1,600 research outputs found
Electromagnetic multipole theory for optical nanomaterials
Optical properties of natural or designed materials are determined by the
electromagnetic multipole moments that light can excite in the constituent
particles. In this work we present an approach to calculate the multipole
excitations in arbitrary arrays of nanoscatterers in a dielectric host medium.
We introduce a simple and illustrative multipole decomposition of the electric
currents excited in the scatterers and link this decomposition to the classical
multipole expansion of the scattered field. In particular, we find that
completely different multipoles can produce identical scattered fields. The
presented multipole theory can be used as a basis for the design and
characterization of optical nanomaterials
Dynamics of Electric Field Domains and Oscillations of the Photocurrent in a Simple Superlattice Model
A discrete model is introduced to account for the time-periodic oscillations
of the photocurrent in a superlattice observed by Kwok et al, in an undoped 40
period AlAs/GaAs superlattice. Basic ingredients are an effective negative
differential resistance due to the sequential resonant tunneling of the
photoexcited carriers through the potential barriers, and a rate equation for
the holes that incorporates photogeneration and recombination. The
photoexciting laser acts as a damping factor ending the oscillations when its
power is large enough. The model explains: (i) the known oscillatory static I-V
characteristic curve through the formation of a domain wall connecting high and
low electric field domains, and (ii) the photocurrent and photoluminescence
time-dependent oscillations after the domain wall is formed. In our model, they
arise from the combined motion of the wall and the shift of the values of the
electric field at the domains. Up to a certain value of the photoexcitation,
the non-uniform field profile with two domains turns out to be metastable:
after the photocurrent oscillations have ceased, the field profile slowly
relaxes toward the uniform stationary solution (which is reached on a much
longer time scale). Multiple stability of stationary states and hysteresis are
also found. An interpretation of the oscillations in the photoluminescence
spectrum is also given.Comment: 34 pages, REVTeX 3.0, 10 figures upon request, MA/UC3M/07/9
Electrically tunable GHz oscillations in doped GaAs-AlAs superlattices
Tunable oscillatory modes of electric-field domains in doped semiconductor
superlattices are reported. The experimental investigations demonstrate the
realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the
temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes
is determined using an analytical and a numerical modeling of the dynamics of
domain formation. Three different oscillatory modes are found. Their presence
depends on the actual shape of the drift velocity curve, the doping density,
the boundary condition, and the length of the superlattice. For most bias
regions, the self-sustained oscillations are due to the formation, motion, and
recycling of the domain boundary inside the superlattice. For some biases, the
strengths of the low and high field domain change periodically in time with the
domain boundary being pinned within a few quantum wells. The dependency of the
frequency on the coupling leads to the prediction of a new type of tunable GHz
oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure
Mucopolysaccharidosis VI in cats - clarification regarding genetic testing
Debate.
Published online: 02 July 2016.The release of new DNA-based diagnostic tools has increased tremendously in companion animals. Over 70 different DNA variants are now known for the cat, including DNA variants in disease-associated genes and genes causing aesthetically interesting traits. The impact genetic tests have on animal breeding and health management is significant because of the ability to control the breeding of domestic cats, especially breed cats. If used properly, genetic testing can prevent the production of diseased animals, causing the reduction of the frequency of the causal variant in the population, and, potentially, the eventual eradication of the disease. However, testing of some identified DNA variants may be unwarranted and cause undo strife within the cat breeding community and unnecessary reduction of gene pools and availability of breeding animals. Testing for mucopolysaccharidosis Type VI (MPS VI) in cats, specifically the genetic testing of the L476P (c.1427T>C) and the D520N (c.1558G>A) variants in arylsulfatase B (ARSB), has come under scrutiny. No health problems are associated with the D520N (c.1558G>A) variant, however, breeders that obtain positive results for this variant are speculating as to possible correlation with health concerns. Birman cats already have a markedly reduced gene pool and have a high frequency of the MPS VI D520N variant. Further reduction of the gene pool by eliminating cats that are heterozygous or homozygous for only the MPS VI D520N variant could lead to more inbreeding depression effects on the breed population. Herein is debated the genetic testing of the MPS VI D520N variant in cats. Surveys from different laboratories suggest the L476P (c.1427T>C) disease-associated variant should be monitored in the cat breed populations, particularly breeds with Siamese derivations and outcrosses. However, the D520N has no evidence of association with disease in cats and testing is not recommended in the absence of L476P genotyping. Selection against the D520N is not warranted in cat populations. More rigorous guidelines may be required to support the genetic testing of DNA variants in all animal species.Leslie A. Lyons, Robert A. Grahn, Francesca Genova, Michela Beccaglia, John J. Hopwood and Maria Longer
