Thick-medium model of transverse pattern formation in optically excited cold two-level atoms with a feedback mirror

We study a pattern-forming instability in a laser-driven optically thick cloud of cold two-level atoms with a planar feedback mirror. We develop a theoretical model, enabling a full analysis of transverse patterns in a medium with saturable nonlinearity, taking into account diffraction within the medium, and both the transmission and reflection gratings. The focus of the analysis is on the combined treatment of nonlinear propagation in a diffractively and optically thick medium and the boundary condition given by feedback. We demonstrate explicitly how diffraction within the medium breaks the degeneracy of Talbot modes inherent in thin-slice models. We predict the existence of envelope curves bounding all possible pattern-formation thresholds and illustrate their interaction with threshold curves by experimental observation of a sudden transition between length scales as mirror displacement is varied

Multiple sclerosis or neuromyelitis optica? Re-evaluating an 18th-century illness using 21st-century software

In this paper we report the application of an extensive database of symptoms, signs, laboratory findings and illnesses, to the diagnosis of an historical figure. The medical diagnosis of Augustus d'Este (1794–1848) – widely held to be the first documented case of multiple sclerosis – is reviewed, using the detailed symptom diary, which he kept over many years, as clinical data. Some of the reported features prompted the competing claim that d'Este suffered from acute porphyria, which in turn was used in support of the hypothesis that his grandfather, King George III, also suffered from the disease. We find that multiple sclerosis is statistically the most likely diagnosis, with neuromyelitis optica a strong alternative possibility. The database did not support a diagnosis of any of the acute porphyrias

A case for a negative-strand coding sequence in a group of positive-sense RNA viruses.

Positive-sense single-stranded RNA viruses form the largest and most diverse group of eukaryote-infecting viruses. Their genomes comprise one or more segments of coding-sense RNA that function directly as messenger RNAs upon release into the cytoplasm of infected cells. Positive-sense RNA viruses are generally accepted to encode proteins solely on the positive strand. However, we previously identified a surprisingly long (∼1,000-codon) open reading frame (ORF) on the negative strand of some members of the family Narnaviridae which, together with RNA bacteriophages of the family Leviviridae, form a sister group to all other positive-sense RNA viruses. Here, we completed the genomes of three mosquito-associated narnaviruses, all of which have the long reverse-frame ORF. We systematically identified narnaviral sequences in public data sets from a wide range of sources, including arthropod, fungal, and plant transcriptomic data sets. Long reverse-frame ORFs are widespread in one clade of narnaviruses, where they frequently occupy >95 per cent of the genome. The reverse-frame ORFs correspond to a specific avoidance of CUA, UUA, and UCA codons (i.e. stop codon reverse complements) in the forward-frame RNA-dependent RNA polymerase ORF. However, absence of these codons cannot be explained by other factors such as inability to decode these codons or GC3 bias. Together with other analyses, we provide the strongest evidence yet of coding capacity on the negative strand of a positive-sense RNA virus. As these ORFs comprise some of the longest known overlapping genes, their study may be of broad relevance to understanding overlapping gene evolution and de novo origin of genes

Spatial correlations in hexagons generated via a Kerr nonlinearity

We consider the hexagonal pattern forming in the cross-section of an optical beam produced by a Kerr cavity, and we study the quantum correlations characterizing this structure. By using arguments related to the symmetry broken by the pattern formation, we identify a complete scenario of six-mode entanglement. Five independent phase quadratures combinations, connecting the hexagonal modes, are shown to exhibit sub-shot-noise fluctuations. By means of a non-linear quantum calculation technique, quantum correlations among the mode photon numbers are demonstrated and calculated.Comment: ReVTeX file, 20 pages, 7 eps figure

Mpemba Effect, Shechtman's Quasicrystals and Students' Exploring Activities

In the 1960s, Tanzanian student Erasto Mpemba and his teacher published an article with the title "Cool" in the journal Physics Education (Mpemba, E. B. - Osborne, D. G.: Cool?. In: Physics Education, vol.4, 1969, pp. 172-175.). In this article they claimed that hot water freezes faster than cold water. The article raised not only a wave of discussions, and other articles about this topic, but also a whole series of new experiments, which should verify this apparent thermodynamic absurdity and find an adequate explanation. Here we give a review with references to explanations and we bring some proposals for experimental student work in this area. We introduce Mpemba Effect not only as a paradoxical physics phenomenon, but we shall present a strong educational message that the Mpemba story brings to the teachers and their students. This message also creates a bridge between this phenomenon and the discovery for which the 2011 Nobel Prize in Chemistry was awarded. It leads to critical adoption of traditional knowledge and encourages resilience in investigative exploration of new things

NON-FREE VORTEX FLOW EFFECTS IN AN AXIAL FLOW ROTOR

This paper presents the analysis of three-dimensional flow field developing through an industrial axial flow fan rotor of non-free vortex design carried out on the basis of concerted experimental and numerical investigations. The paper focuses on non-free vortex flow effects requiring consideration in rotor blade design theories. The distortion of stream surfaces through the blading has been observed and a quantitative analysis of its effects on the blade loading condition has been carried out in the through-flow core region. The streamlines close to the blades have been modelled fitting to outward and inward conical stream tubes on the blade suction and pressure sides, respectively - termed `cone couple model´. It has been pointed out that the blade lift can be satisfactorily described at midspan on the basis of the cone couple model, with use of pitch-averaged flow data. The loading aspects of the tested axial rotor have been discussed on the basis of both the pressure distribution in the vicinity of the blades and pitch-averaged flow data obtained upstream and downstream of the rotor. Sample calculations suggested that separate optimisation of the blade suction and pressure sides is essential if the three-dimensional blade-to-blade flow is intended to be considered in NFV fan design with use of two-dimensional cascade data

Bilateral symmetry breaking in a nonlinear Fabry-Perot cavity exhibiting optical tristability

We show the existence of a region in the parameter space that defines the field dynamics in a Fabry-Perot cylindrical cavity, where three output stable stationary states of the light are possible for a given localized incident field. Two of these states do not preserve the bilateral (i.e. left-right) symmetry of the entire system. These broken-symmetry states are the high-transmission nonlinear modes of the system. We also discuss how to excite these states.Comment: 5 pages, 5 figure

Instabilities of Higher-Order Parametric Solitons. Filamentation versus Coalescence

We investigate stability and dynamics of higher-order solitary waves in quadratic media, which have a central peak and one or more surrounding rings. We show existence of two qualitatively different behaviours. For positive phase mismatch the rings break up into filaments which move radially to initial ring. For sufficient negative mismatches rings are found to coalesce with central peak, forming a single oscillating filament.Comment: 5 pages, 7 figure