104 research outputs found
Pairing of Cooper pairs in a Josephson junction network containing an impurity
We show how to induce pairing of Cooper pairs (and, thus,
superconductivity) as a result of local embedding of a quantum impurity in a
Josephson network fabricable with conventional junctions. We find that a
boundary double Sine-Gordon model provides an accurate description of the dc
Josephson current patterns, as well as of the stable phases accessible to the
network. We point out that tunneling of pairs of Cooper pairs is robust against
quantum fluctuations, as a consequence of the time reversal invariance, arising
when the central region of the network is pierced by a dimensionless magnetic
flux . We find that, for , a stable attractive finite
coupling fixed point emerges and point out its relevance for engineering a two
level quantum system with enhanced coherence.Comment: 5 Pages, 5 Figures. Small modifications, ref.[11] added. To appear in
EP
Low Energy Properties of the (n,n) Carbon Nanotubes
According to band theory, an ideal undoped (n,n) carbon nanotube is metallic.
We show that the electron-electron interaction causes it to become Mott
insulating with a spin gap. More interestingly, upon doping it develops
superconducting fluctuations.Comment: 5pages, 2eps figures, one reference added, final version, accepted to
PR
Unified analysis of terminal-time control in classical and quantum systems
Many phenomena in physics, chemistry, and biology involve seeking an optimal
control to maximize an objective for a classical or quantum system which is
open and interacting with its environment. The complexity of finding an optimal
control for maximizing an objective is strongly affected by the possible
existence of sub-optimal maxima. Within a unified framework under specified
conditions, control objectives for maximizing at a terminal time physical
observables of open classical and quantum systems are shown to be inherently
free of sub-optimal maxima. This attractive feature is of central importance
for enabling the discovery of controls in a seamless fashion in a wide range of
phenomena transcending the quantum and classical regimes.Comment: 10 page
What happens for informal caregivers during transition to increased levels of care for the person with dementia? A systematic review protocol
Abstract Background Dementia is a globally prevalent disease that requires ongoing and increasing levels of care, often provided in the first instance by informal caregivers. Supporting transitions in informal caregiving in dementia is a pertinent issue for caregivers, care providers and governments. There is no existing systematic review that seeks to identify and map the body of literature regarding the review question: ‘What happens for informal caregivers during transition to increased levels of care for the person with dementia?’ Methods/design ASSIA, CINAHL+, MEDLINE, PsycINFO, SCIE, Social Service Abstracts and Web of Science will be systematically searched. Specialist dementia research libraries will be contacted. Reviews identified as relevant during the search process, their reference lists, and reference lists of accepted papers will be hand-searched. Qualitative, quantitative and mixed methods studies that seek to represent the experiences of, or examine the impact upon, informal caregivers during transition to increased formal care for the person with dementia will be eligible for inclusion. Synthesis will be segregated into qualitative and quantitative papers. Findings will be summarised, and the review will be prepared for publication. Discussion The review will seek to identify potentially vulnerable groups in need of support and as such, inform the practice of those offering support. It will also inform future research by highlighting areas in which current literature is insubstantial. Systematic review registration PROSPERO CRD4201706724
The Central Velocity Field in NGC 253 : Possible Indication of a Bar
We have investigated whether motion of gas in a bar-like potential can
account for the peculiar but systematic velocity field observed in the nuclear
region of the starburst galaxy NGC 253. This unusual velocity field with
gradients along both major and minor axes was revealed in a high resolution
(1.8^{\arcsec}\times 1.0^{\arcsec}) H92 recombination line
observation by Anantharamiah and Goss (1996). A simple logarithmic potential is
used to model the bar. Assuming that the bulk of the gas flows along closed and
non-intersecting x1 (bar) and x2 (anti-bar) orbits of the bar potential, we
have computed the expected velocity field and position-velocity diagrams and
compared them with the observations. A comparison of the integrated CO
intensity maps with the spatial distribution of the x1 and x2 orbits in the
model indicates that the nuclear molecular gas in NGC 253 lies mainly on the x2
orbits. We also find that the velocity field observed in the central 100 pc
region in the H92 recombination line is well accounted for by the bar
model if most of the ionized gas resides in the inner x2 orbits. However, the
model is unable to explain the velocity field on a larger scale of
pc observed using the OVRO interferometer with a resolution of
5^{\arcsec}\times 3^{\arcsec}. The direction of the observed CO velocity
field appears twisted compared to the model. We suggest that this perturbation
in the velocity field may be due to an accretion event that could have occurred
years ago.Comment: 26 latex pages, 7 figures, accepted in Astrophysical Journa
Fragility of the Free-Energy Landscape of a Directed Polymer in Random Media
We examine the sensitiveness of the free-energy landscape of a directed
