bridge" connecting the drops evolves with self-similar dynamics, providing a new perspective on the coalescence of sessile drops. We show that the universal shape of the bridge is accurately described by similarity solutions of the one-dimensional lubrication equation. Our theory predicts a bridge that grows linearly in time and stresses the strong dependence on the contact angles. Without any adjustable parameters, we find quantitative agreement with all experimental observations.Comment: 5 pages, 4 figure

An investigation into the perspectives of providers and learners on MOOC accessibility

An effective open eLearning environment should consider the target learner’s abilities, learning goals, where learning takes place, and which specific device(s) the learner uses. MOOC platforms struggle to take these factors into account and typically are not accessible, inhibiting access to environments that are intended to be open to all. A series of research initiatives are described that are intended to benefit MOOC providers in achieving greater accessibility and disabled learners to improve their lifelong learning and re-skilling. In this paper, we first outline the rationale, the research questions, and the methodology. The research approach includes interviews, online surveys and a MOOC accessibility audit; we also include factors such the risk management of the research programme and ethical considerations when conducting research with vulnerable learners. Preliminary results are presented from interviews with providers and experts and from analysis of surveys of learners. Finally, we outline the future research opportunities. This paper is framed within the context of the Doctoral Consortium organised at the TEEM'17 conference

Passive harmonic mode-locking in a fiber laser with nonlinear polarization rotation

On basis of numerical simulation and analytical treatment, we investigate the multiple pulse passive mode-locking of a fiber laser with nonlinear loss due to the nonlinear polarization rotation technique. Various additional mechanisms resulting in the ordering of ultrashort pulses inside the laser resonator and in the realization of harmonic passive mode-locking are analyzed. Among which are the active harmonic modulation of the intracavity loss or of the refractive index, the passive modulation of the index due to the inertial properties of intracavity elements, and the amplification modulation due to the depletion and the relaxation of the gain. The velocity of pulses relative to each other is determined, it results in attractive or repulsive pulse–pulse interaction, and allows to evaluate the conditions of ordering of the pulses inside the cavity

Spectral-selective management of dissipative solitons in passive mode-locked fibre lasers

We study properties of dissipative solitons in passive mode-locked fibre lasers due to additional intracavity narrow spectral selection. Such selection allows us to control the soliton velocity, the nature of the interaction between solitons (attraction or repulsion), to create a bistable state in which the velocity of a single soliton can have two different values, to realize an elastic particle-like collision between single solitons. In the investigated laser system, the soliton interaction distance can be considerably larger than the soliton duration, which allows the realization of passive harmonic mode-locking

Brillouin instabilities in continuously pumped high power fiber lasers

The effect of Brillouin backscattering on the stability of a high power continuously pumped fiber laser is theoretically analyzed in the general framework of two-coupled modes laser model. It is demonstrated that, depending on the cavity losses, different type of instabilities can arise. Low loss cavity favours stable continuous regime in a large range of pumping rates while high loss configuration permits the emergence of different self-pulsing instabilities

Instabilities in high power fiber lasers induced by stimulated Brillouin scattering

We investigate numerically different types of instabilities in a high loss Fabry–Perot laser cavity in presence of the stimulated Brillouin scattering. Our results reveal many interesting dynamical behaviors such as periodic, quasi-periodic and chaotic

Uncertainties in gas kinematics arising from stellar continuum modelling in integral field spectroscopy data: the case of NGC2906 observed with MUSE/VLT

We study how the use of several stellar subtraction methods and line fitting approaches can affect the derivation of the main kinematic parameters (velocity and velocity dispersion fields) of the ionized gas component. The target of this work is the nearby galaxy NGC 2906, observed with the MUSE instrument at Very Large Telescope. A sample of twelve spectra is selected from the inner (nucleus) and outer (spiral arms) regions, characterized by different ionization mechanisms. We compare three different methods to subtract the stellar continuum (FIT3D, STARLIGHT and pPXF), combined with one of the following stellar libraries: MILES, STELIB and GRANADA+MILES. The choice of the stellar subtraction method is the most important ingredient affecting the derivation of the gas kinematics, followed by the choice of the stellar library and by the line fitting approach. In our data, typical uncertainties in the observed wavelength and width of the H\alpha and [NII] lines are of the order of _rms \sim 0.1\AA\ and _rms \sim 0.2\AA\ (\sim 5 and 10km/s, respectively). The results obtained from the [NII] line seem to be slightly more robust, as it is less affected by stellar absorption than H\alpha. All methods considered yield statistically consistent measurements once a mean systemic contribution \Delta\bar\lambda=\Delta\bar\sigma=0.2xDelta_{MUSE} is added in quadrature to the line fitting errors, where \Delta_{MUSE} = 1.1\AA\ \sim 50 km/s denotes the instrumental resolution of the MUSE spectra. Although the subtraction of the stellar continuum is critical in order to recover line fluxes, any method (including none) can be used in order to measure the gas kinematics, as long as an additional component of 0.2 x Delta_MUSE is added to the error budget.Comment: 20 pages, 14 figure