8,981 research outputs found
Accumulation of embedded solitons in systems with quadratic nonlinearity
Previous numerical studies have revealed the existence of embedded solitons
(ESs) in a class of multi-wave systems with quadratic nonlinearity, families of
which seem to emerge from a critical point in the parameter space, where the
zero solution has a fourfold zero eigenvalue. In this paper, the existence of
such solutions is studied in a three-wave model. An appropriate rescaling casts
the system in a normal form, which is universal for models supporting ESs
through quadratic nonlinearities. The normal-form system contains a single
irreducible parameter , and is tantamount to the basic model of
type-I second-harmonic generation. An analytical approximation of WKB type
yields an asymptotic formula for the distribution of discrete values of
at which the ESs exist. Comparison with numerical results shows
that the asymptotic formula yields an exact value of the scaling index, -6/5,
and a fairly good approximation for the numerical factor in front of the
scaling term.Comment: 25 pages, 4 figure
Threonine 57 is required for the post-translational activation of Escherichia coli aspartate α-decarboxylase.
Aspartate α-decarboxylase is a pyruvoyl-dependent decarboxylase required for the production of β-alanine in the bacterial pantothenate (vitamin B5) biosynthesis pathway. The pyruvoyl group is formed via the intramolecular rearrangement of a serine residue to generate a backbone ester intermediate which is cleaved to generate an N-terminal pyruvoyl group. Site-directed mutagenesis of residues adjacent to the active site, including Tyr22, Thr57 and Tyr58, reveals that only mutation of Thr57 leads to changes in the degree of post-translational activation. The crystal structure of the site-directed mutant T57V is consistent with a non-rearranged backbone, supporting the hypothesis that Thr57 is required for the formation of the ester intermediate in activation
The 'At-risk mental state' for psychosis in adolescents : clinical presentation, transition and remission.
Despite increased efforts over the last decade to prospectively identify individuals at ultra-high risk of developing a psychotic illness, limited attention has been specifically directed towards adolescent populations (<18 years). In order to evaluate how those under 18 fulfilling the operationalised criteria for an At-Risk Mental State (ARMS) present and fare over time, we conducted an observational study. Participants (N = 30) generally reported a high degree of functional disability and frequent and distressing perceptual disturbance, mainly in the form of auditory hallucinations. Seventy percent (21/30) were found to fulfil the criteria for a co-morbid ICD-10 listed mental health disorder, with mood (affective; 13/30) disorders being most prevalent. Overall transition rates to psychosis were low at 24 months follow-up (2/28; 7.1 %) whilst many participants demonstrated a significant reduction in psychotic-like symptoms. The generalisation of these findings may be limited due to the small sample size and require replication in a larger sample
Gamma and beta frequency oscillations in response to novel auditory stimuli: A comparison of human electroencephalogram (EEG) data with in vitro models
Investigations using hippocampal slices maintained in vitro have demonstrated that bursts of oscillatory field potentials in the gamma frequency range (30-80 Hz) are followed by a slower oscillation in the beta 1 range (12-20 Hz). In this study, we demonstrate that a comparable gamma-to-beta transition is seen in the human electroencephalogram (EEG) in response to novel auditory stimuli. Correlations between gamma and beta 1 activity revealed a high degree of interdependence of synchronized oscillations in these bands in the human EEG. Evoked (stimulus-locked) gamma oscillations preceded beta 1 oscillations in response to novel stimuli, suggesting that this may be analogous to the gamma-to-beta shift observed in vitro. Beta 1 oscillations were the earliest discriminatory responses to show enhancement to novel stimuli, preceding changes in the broad-band event-related potential (mismatch negativity). Later peaks of induced beta activity over the parietal cortex were always accompanied by an underlying gamma frequency oscillation as seen in vitro. A further analogy between in vitro and human recordings was that both gamma and beta oscillations habituated markedly after the initial novel stimulus presentation
Structure and diffusive dynamics of aspartate α-decarboxylase (ADC) liganded with D-serine in aqueous solution.
