6,885 research outputs found
Flat Dielectric Grating Reflectors with High Focusing Power
Sub-wavelength dielectric gratings (SWG) have emerged recently as a promising
alternative to distributed-Bragg-reflection (DBR) dielectric stacks for
broadband, high-reflectivity filtering applications. A SWG structure composed
of a single dielectric layer with the appropriate patterning can sometimes
perform as well as thirty or forty dielectric DBR layers, while providing new
functionalities such as polarization control and near-field amplification. In
this paper, we introduce a remarkable property of grating mirrors that cannot
be realized by their DBR counterpart: we show that a non-periodic patterning of
the grating surface can give full control over the phase front of reflected
light while maintaining a high reflectivity. This new feature of dielectric
gratings could have a substantial impact on a number of applications that
depend on low-cost, compact optical components, from laser cavities to CD/DVD
read/write heads.Comment: submitted to Nature Photonic
Awake chronic mouse model of targeted pial vessel occlusion via photothrombosis
Animal models of stroke are used extensively to study the mechanisms involved in the acute and chronic phases of recovery following stroke. A translatable animal model that closely mimics the mechanisms of a human stroke is essential in understanding recovery processes as well as developing therapies that improve functional outcomes. We describe a photothrombosis stroke model that is capable of targeting a single distal pial branch of the middle cerebral artery with minimal damage to the surrounding parenchyma in awake head-fixed mice. Mice are implanted with chronic cranial windows above one hemisphere of the brain that allow optical access to study recovery mechanisms for over a month following occlusion. Additionally, we study the effect of laser spot size used for occlusion and demonstrate that a spot size with small axial and lateral resolution has the advantage of minimizing unwanted photodamage while still monitoring macroscopic changes to cerebral blood flow during photothrombosis. We show that temporally guiding illumination using real-time feedback of blood flow dynamics also minimized unwanted photodamage to the vascular network. Finally, through quantifiable behavior deficits and chronic imaging we show that this model can be used to study recovery mechanisms or the effects of therapeutics longitudinally.R01 EB021018 - NIBIB NIH HHS; R01 MH111359 - NIMH NIH HHS; R01 NS108472 - NINDS NIH HHSPublished versio
Monopoles and Holography
We present a holographic theory in AdS_4 whose zero temperature ground state
develops a crystal structure, spontaneously breaking translational symmetry.
The crystal is induced by a background magnetic field, but requires no chemical
potential. This lattice arises from the existence of 't Hooft-Polyakov monopole
solitons in the bulk which condense to form a classical object known as a
monopole wall. In the infra-red, the magnetic field is screened and there is an
emergent SU(2) global symmetry.Comment: 33 pages, 16 figures; v2: ref adde
Higher spin fermions in the BTZ black hole
Recently it has been shown that the wave equations of bosonic higher spin
fields in the BTZ background can be solved exactly. In this work we extend this
analysis to fermionic higher spin fields. We solve the wave equations for
arbitrary half-integer spin fields in the BTZ black hole background and obtain
exact expressions for their quasinormal modes. These quasinormal modes are
shown to agree precisely with the poles of the corresponding two point function
in the dual conformal field theory as predicted by the AdS/CFT correspondence.
We also obtain an expression for the 1-loop determinant in terms of the
quasinormal modes and show it agrees with that obtained by integrating the heat
kernel found by group theoretic methods.Comment: 29 page
Flowing Between Fermionic Fixed Points
We study holographic Wilsonian renormalization group flows for bulk spinor
fields in AdS. We use this to compute the all-loop beta function for fermionic
double trace operators in the dual conformal field theory.Comment: 21 pages. V2: Acknowledgement added; v3: Typo correcte
Photoemission "experiments" on holographic superconductors
We study the effects of a superconducting condensate on holographic Fermi
surfaces. With a suitable coupling between the fermion and the condensate,
there are stable quasiparticles with a gap. We find some similarities with the
phenomenology of the cuprates: in systems whose normal state is a non-Fermi
liquid with no stable quasiparticles, a stable quasiparticle peak appears in
the condensed phase.Comment: 14 pages, 13 figures; v2: typos corrected and some clarification
adde
Higher spin quasinormal modes and one-loop determinants in the BTZ black hole
We solve the wave equations of arbitrary integer spin fields in the BTZ black
hole background and obtain exact expressions for their quasinormal modes. We
show that these quasinormal modes precisely agree with the location of the
poles of the corresponding two point function in the dual conformal field
theory as predicted by the AdS/CFT correspondence. We then use these
quasinormal modes to construct the one-loop determinant of the higher spin
field in the thermal BTZ background. This is shown to agree with that obtained
from the corresponding heat kernel constructed recently by group theoretic
methods.Comment: 47 page
Stellar spectroscopy: Fermions and holographic Lifshitz criticality
Electron stars are fluids of charged fermions in Anti-de Sitter spacetime.
