310 research outputs found
PT-symmetry in honeycomb photonic lattices
We apply gain/loss to honeycomb photonic lattices and show that the
dispersion relation is identical to tachyons - particles with imaginary mass
that travel faster than the speed of light. This is accompanied by PT-symmetry
breaking in this structure. We further show that the PT-symmetry can be
restored by deforming the lattice
Fractoluminescence characterization of the energy dissipated during fast fracture of glass
Fractoluminescence experiments are performed on two kinds of silicate
glasses. All the light spectra collected during dynamic fracture reveal a black
body radiator behaviour, which is interpreted as a crack velocity-dependent
temperature rise close to the crack tip. Crack velocities are estimated to be
of the order of 1300 m.s and fracture process zones are shown to extend
over a few nanometers.Comment: Accepted for publication in Europhysics Letters; 5 pages; 4 figure
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Normally-off GaN transistors for power applications
Normally-off high voltage GaN-HFETs for switching applications are presented. Normally-off operation with threshold voltages of 1 V and more and with 5 V gate swing has been obtained by using p-type GaN as gate. Different GaN-based buffer types using doping and backside potential barriers have been used to obtain blocking strengths up to 1000 V. The increase of the dynamic on-state resistance is analyzed for the different buffer types. The best trade-off between low dispersion and high blocking strength was obtained for a modified carbon-doped GaN-buffer that showed a 2.6x increase of the dynamic on-state resistance for 500 V switching as compared to switching from 20 V off-state drain bias. Device operation up to 200 °C ambient temperature without any threshold voltage shift is demonstrated
Managing Injuries of the Neck Trial (MINT) : design of a randomised controlled trial of treatments for whiplash associated disorders
Background: A substantial proportion of patients with whiplash injuries develop chronic
symptoms. However, the best treatment of acute injuries to prevent long-term problems is
uncertain. A stepped care treatment pathway has been proposed, in which patients are given advice
and education at their initial visit to the emergency department (ED), followed by review at three
weeks and physiotherapy for those with persisting symptoms. MINT is a two-stage randomised
controlled trial to evaluate two components of such a pathway: 1. use of The Whiplash Book versus
usual advice when patients first attend the emergency department; 2. referral to physiotherapy
versus reinforcement of advice for patients with continuing symptoms at three weeks.
Methods: Evaluation of the Whiplash Book versus usual advice uses a cluster randomised design
in emergency departments of eight NHS Trusts. Eligible patients are identified by clinicians in
participating emergency departments and are sent a study questionnaire within a week of their ED
attendance. Three thousand participants will be included. Patients with persisting symptoms three
weeks after their ED attendance are eligible to join an individually randomised study of
physiotherapy versus reinforcement of the advice given in ED. Six hundred participants will be
randomised. Follow-up is at 4, 8 and 12 months after their ED attendance. Primary outcome is the
Neck Disability Index (NDI), and secondary outcomes include quality of life and time to return to
work and normal activities. An economic evaluation is being carried out.
Conclusion: This paper describes the protocol and operational aspects of a complex intervention
trial based in NHS emergency and physiotherapy departments, evaluating two components of a
stepped-care approach to the treatment of whiplash injuries. The trial uses two randomisations,
with the first stage being cluster randomised and the second individually randomised
Leptogenesis from Soft Supersymmetry Breaking (Soft Leptogenesis)
Soft leptogenesis is a scenario in which the cosmic baryon asymmetry is
produced from a lepton asymmetry generated in the decays of heavy sneutrinos
(the partners of the singlet neutrinos of the seesaw) and where the relevant
sources of CP violation are the complex phases of soft supersymmetry-breaking
terms. We explain the motivations for soft leptogenesis, and review its basic
ingredients: the different CP-violating contributions, the crucial role played
by thermal corrections, and the enhancement of the efficiency from lepton
flavour effects. We also discuss the high temperature regime GeV in
which the cosmic baryon asymmetry originates from an initial asymmetry of an
anomalous -charge, and soft leptogenesis reembodies in -genesis.Comment: References updated. Some minor corrections to match the published
versio
Dirac Neutrinos, Dark Energy and Baryon Asymmetry
We explore a new origin of neutrino dark energy and baryon asymmetry in the
universe. The neutrinos acquire small masses through the Dirac seesaw
mechanism. The pseudo-Nambu-Goldstone boson associated with neutrino
mass-generation provides a candidate for dark energy. The puzzle of
cosmological baryon asymmetry is resolved via neutrinogenesis.Comment: 6 pages, 1 figure. Accepted by JCAP (only minor rewordings, refs
added
Strain-induced pseudomagnetic field and Landau levels in photonic structures
Magnetic effects at optical frequencies are notoriously weak. This is
evidenced by the fact that the magnetic permeability of nearly all materials is
unity in the optical frequency range, and that magneto-optical devices (such as
