616 research outputs found
Enhanced Transmission of Light and Particle Waves through Subwavelength Nanoapertures by Far-Field Interference
Subwavelength aperture arrays in thin metal films can enable enhanced
transmission of light and matter (atom) waves. The phenomenon relies on
resonant excitation and interference of the plasmon or matter waves on the
metal surface. We show a new mechanism that could provide a great resonant and
nonresonant transmission enhancement of the light or de Broglie particle waves
passed through the apertures not by the surface waves, but by the constructive
interference of diffracted waves (beams generated by the apertures) at the
detector placed in the far-field zone. In contrast to other models, the
mechanism depends neither on the nature (light or matter) of the beams
(continuous waves or pulses) nor on material and shape of the multiple-beam
source (arrays of 1-D and 2-D subwavelength apertures, fibers, dipoles or
atoms). The Wood anomalies in transmission spectra of gratings, a long standing
problem in optics, follow naturally from the interference properties of our
model. The new point is the prediction of the Wood anomaly in a classical
Young-type two-source system. The new mechanism could be interpreted as a
non-quantum analog of the superradiance emission of a subwavelength ensemble of
atoms (the light power and energy scales as the number of light-sources
squared, regardless of periodicity) predicted by the well-known Dicke quantum
model.Comment: Revised version of MS presented at the Nanoelectronic Devices for
Defense and Security (NANO-DDS) Conference, 18-21 June, 2007, Washington, US
Influence of the anion potential on the charge ordering in quasi-one dimensional charge transfer salts
We examine the various instabilities of quarter-filled strongly correlated
electronic chains in the presence of a coupling to the underlying lattice. To
mimic the physics of the (TMTTF)X Bechgaard-Fabre salts we also include
electrostatic effects of intercalated anions. We show that small displacements
of the anion can stabilize new mixed Charged Density Wave-Bond Order Wave
phases in which central symmetry centers are suppressed. This finding is
discussed in the context of recent experiments. We suggest that the recently
observed charge ordering is due to a cooperative effect between the Coulomb
interaction and the coupling of the electronic stacks to the anions. On the
other hand, the Spin-Peierls instability at lower temperature requires a
Peierls-like lattice coupling.Comment: Latex, 4 pages, 4 postscript figure
Calibration and Stokes Imaging with Full Embedded Element Primary Beam Model for the Murchison Widefield Array
15 pages, 11 figures. Accepted for publication in PASA. © Astronomical Society of Australia 2017The Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science program, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programs for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescope's primary beam. Recent results from the MWA GaLactic Extragalactic All-sky MWA (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10-20 % in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWA's primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 x 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200 - 231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.Peer reviewedFinal Accepted Versio
Optogenetic strategies to investigate neural circuitry engaged by stress
Optogenetic techniques have given researchers unprecedented access to the function of discrete neural circuit elements and have been instrumental in the identification of novel brain pathways that become dysregulated in neuropsychiatric diseases. For example, stress is integrally linked to the manifestation and pathophysiology of neuropsychiatric illness, including anxiety, addiction and depression. Due to the heterogeneous populations of genetically and neurochemically distinct neurons in areas such as the bed nucleus of the stria terminalis (BNST), as well as their substantial number of projections, our understanding of how neural circuits become disturbed after stress has been limited. Using optogenetic tools, we are now able to selectively isolate distinct neural circuits that contribute to these disorders and perturb these circuits in vivo, which in turn may lead to the normalization of maladaptive behavior. This review will focus on current optogenetic strategies to identify, manipulate, and record from discrete neural circuit elements in vivo as well as highlight recent optogenetic studies that have been utilized to parcel out BNST function
Holographic two dimensional QCD and Chern-Simons term
We present a holographic realization of large Nc massless QCD in two
dimensions using a D2/D8 brane construction. The flavor axial anomaly is dual
to a three dimensional Chern-Simons term which turns out to be of leading
order, and it affects the meson spectrum and holographic renormalization in
crucial ways. The massless flavor bosons that exist in the spectrum are found
to decouple from the heavier mesons, in agreement with the general lore of
non-Abelian bosonization. We also show that an external dynamical photon
acquires a mass through the three dimensional Chern-Simons term as expected
from the Schwinger mechanism. Massless two dimensional QCD at large Nc exhibits
anti-vector-meson dominance due to the axial anomaly.Comment: 22 page
Re-integerization of fractional charges in the correlated quarter-filled band
Previous work has demonstrated the existence of soliton defect states with
charges +/- e/2 in the limits of zero and of infinite on-site Coulomb
interactions in the one-dimensional (1D) quarter-filled band. For large but
finite on-site Coulomb interaction, the low temperature 2k_F bond distortion
that occurs within the 4k_F bond-distorted phase is accompanied by
charge-ordering on the sites. We show that a ``re-integerization'' of the
defect charge occurs in this bond-charge density wave (BCDW) state due to a
``binding'' of the fractional charges. We indicate briefly possible
implications of this result for mechanisms of organic superconductivity.Comment: 4 eps figure