Music as a scaffold for listening to speech: Better neural phase-locking to song than speech
© 2020 The Authors Neural activity synchronizes with the rhythmic input of many environmental signals, but the capacity of neural activity to entrain to the slow rhythms of speech is particularly important for successful communication. Compared to speech, song has greater rhythmic regularity, a more stable fundamental frequency, discrete pitch movements, and a metrical structure, this may provide a temporal framework that helps listeners neurally track information better than the rhythmically irregular rhythms of speech. The current study used EEG to examine whether entrainment to the syllable rate of linguistic utterances, as indexed by cerebro-acoustic phase coherence, was greater when listeners heard sung than spoken sentences. We assessed listeners phase-locking in both easy (no time compression) and hard (50% time-compression) utterance conditions. Adults phase-locked equally well to speech and song in the easy listening condition. However, in the time-compressed condition, phase-locking was greater for sung than spoken utterances in the theta band (3.67–5 Hz). Thus, the musical temporal and spectral characteristics of song related to better phase-locking to the slow phrasal and syllable information (4–7 Hz) in the speech stream. These results highlight the possibility of using song as a tool for improving speech processing in individuals with language processing deficits, such as dyslexia
Current-voltage characteristic and stability in resonant-tunneling n-doped semiconductor superlattices
We review the occurrence of electric-field domains in doped superlattices
within a discrete drift model. A complete analysis of the construction and
stability of stationary field profiles having two domains is carried out. As a
consequence, we can provide a simple analytical estimation for the doping
density above which stable stable domains occur. This bound may be useful for
the design of superlattices exhibiting self-sustained current oscillations.
Furthermore we explain why stable domains occur in superlattices in contrast to
the usual Gunn diode.Comment: Tex file and 3 postscript figure
Familiarity modulates neural tracking of sung and spoken utterances
Music is often described in the laboratory and in the classroom as a beneficial tool for memory encoding and retention, with a particularly strong effect when words are sung to familiar compared to unfamiliar melodies. However, the neural mechanisms underlying this memory benefit, especially for benefits related to familiar music are not well understood. The current study examined whether neural tracking of the slow syllable rhythms of speech and song is modulated by melody familiarity. Participants became familiar with twelve novel melodies over four days prior to MEG testing. Neural tracking of the same utterances spoken and sung revealed greater cerebro-acoustic phase coherence for sung compared to spoken utterances, but did not show an effect of familiar melody when stimuli were grouped by their assigned (trained) familiarity. However, when participant\u27s subjective ratings of perceived familiarity were used to group stimuli, a large effect of familiarity was observed. This effect was not specific to song, as it was observed in both sung and spoken utterances. Exploratory analyses revealed some in-session learning of unfamiliar and spoken utterances, with increased neural tracking for untrained stimuli by the end of the MEG testing session. Our results indicate that top-down factors like familiarity are strong modulators of neural tracking for music and language. Participants’ neural tracking was related to their perception of familiarity, which was likely driven by a combination of effects from repeated listening, stimulus-specific melodic simplicity, and individual differences. Beyond simply the acoustic features of music, top-down factors built into the music listening experience, like repetition and familiarity, play a large role in the way we attend to and encode information presented in a musical context
Chaos in resonant-tunneling superlattices
Spatio-temporal chaos is predicted to occur in n-doped semiconductor
superlattices with sequential resonant tunneling as their main charge transport
mechanism. Under dc voltage bias, undamped time-dependent oscillations of the
current (due to the motion and recycling of electric field domain walls) have
been observed in recent experiments. Chaos is the result of forcing this
natural oscillation by means of an appropriate external microwave signal.Comment: 3 pages, LaTex, RevTex, 3 uuencoded figures (1.2M) are available upon
request from [email protected], to appear in Phys.Rev.
Measurement of miniband parameters of a doped superlattice by photoluminescence in high magnetic fields
We have studied a 50/50\AA superlattice of GaAs/AlGaAs
composition, modulation-doped with Si, to produce
cm electrons per superlattice period. The modulation-doping was tailored
to avoid the formation of Tamm states, and photoluminescence due to interband
transitions from extended superlattice states was detected. By studying the
effects of a quantizing magnetic field on the superlattice photoluminescence,
the miniband energy width, the reduced effective mass of the electron-hole
pair, and the band gap renormalization could be deduced.Comment: minor typing errors (minus sign in eq. (5)
- …