Faraday isolators) must be large in order to allow for a sufficiently strong
effect. In graphene, however, it has been shown that inhomogeneous strains can
induce 'pseudomagnetic fields' that behave very similarly to real fields. Here,
we show experimentally and theoretically that, by properly structuring a
dielectric lattice, it is possible to induce a pseudomagnetic field at optical
frequencies in a photonic lattice, where the propagation dynamics is equivalent
to the evolution of an electronic wavepacket in graphene. To our knowledge,
this is the first realization of a pseudomagnetic field in optics. The induced
field gives rise to multiple photonic Landau levels (singularities in the
density of states) separated by band gaps. We show experimentally and
numerically that the gaps between these Landau levels give rise to transverse
confinement of the optical modes. The use of strain allows for the exploration
of magnetic effects in a non-resonant way that would be otherwise inaccessible
in optics. Employing inhomogeneous strain to induce pseudomagnetism suggests
the possibility that aperiodic photonic crystal structures can achieve greater
field-enhancement and slow-light effects than periodic structures via the high
density-of-states at Landau levels. Generalizing these concepts to other
systems beyond optics, for example with matter waves in optical potentials,
offers new intriguing physics that is fundamentally different from that in
purely periodic structures.Comment: 24 pages including supplementary information section, 4 figure
Klein tunneling in graphene: optics with massless electrons
This article provides a pedagogical review on Klein tunneling in graphene,
i.e. the peculiar tunneling properties of two-dimensional massless Dirac
electrons. We consider two simple situations in detail: a massless Dirac
electron incident either on a potential step or on a potential barrier and use
elementary quantum wave mechanics to obtain the transmission probability. We
emphasize the connection to related phenomena in optics, such as the
Snell-Descartes law of refraction, total internal reflection, Fabry-P\'erot
resonances, negative refraction index materials (the so called meta-materials),
etc. We also stress that Klein tunneling is not a genuine quantum tunneling
effect as it does not necessarily involve passing through a classically
forbidden region via evanescent waves. A crucial role in Klein tunneling is
played by the conservation of (sublattice) pseudo-spin, which is discussed in
detail. A major consequence is the absence of backscattering at normal
incidence, of which we give a new shorten proof. The current experimental
status is also thoroughly reviewed. The appendix contains the discussion of a
one-dimensional toy model that clearly illustrates the difference in Klein
tunneling between mono- and bi-layer graphene.Comment: short review article, 18 pages, 14 figures; v3: references added,
several figures slightly modifie
U-Pb baddeleyite age of a ne trending doleritic dyke swarm in the Tagragra of Akka Inlier: additional constraints on the ca. 1416-1360 Ma Mesoproterozoic magmatic event(s) in the anti-atlas of Morocco (West African Craton)
Protecting older patients with cardiovascular diseases from COVID-19 complications using current medications
Purpose In the pathogenesis of severe COVID-19 complications, derangements of renin-angiotensin-aldosterone system (RAAS), vascular endothelial dysfunction leading to inflammation and coagulopathy, and arrhythmias play an important role. Therefore, it is worth considering the use of currently available drugs to protect COVID-19 patients with cardiovascular diseases. Methods We review the current experience of conventional cardiovascular drugs [angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers, anticoagulants, acetosalicylic acid, antiarrhythmic drugs, statins] as well as some other drug classes (antidiabetic drugs, vitamin D and NSAIDs) frequently used by older patients with cardiovascular diseases. Data were sought from clinical databases for COVID-19 and appropriate key words. Conclusions and recommendations are based on a consensus among all authors. Results Several cardiovascular drugs have a potential to protect patients with COVID-19, although evidence is largely based on retrospective, observational studies. Despite propensity score adjustments used in many analyses observational studies are not equivalent to randomised controlled trials (RCTs). Ongoing RCTs include treatment with antithrombotics, pulmonary vasodilators, RAAS-related drugs, and colchicine. RCTs in the acute phase of COVID-19 may not, however, recognise the benefits of long term anti-atherogenic therapies, such as statins. Conclusions Most current cardiovascular drugs can be safely continued during COVID-19. Some drug classes may even be protective. Age-specific data are scarce, though, and conditions which are common in older patients (frailty, comorbidities, polypharmacy) must be individually considered for each drug group. Key summary pointsAim To review current cardiovascular medications for benefits and potential harms during COVID-19. Findings Several cardiovascular drugs have a potential to protect patients with COVID-19, although evidence is largely based on observational studies and age-specific data are scarce. Message Most current cardiovascular drugs can be safely continued during COVID-19, but general conditions common in older patients must be considered.Peer reviewe
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