107 research outputs found
XPS as a Probe of Gap Opening in Many Electron Systems
Core hole photoemission (XPS) provides a powerful indirect probe of the low
energy excitations of a many electron system. We argue that XPS can be used to
study the way in which a gap opens at a metal-superconductor or metal-
insulator transition. We consider the "universal" physics of how the loss of
low energy excitations modifies XPS spectra in the context of several simple
models, considering in particular the case of a two dimensional d-wave
superconductor.Comment: 8 pages, 9 eps figure
The Computational Structure of Spike Trains
Neurons perform computations, and convey the results of those computations
through the statistical structure of their output spike trains. Here we present
a practical method, grounded in the information-theoretic analysis of
prediction, for inferring a minimal representation of that structure and for
characterizing its complexity. Starting from spike trains, our approach finds
their causal state models (CSMs), the minimal hidden Markov models or
stochastic automata capable of generating statistically identical time series.
We then use these CSMs to objectively quantify both the generalizable structure
and the idiosyncratic randomness of the spike train. Specifically, we show that
the expected algorithmic information content (the information needed to
describe the spike train exactly) can be split into three parts describing (1)
the time-invariant structure (complexity) of the minimal spike-generating
process, which describes the spike train statistically; (2) the randomness
(internal entropy rate) of the minimal spike-generating process; and (3) a
residual pure noise term not described by the minimal spike-generating process.
We use CSMs to approximate each of these quantities. The CSMs are inferred
nonparametrically from the data, making only mild regularity assumptions, via
the causal state splitting reconstruction algorithm. The methods presented here
complement more traditional spike train analyses by describing not only spiking
probability and spike train entropy, but also the complexity of a spike train's
structure. We demonstrate our approach using both simulated spike trains and
experimental data recorded in rat barrel cortex during vibrissa stimulation.Comment: Somewhat different format from journal version but same conten
c-axis Josephson Tunneling in Twinned YBCO Crystals
Josephson tunneling between YBCO and Pb with the current flowing along the
c-axis of the YBCO is persumed to come from an s-wave component of the
superconductivity of the YBCO. Experiments on multi-twin samples are not
entirely consistent with this hypothesis. The sign change of the s-wave order
parameter across the N_T twin boundaries should give cancelations, resulting in
a small tunneling current. The actual current is larger than this.
We present a theory of this unexpectedly large current based upon a surface
effect: disorder-induced supression of the d-wave component at the (001)
surface leads to s-wave coherence across the twin boundaries and a non-random
tunneling current. We solve the case of an ordered array of d+s and d-s twins,
and estimate that the twin size at which s-wave surface coherence occurs is
consistent with typical sizes observed in experiments. In this picture, there
is a phase difference of between different surfaces of the material. We
propose a corner junction experiment to test this picture.Comment: 5 pages, 4 eps figure
Scalable Bayesian modeling, monitoring and analysis of dynamic network flow data
Traffic flow count data in networks arise in many applications, such as
automobile or aviation transportation, certain directed social network
contexts, and Internet studies. Using an example of Internet browser traffic
flow through site-segments of an international news website, we present
Bayesian analyses of two linked classes of models which, in tandem, allow fast,
scalable and interpretable Bayesian inference. We first develop flexible
state-space models for streaming count data, able to adaptively characterize
and quantify network dynamics efficiently in real-time. We then use these
models as emulators of more structured, time-varying gravity models that allow
formal dissection of network dynamics. This yields interpretable inferences on
traffic flow characteristics, and on dynamics in interactions among network
nodes. Bayesian monitoring theory defines a strategy for sequential model
assessment and adaptation in cases when network flow data deviates from
model-based predictions. Exploratory and sequential monitoring analyses of
evolving traffic on a network of web site-segments in e-commerce demonstrate
the utility of this coupled Bayesian emulation approach to analysis of
streaming network count data.Comment: 29 pages, 16 figure
Thalamic activity that drives visual cortical plasticity
Manipulations of activity in one retina can profoundly affect binocular connections in the visual cortex. Retinal activity is relayed to the cortex by the dorsal lateral geniculate nucleus (dLGN). We compared the qualities and amount of activity in the dLGN following monocular eyelid closure and monocular retinal inactivation in awake mice. Our findings substantially alter the interpretation of previous studies and define the afferent activity patterns that trigger cortical plasticity.National Eye InstituteNational Institute of Neurological Disorders and Stroke (U.S.) (National Research Service Award fellowship
Condensation energy in strongly coupled superconductors
We consider the condensation energy in superconductors where the pairing is
electronic in origin and is mediated by a collective bosonic mode.
We use magnetically-mediated superconductivity as an example, and show that
for large spin-fermion couplings, the physics is qualitatively different from
the BCS theory as the condensation energy results from the feedback on spin
excitations, while the electronic contribution to the condensation energy is
positive due to an ``undressing'' feedback on the fermions. The same feedback
effect accounts for the gain of the kinetic energy at strong couplings.Comment: 4 pages, revtex 4, 3 eps figure
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