polymer in random media with respect to various kinds of infinitesimally weak
perturbation including the intriguing case of temperature-chaos. To this end,
we combine the replica Bethe ansatz approach outlined in cond-mat/0112384, the
mapping to a modified Sinai model and numerically exact calculations by the
transfer-matrix method. Our results imply that for all the perturbations under
study there is a slow crossover from a weakly perturbed regime where rare
events take place to a strongly perturbed regime at larger length scales beyond
the so called overlap length where typical events take place leading to chaos,
i.e. a complete reshuffling of the free-energy landscape. Within the replica
space, the evidence for chaos is found in the factorization of the replicated
partition function induced by infinitesimal perturbations. This is the reflex
of explicit replica symmetry breaking.Comment: 29 pages, Revtex4, ps figure
Quasiparticle Band Structure and Density Functional Theory: Single-Particle Excitations and Band Gaps in Lattice Models
We compare the quasiparticle band structure for a model insulator obtained
from the fluctuation exchange approximation (FEA) with the eigenvalues of the
corresponding density functional theory (DFT) and local density approximation
(LDA). The discontinuity in the exchange-correlation potential for this model
is small and the FEA and DFT band structures are in good agreement. In contrast
to conventional wisdom, the LDA for this model overestimates the size of the
band gap. We argue that this is a consequence of an FEA self-energy that is
strongly frequency dependent, but essentially local.Comment: 8 pages, and 5 figure
Surface impedance anisotropy of YBaCuO single crystals: electrodynamic basis of the measurements
An electrodynamic technique is developed for determining the components of
surface impedance and complex conductivity tensors of HTSC single crystals on
the basis of measured quantities of a quality factor and a resonator frequency
shift. A simple formula is obtained for a geometrical factor of a crystal in
the form of a plate with dimensions in a microwave magnetic field
. To obtain the c-axis complex conductivity from
measurements at we propose a procedure which
takes account of sample size effects. With the aid of the technique involved
temperature dependences of all impedance and conductivity tensors components of
YBaCuO single crystal, grown in BaZrO crucible, are
determined at a frequency of GHz in its normal and superconducting
states. All of them proved to be linear at , and their extrapolation
to zero temperature gives the values of residual surface resistance
and m and
magnetic field penetration depth nm and
m.Comment: 9 pages, 7 figures. Submitted to Phys.Rev.B 05Jun2002; accepted for
publication 21Febr200
On the Application of the Non Linear Sigma Model to Spin Chains and Spin Ladders
We review the non linear sigma model approach (NLSM) to spin chains and spin
ladders, presenting new results. The generalization of the Haldane's map to
ladders in the Hamiltonian approach, give rise to different values of the
parameter depending on the spin S, the number of legs and
the choice of blocks needed to built up the NLSM fields. For rectangular blocks
we obtain or depending on wether , is even or
odd, while for diagonal blocks we obtain . Both
results agree modulo , and yield the same prediction, namely that even (
resp. odd) ladders are gapped (resp. gapless). For even legged ladders we show
that the spin gap collapses exponentially with and we propose a
finite size correction to the gap formula recently derived by Chakravarty using
the 2+1 NSLM, which gives a good fit of numerical results. We show the
existence of a Haldane phase in the two legged ladder using diagonal blocks and
finally we consider the phase diagram of dimerized ladders.Comment: 25 pages, Latex, 7 figures in postscript files, Proc. of the 1996 El
Escorial Summer School on "Strongly Correlated Magnetic and Superconducting
Systems". Some more references are adde
Coordinated optimization of visual cortical maps (II) Numerical studies
It is an attractive hypothesis that the spatial structure of visual cortical
architecture can be explained by the coordinated optimization of multiple
visual cortical maps representing orientation preference (OP), ocular dominance
(OD), spatial frequency, or direction preference. In part (I) of this study we
defined a class of analytically tractable coordinated optimization models and
solved representative examples in which a spatially complex organization of the
orientation preference map is induced by inter-map interactions. We found that
attractor solutions near symmetry breaking threshold predict a highly ordered
map layout and require a substantial OD bias for OP pinwheel stabilization.
Here we examine in numerical simulations whether such models exhibit
biologically more realistic spatially irregular solutions at a finite distance
from threshold and when transients towards attractor states are considered. We
also examine whether model behavior qualitatively changes when the spatial
periodicities of the two maps are detuned and when considering more than 2
feature dimensions. Our numerical results support the view that neither minimal
energy states nor intermediate transient states of our coordinated optimization
models successfully explain the spatially irregular architecture of the visual
cortex. We discuss several alternative scenarios and additional factors that
may improve the agreement between model solutions and biological observations.Comment: 55 pages, 11 figures. arXiv admin note: substantial text overlap with
arXiv:1102.335
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