2,280 research outputs found
The vortex state in the BEC to BCS crossover: a path-integral description
We derive a path-integral description of the vortex state of a fermionic
superfluid in the crossover region between the molecular condensate (BEC)
regime and the Cooper pairing (BCS) regime. This path-integral formalism,
supplemented by a suitable choice for the saddle point value of the pairing
field in the presence of a vortex, offers a unified description that
encompasses both the BEC and BCS limits. The vortex core size is studied as a
function of the tunable interaction strength between the fermionic atoms. We
find that in the BEC regime, the core size is determined by the molecular
healing length, whereas in the BCS regime, the core size is proportional only
to the Fermi wave length. The observation of such quantized vortices in dilute
Fermi gases would provide an unambiguous proof of the realization of
superfluidity in these gases.Comment: 10 pages, 2 figure
End-to-End QoS Support for a Medical Grid Service Infrastructure
Quality of Service support is an important prerequisite for the adoption of Grid technologies for medical applications. The GEMSS Grid infrastructure addressed this issue by offering end-to-end QoS in the form of explicit timeliness guarantees for compute-intensive medical simulation services. Within GEMSS, parallel applications installed on clusters or other HPC hardware may be exposed as QoS-aware Grid services for which clients may dynamically negotiate QoS constraints with respect to response time and price using Service Level Agreements. The GEMSS infrastructure and middleware is based on standard Web services technology and relies on a reservation based approach to QoS coupled with application specific performance models. In this paper we present an overview of the GEMSS infrastructure, describe the available QoS and security mechanisms, and demonstrate the effectiveness of our methods with a Grid-enabled medical imaging service
Short-coherence length superconductivity in the Attractive Hubbard Model in three dimensions
We study the normal state and the superconducting transition in the
Attractive Hubbard Model in three dimensions, using self-consistent
diagrammatics. Our results for the self-consistent -matrix approximation are
consistent with 3D-XY power-law critical scaling and finite-size scaling. This
is in contrast to the exponential 2D-XY scaling the method was able to capture
in our previous 2D calculation. We find the 3D transition temperature at
quarter-filling and to be . The 3D critical regime is much
narrower than in 2D and the ratio of the mean-field transition to is
about 5 times smaller than in 2D. We also find that, for the parameters we
consider, the pseudogap regime in 3D (as in 2D) coincides with the critical
scaling regime.Comment: 4 pages, 5 figure
BCS-to-BEC crossover from the exact BCS solution
The BCS-to-BEC crossover, as well as the nature of Cooper pairs, in a
superconducting and Fermi superfluid medium is studied from the exact ground
state wavefunction of the reduced BCS Hamiltonian. As the strength of the
interaction increases, the ground state continuously evolves from a
mixed-system of quasifree fermions and pair resonances (BCS), to pair
resonances and quasibound molecules (pseudogap), and finally to a system of
quasibound molecules (BEC). A single unified scenario arises where the
Cooper-pair wavefunction has a unique functional form. Several exact analytic
expressions, such as the binding energy and condensate fraction, are derived.
We compare our results with recent experiments in ultracold atomic Fermi gases.Comment: 5 pages, 4 figures. Revised version with one figure adde
Dynamical Phase Transitions In Driven Integrate-And-Fire Neurons
We explore the dynamics of an integrate-and-fire neuron with an oscillatory
stimulus. The frustration due to the competition between the neuron's natural
firing period and that of the oscillatory rhythm, leads to a rich structure of
asymptotic phase locking patterns and ordering dynamics. The phase transitions
between these states can be classified as either tangent or discontinuous
bifurcations, each with its own characteristic scaling laws. The discontinuous
bifurcations exhibit a new kind of phase transition that may be viewed as
intermediate between continuous and first order, while tangent bifurcations
behave like continuous transitions with a diverging coherence scale.Comment: 4 pages, 5 figure
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