The Inhibitory Circuit Architecture of the Lateral Hypothalamus Orchestrates Feeding
The growing prevalence of overeating disorders is a key contributor to the worldwide obesity epidemic. Dysfunction of particular neural circuits may trigger deviations from adaptive feeding behaviors. The lateral hypothalamus (LH) is a crucial neural substrate for motivated behavior including feeding, but the precise functional neurocircuitry that controls LH neuronal activity to engage feeding has not been defined. We observed that inhibitory synaptic inputs from the extended amygdala preferentially innervate and suppress the activity of LH glutamatergic neurons to control food intake. These findings help explain how dysregulated activity at a number of unique nodes can result in a cascading failure within a defined brain network to produce maladaptive feeding
Activation of Prefrontal Cortical Parvalbumin Interneurons Facilitates Extinction of Reward-Seeking Behavior
Forming and breaking associations between emotionally salient environmental stimuli and rewarding or aversive outcomes is an essential component of learned adaptive behavior. Importantly, when cue-reward contingencies degrade, animals must exhibit behavioral flexibility to extinguish prior learned associations. Understanding the specific neural circuit mechanisms that operate during the formation and extinction of conditioned behaviors is critical because dysregulation of these neural processes is hypothesized to underlie many of the maladaptive and pathological behaviors observed in various neuropsychiatric disorders in humans. The medial prefrontal cortex (mPFC) participates in the behavioral adaptations seen in both appetitive and aversive-cue-mediated responding, but the precise cell types and circuit mechanisms sufficient for driving these complex behavioral states remain largely unspecified. Here, we recorded and manipulated the activity of parvalbumin-positive fast spiking interneurons (PV+ FSIs) in the prelimbic area (PrL) of the mPFC in mice. In vivo photostimulation of PV+ FSIs resulted in a net inhibition of PrL neurons, providing a circuit blueprint for behavioral manipulations. Photostimulation of mPFC PV+ cells did not alter anticipatory or consummatory licking behavior during reinforced training sessions. However, optical activation of these inhibitory interneurons to cues associated with reward significantly accelerated the extinction of behavior during non-reinforced test sessions. These data suggest that suppression of excitatory mPFC networks via increased activity of PV+ FSIs may enhance reward-related behavioral flexibility
Bond and charge density waves in the isotropic interacting two-dimensional quarter-filled band and the insulating state proximate to organic superconductivity
We report two surprising results regarding the nature of the spatial broken
symmetries in the two-dimensional (2D), quarter-filled band with strong
electron-electron interactions. First, in direct contradiction to the
predictions of one-electron theory, we find a coexisting ``bond-order and
charge density wave'' (BCDW) insulating ground state in the 2D rectangular
lattice for all anisotropies, including the isotropic limit. Second, we find
that the BCDW further coexists with a spin-density wave (SDW) in the range of
large anisotropy. Further, in contrast to the interacting half-filled band, in
the interacting quarter-filled band there are two transitions: first, a similar
singlet-to-AFM/SDW transition for large anisotropy and second, an
AFM/SDW-to-singlet transition at smaller anisotropy. We discuss how these
theoretical results apply to the insulating states that are proximate to the
superconducting states of 2:1 cationic charge-transfer solids (CTS).
An important consequence of this work is the suggestion that organic
superconductivity is related to the proximate Coulomb-induced BCDW, with the
SDW that coexists for large anisotropies being also a consequence of the BCDW,
rather than the driver of superconductivity.Comment: 29 pages, 18 eps figures. Revised with new appendices; to appear in
Phys. Rev. B 62, Nov 15, 200
Distinct extended amygdala circuits for divergent motivational states
SummaryThe comorbidity of anxiety and dysfunctional reward processing in illnesses such as addiction1 and depression2 suggests that common neural circuitry contributes to these disparate neuropsychiatric symptoms. The extended amygdala, including the bed nucleus of the stria terminalis (BNST), modulates fear and anxiety3,4, but also projects to the ventral tegmental area (VTA) 5,6, a region implicated in reward and aversion7–13, thus providing a candidate neural substrate for integrating diverse emotional states. However, the precise functional connectivity between distinct BNST projection neurons and their postsynaptic targets in the VTA, as well as the role of this circuit in controlling motivational states have not been described. Here, we recorded and manipulated the activity of genetically and neurochemically identified VTA-projecting BNST neurons in freely behaving mice. Collectively, aversive stimuli exposure produced heterogeneous firing patterns in VTA-projecting BNST neurons. In contrast, in vivo optically-identified glutamatergic projection neurons displayed a net enhancement of activity to aversive stimuli, whereas the firing rate of identified GABAergic projection neurons was suppressed. Channelrhodopsin-2 (ChR2) assisted circuit mapping revealed that both BNST glutamatergic and GABAergic projections preferentially innervate postsynaptic non-dopaminergic VTA neurons, thus providing a mechanistic framework for in vivo circuit perturbations. In vivo photostimulation of BNST glutamatergic projections resulted in aversive and anxiogenic behavioral phenotypes. In contrast, activation of BNST GABAergic projections produced rewarding and anxiolytic phenotypes, which were also recapitulated by direct inhibition of VTA GABAergic neurons. These data demonstrate that functionally opposing BNST to VTA circuits regulate rewarding and aversive motivational states and may serve as a critical circuit node for bidirectionally normalizing maladaptive behaviors
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