Incoherent neutron spectroscopy, in combination with dynamic light scattering, was used to investigate the effect of ligand binding on the center-of-mass self-diffusion and internal diffusive dynamics of Escherichia coli aspartate α-decarboxylase (ADC). The X-ray crystal structure of ADC in complex with the D-serine inhibitor was also determined, and molecular dynamics simulations were used to further probe the structural rearrangements that occur as a result of ligand binding. These experiments reveal that D-serine forms hydrogen bonds with some of the active site residues, that higher order oligomers of the ADC tetramer exist on ns-ms time-scales, and also show that ligand binding both affects the ADC internal diffusive dynamics and appears to further increase the size of the higher order oligomers
Entangled-State Cycles of Atomic Collective-Spin States
We study quantum trajectories of collective atomic spin states of
effective two-level atoms driven with laser and cavity fields. We show that
interesting ``entangled-state cycles'' arise probabilistically when the (Raman)
transition rates between the two atomic levels are set equal. For odd (even)
, there are () possible cycles. During each cycle the
-qubit state switches, with each cavity photon emission, between the states
, where is a Dicke state in a rotated
collective basis. The quantum number (), which distinguishes the
particular cycle, is determined by the photon counting record and varies
randomly from one trajectory to the next. For even it is also possible,
under the same conditions, to prepare probabilistically (but in steady state)
the Dicke state , i.e., an -qubit state with excitations,
which is of particular interest in the context of multipartite entanglement.Comment: 10 pages, 9 figure
Soliton pair dynamics in patterned ferromagnetic ellipses
Confinement alters the energy landscape of nanoscale magnets, leading to the
appearance of unusual magnetic states, such as vortices, for example. Many
basic questions concerning dynamical and interaction effects remain unanswered,
and nanomagnets are convenient model systems for studying these fundamental
physical phenomena. A single vortex in restricted geometry, also known as a
non-localized soliton, possesses a characteristic translational excitation mode
that corresponds to spiral-like motion of the vortex core around its
equilibrium position. Here, we investigate, by a microwave reflection
technique, the dynamics of magnetic soliton pairs confined in lithographically
defined, ferromagnetic Permalloy ellipses. Through a comparison with
micromagnetic simulations, the observed strong resonances in the subgigahertz
frequency range can be assigned to the translational modes of vortex pairs with
parallel or antiparallel core polarizations. Vortex polarizations play a
negligible role in the static interaction between two vortices, but their
effect dominates the dynamics.Comment: supplemental movies on
http://www.nature.com/nphys/journal/v1/n3/suppinfo/nphys173_S1.htm
A survey of statistics in three UK general practice journal
Background
Many medical specialities have reviewed the statistical content of their journals. To our knowledge this has not been done in general practice. Given the main role of a general practitioner as a diagnostician we thought it would be of interest to see whether the statistical methods reported reflect the diagnostic process.
Methods
Hand search of three UK journals of general practice namely the British Medical Journal (general practice section), British Journal of General Practice and Family Practice over a one-year period (1 January to 31 December 2000).
Results
A wide variety of statistical techniques were used. The most common methods included t-tests and Chi-squared tests. There were few articles reporting likelihood ratios and other useful diagnostic methods. There was evidence that the journals with the more thorough statistical review process reported a more complex and wider variety of statistical techniques.
Conclusions
The BMJ had a wider range and greater diversity of statistical methods than the other two journals. However, in all three journals there was a dearth of papers reflecting the diagnostic process. Across all three journals there were relatively few papers describing randomised controlled trials thus recognising the difficulty of implementing this design in general practice
Topological Crystalline Insulators in the SnTe Material Class
Topological crystalline insulators are new states of matter in which the
topological nature of electronic structures arises from crystal symmetries.
Here we predict the first material realization of topological crystalline
insulator in the semiconductor SnTe, by identifying its nonzero topological
index. We predict that as a manifestation of this nontrivial topology, SnTe has
metallic surface states with an even number of Dirac cones on high-symmetry
crystal surfaces such as {001}, {110} and {111}. These surface states form a
new type of high-mobility chiral electron gas, which is robust against disorder
and topologically protected by reflection symmetry of the crystal with respect
to {110} mirror plane. Breaking this mirror symmetry via elastic strain
engineering or applying an in-plane magnetic field can open up a continuously
tunable band gap on the surface, which may lead to wide-ranging applications in
thermoelectrics, infrared detection, and tunable electronics. Closely related
semiconductors PbTe and PbSe also become topological crystalline insulators
after band inversion by pressure, strain and alloying.Comment: submitted on Feb. 10, 2012; to appear in Nature Communications; 5
pages, 4 figure
- …