32,917 research outputs found
Simple stochastic models showing strong anomalous diffusion
We show that {\it strong} anomalous diffusion, i.e. \mean{|x(t)|^q} \sim
t^{q \nu(q)} where is a nonlinear function of , is a generic
phenomenon within a class of generalized continuous-time random walks. For such
class of systems it is possible to compute analytically nu(2n) where n is an
integer number. The presence of strong anomalous diffusion implies that the
data collapse of the probability density function P(x,t)=t^{-nu}F(x/t^nu)
cannot hold, a part (sometimes) in the limit of very small x/t^\nu, now
nu=lim_{q to 0} nu(q). Moreover the comparison with previous numerical results
shows that the shape of F(x/t^nu) is not universal, i.e., one can have systems
with the same nu but different F.Comment: Final versio
Three-loop HTLpt thermodynamics at finite temperature and chemical potential
In this proceedings we present a state-of-the-art method of calculating
thermodynamic potential at finite temperature and finite chemical potential,
using Hard Thermal Loop perturbation theory (HTLpt) up to
next-to-next-leading-order (NNLO). The resulting thermodynamic potential
enables us to evaluate different thermodynamic quantities including pressure
and various quark number susceptibilities (QNS). Comparison between our
analytic results for those thermodynamic quantities with the available lattice
data shows a good agreement.Comment: 5 pages, 6 figures, conference proceedings of XXI DAE-BRNS HEP
Symposium, IIT Guwahati, December 2014; to appear in 'Springer Proceedings in
Physics Series
Signatures of orbital loop currents in the spatially resolved local density of states
Polarized neutron scattering measurements have suggested that intra-unit cell
antiferromagnetism may be associated with the pseudogap phase. Assuming that
loop current order is responsible for the observed magnetism, we calculate some
signatures of such circulating currents in the local density of states around a
single non-magnetic impurity in a coexistence phase with superconductivity. We
find a distinct C4 symmetry breaking near the disorder which is also detectable
in the resulting quasi-particle interference patterns.Comment: 5 pages, 3 figure
Model of Electronic Structure and Superconductivity in Orbitally Ordered FeSe
We provide a band structure with low-energy properties consistent with recent
photoemission and quantum oscillations measurements on FeSe, assuming
mean-field like s and/or d-wave orbital ordering at the structural transition.
We show how the resulting model provides a consistent explanation of the
temperature dependence of the measured Knight shift and the spin-relaxation
rate. Furthermore, the superconducting gap structure obtained from spin
fluctuation theory exhibits nodes on the electron pockets, consistent with the
'V'-shaped density of states obtained by tunneling spectroscopy on this
material, and the temperature dependence of the London penetration depth. Our
studies prove that the recent experimental observations of the electronic
properties of FeSe are consistent with orbital order, but leave open the
microscopic origin of the unusual band structure of this material.Comment: 12 pages, 15 figures, T.B hopping error corrected, d-wave orbital
order added, real space hoppings included in tex fil
A framework for detection and classification of events in neural activity
We present a method for the real time prediction of punctate events in neural
activity, based on the time-frequency spectrum of the signal, applicable both
to continuous processes like local field potentials (LFP) as well as to spike
trains. We test it on recordings of LFP and spiking activity acquired
previously from the lateral intraparietal area (LIP) of macaque monkeys
performing a memory-saccade task. In contrast to earlier work, where trials
with known start times were classified, our method detects and classifies
trials directly from the data. It provides a means to quantitatively compare
and contrast the content of LFP signals and spike trains: we find that the
detector performance based on the LFP matches the performance based on spike
rates. The method should find application in the development of neural
prosthetics based on the LFP signal. Our approach uses a new feature vector,
which we call the 2D cepstrum.Comment: 30 pages, 6 figures; This version submitted to the IEEE Transactions
in Biomedical Engineerin
Temporal structure in neuronal activity during working memory in Macaque parietal cortex
A number of cortical structures are reported to have elevated single unit
firing rates sustained throughout the memory period of a working memory task.
How the nervous system forms and maintains these memories is unknown but
reverberating neuronal network activity is thought to be important. We studied
the temporal structure of single unit (SU) activity and simultaneously recorded
local field potential (LFP) activity from area LIP in the inferior parietal
lobe of two awake macaques during a memory-saccade task. Using multitaper
techniques for spectral analysis, which play an important role in obtaining the
present results, we find elevations in spectral power in a 50--90 Hz (gamma)
frequency band during the memory period in both SU and LFP activity. The
activity is tuned to the direction of the saccade providing evidence for
temporal structure that codes for movement plans during working memory. We also
find SU and LFP activity are coherent during the memory period in the 50--90 Hz
gamma band and no consistent relation is present during simple fixation.
Finally, we find organized LFP activity in a 15--25 Hz frequency band that may
be related to movement execution and preparatory aspects of the task. Neuronal
activity could be used to control a neural prosthesis but SU activity can be
hard to isolate with cortical implants. As the LFP is easier to acquire than SU
activity, our finding of rich temporal structure in LFP activity related to
movement planning and execution may accelerate the development of this medical
application.Comment: Originally submitted to the neuro-sys archive which was never
publicly announced (was 0005002
The Dreaming Variational Autoencoder for Reinforcement Learning Environments
Reinforcement learning has shown great potential in generalizing over raw
sensory data using only a single neural network for value optimization. There
are several challenges in the current state-of-the-art reinforcement learning
algorithms that prevent them from converging towards the global optima. It is
likely that the solution to these problems lies in short- and long-term
planning, exploration and memory management for reinforcement learning
algorithms. Games are often used to benchmark reinforcement learning algorithms
as they provide a flexible, reproducible, and easy to control environment.
Regardless, few games feature a state-space where results in exploration,
memory, and planning are easily perceived. This paper presents The Dreaming
Variational Autoencoder (DVAE), a neural network based generative modeling
architecture for exploration in environments with sparse feedback. We further
present Deep Maze, a novel and flexible maze engine that challenges DVAE in
partial and fully-observable state-spaces, long-horizon tasks, and
deterministic and stochastic problems. We show initial findings and encourage
further work in reinforcement learning driven by generative exploration.Comment: Best Student Paper Award, Proceedings of the 38th SGAI International
Conference on Artificial Intelligence, Cambridge, UK, 2018, Artificial
Intelligence XXXV, 201
A Method for Detection and Classification of Events in Neural Activity
We present a method for the real time prediction of punctuate events in neural activity, based on the time-frequency spectrum of the signal, applicable both to continuous processes like local field potentials (LFPs) as well as to spike trains. We test it on recordings of LFP and spiking activity acquired previously from the lateral intraparietal area (LIP) of macaque monkeys performing a memory-saccade task. In contrast to earlier work, where trials with known start times were classified, our method detects and classifies trials directly from the data. It provides a means to quantitatively compare and contrast the content of LFP signals and spike trains: we find that the detector performance based on the LFP matches the performance based on spike rates. The method should find application in the development of neural prosthetics based on the LFP signal. Our approach uses a new feature vector, which we call the 2d cepstrum
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