They are candidate holographic duals for gauge theories at finite charge
density and exhibit emergent Lifshitz scaling at low energies. This paper
computes in detail the field theory Green's function G^R(w,k) of the
gauge-invariant fermionic operators making up the star. The Green's function
contains a large number of closely spaced Fermi surfaces, the volumes of which
add up to the total charge density in accordance with the Luttinger count.
Excitations of the Fermi surfaces are long lived for w <~ k^z. Beyond w ~ k^z
the fermionic quasiparticles dissipate strongly into the critical Lifshitz
sector. Fermions near this critical dispersion relation give interesting
contributions to the optical conductivity.Comment: 38 pages + appendices. 9 figure
Collaborative research and development (R&D) for climate technology transfer and uptake in developing countries: Towards a needs driven approach
While international cooperation to facilitate the transfer and uptake of climate technologies in developing countries is an ongoing part of climate policy conversations, international collaborative R&D has received comparatively little attention. Collaborative R&D, however, could be a potentially important contributor to facilitating the transfer and uptake of climate technologies in developing countries. But the complexities of international collaborative R&D options and their distributional consequences have been given little attention to date. This paper develops a systematic approach to informing future empirical research and policy analysis on this topic. Building on insights from relevant literature and analysis of empirical data based on a sample of existing international climate technology R&D initiatives, three contributions are made. First, the paper analyses the coverage of existing collaborative R&D efforts in relation to climate technologies, highlighting some important concerns, such as a lack of coverage of lower-income countries or adaptation technologies. Second, it provides a starting point for further systematic research and policy thinking via the development of a taxonomic approach for analysing collaborative designs. Finally, it matches characteristics of R&D collaborations against developing countries’ climate technology needs to provide policymakers with guidance on how to Configure R&D collaborations to meet these needs
Self-harm and suicidal acts: a suitable case for treatment of impulsivity-driven behaviour with repetitive transcranial magnetic stimulation (rTMS).
SUMMARY: Suicidal thinking, self-harm and suicidal acts are common, although determining their precise prevalence is complex. Epidemiological work has identified a number of associated demographic and clinical factors, though, with the exception of past acts of self-harm, these are non-specific and weak future predictors. There is a critical need shift focus from managing 'suicidality-by-proxy' through general mental health treatments, to better understand the neuropsychology and neurophysiology of such behaviour to guide targeted interventions. The model of the cognitive control of emotion (MCCE) offers such a paradigm, with an underlying pan-diagnostic pathophysiology of a hypoactive prefrontal cortex failing to suitably inhibit an overactive threat-responding limbic system. The result is a phenotype - from any number of causative gene-environment interactions - primed to impulsively self-harm. We argue that such neural dysconnectivity is open to potential therapeutic modification from repetitive transcranial magnetic stimulation (rTMS). The current evidence base for this is undoubtedly extremely limited, but the societal and clinical burden self-harm and suicide pose warrants such investigation. DECLARATION OF INTEREST: K.B. is the Editor of BJPsych Open, but had no editorial involvement in the review or decision process regarding this paper. COPYRIGHT AND USAGE: © The Royal College of Psychiatrists 2015. This is an open access article distributed under the terms of the Creative Commons Non-Commercial, No Derivatives (CC BY-NC-ND) licence.D.K.T., S.S.S. and A.S.D. are supported by the National Institute of Health Research Biomedical
Research Centre (BRC) at the South London & Maudsley NHS Foundation Trust and the
Institute of Psychiatry, Psychology and Neuroscience, King’s College London
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