5,623 research outputs found
Conformal Field Theory and the Exact Solution of the BCS Hamiltonian
We propose a connection between conformal field theory (CFT) and the exact
solution and integrability of the reduced BCS model of superconductivity. The
relevant CFT is given by the -WZW model in the singular limit when the
level k goes to -2. This theory has to be perturbed by an operator proportional
to the inverse of the BCS coupling constant. Using the free field realization
of this perturbed Wess-Zumino-Witten model, we derive the exact Richardson's
wave function and the integrals of motion of the reduced BCS model in the
saddle point approximation. The construction is reminiscent of the CFT approach
to the Fractional Quantum Hall effect.Comment: 10 pages, RevTe
Perceptions of Cheating and Doping in E-Cycling
E-cycling is a growing area of cycling appealing to competitive cyclists and fitness enthusiasts. Zwift is the most popular e-cycling platform, with approx. 1 million subscribers and is a virtual environment that hosts regular races, including the UCI e-cycling world championships. Thepopularity of Zwift has given rise to cases of cheating and hacking the system to gain an advantagein e-racing. As a result, some high-profile professional riders have faced bans. We set out to understand the thoughts and concerns e-cyclists have about cheating, hacking, and doping in e-cycling.A total of 337 females and 1130 males were recruited over a 7-week period via social networkingsites to complete an online survey. Forty-four per cent had experienced cheating during e-racing,which made them feel angry, annoyed, disappointed, and cheated. However, 15% of those whoexperienced cheating said they did not care, possibly because many see e-racing as a game or training tool rather than a competitive event. Eighty-seven per cent of participants were in favour ofenforcing a ban on cheaters in e-cycling, while 34% wanted cheaters to be banned during in-personcycling events too. Results indicate that many e-cyclists have experienced cheating and would likeclearer rules and bans for cheaters during e-races
Neonatal 5,7-DHT Lesions Cause Sex-Specific Changes in Mouse Cortical Morphogenesis
Both monoaminergic and cholinergic afferent
projections to the neocortex putatively modulate
cortical morphogenesis and plasticity. Previously
we showed that neonatal,electrolytic lesions:
the cholinergic nucleus basalis magnocel!ularis
(nBM) projections to the neocortex result in
significant decreases-of cortical layer width that
correlate with cognitive alterations. Such electrolytic
lesions, performed for lack of a selective
neurotoxin in mice, may affect mono- aminergic
fibers of passage. Here, we investigate the effects
of neonatal 5,7 dihydroxytryptamine (5,7-DHT)
focal injections into the nBM region on cortical
laminar morphology in adult male and female
mice. 5,7-DHT lesions on the first postnatal day
resulted in significant cortical depletion of both
serotonin and norepinephrine that attenuated
with age. Generally, cortical layer widths
increased in response to the lesion; the effects
were layer, region, and sex specific. Previous
reports from our laboratories described longterm
behavioral alterations after comparable
focal, neonatal 5,7-DHT lesions. The studies
described here provide an anatomical basis for
such behavioral alterations. Our data suggest
that monoaminergic and cholinergic projections to the cortex may have opposite effects on the
developing cortical neuropil. Jointly, our
morphological and behavioral findings may
have important implications for a variety of
developmental disorders in humans and
provide some insights into sex differences in the
penetrance of these disorders
Exact Study of the Effect of Level Statistics in Ultrasmall Superconducting Grains
The reduced BCS model that is commonly used for ultrasmall superconducting
grains has an exact solution worked out long ago by Richardson in the context
of nuclear physics. We use it to check the quality of previous treatments of
this model, and to investigate the effect of level statistics on pairing
correlations. We find that the ground state energies are on average somewhat
lower for systems with non-uniform than uniform level spacings, but both have
an equally smooth crossover from the bulk to the few-electron regime. In the
latter, statistical fluctuations in ground state energies strongly depend on
the grain's electron number parity.Comment: 4 pages, 3 eps figs, RevTe
Extracting non-linear integrate-and-fire models from experimental data using dynamic I - V curves
The dynamic I-V curve method was recently introduced for the efficient experimental generation of reduced neuron models. The method extracts the response properties of a neuron while it is subject to a naturalistic stimulus that mimics in vivo-like fluctuating synaptic drive. The resulting history-dependent, transmembrane current is then projected onto a one-dimensional current-voltage relation that provides the basis for a tractable non-linear integrate-and-fire model. An attractive feature of the method is that it can be used in spike-triggered mode to quantify the distinct patterns of post-spike refractoriness seen in different classes of cortical neuron. The method is first illustrated using a conductance-based model and is then applied experimentally to generate reduced models of cortical layer-5 pyramidal cells and interneurons, in injected-current and injected- conductance protocols. The resulting low-dimensional neuron models—of the refractory exponential integrate-and-fire type—provide highly accurate predictions for spike-times. The method therefore provides a useful tool for the construction of tractable models and rapid experimental classification of cortical neuron
Superconducting correlations in metallic nanoparticles: exact solution of the BCS model by the algebraic Bethe ansatz
Superconducting pairing of electrons in nanoscale metallic particles with
discrete energy levels and a fixed number of electrons is described by the
reduced BCS model Hamiltonian. We show that this model is integrable by the
algebraic Bethe ansatz. The eigenstates, spectrum, conserved operators,
integrals of motion, and norms of wave functions are obtained. Furthermore, the
quantum inverse problem is solved, meaning that form factors and correlation
functions can be explicitly evaluated. Closed form expressions are given for
the form factors that describe superconducting pairing.Comment: revised version, 5 pages, revtex, no figure
Quantum Entanglement and Order Parameter in a Paired Finite Fermi System
We study the pairing correlations in a finite Fermi system from quantum
entanglement point of view. We investigate the relation between the order
parameter, which has been introduced recently to describe both finite and
infinite superconductors, and the concurrence. For a proper definition of the
concurrence, we argue that a possible generalization of spin flip
transformation is time reversal operation. While for a system with indefinite
number of particles concurrence is a good measure of entanglement, for a finite
system it does not distinguish between normal and superconducting states. We
propose that the expectation value of the radial operator for the total
pseudospin can be used to identify entanglement of pairing.Comment: submitted to Solid State Communications (the Festschrift in the honor
of Prof. Salim Ciraci's 60th birthday
Evolution of helicity in NOAA 10923 over three consecutive solar rotations
We have studied the evolution of magnetic helicity and chirality in an active
region over three consecutive solar rotations. The region when it first
appeared was named NOAA10923 and in subsequent rotations it was numbered NOAA
10930, 10935 and 10941. We compare the chirality of these regions at
photospheric, chromospheric and coronal heights. The observations used for
photospheric and chromospheric heights are taken from Solar Vector Magnetograph
(SVM) and H_alpha imaging telescope of Udaipur Solar Observatory (USO),
respectively. We discuss the chirality of the sunspots and associated H_alpha
filaments in these regions. We find that the twistedness of superpenumbral
filaments is maintained in the photospheric transverse field vectors also. We
also compare the chirality at photospheric and chromospheric heights with the
chirality of the associated coronal loops, as observed from the HINODE X-Ray
Telescope.Comment: 8 pages, 4 figure
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