271,956 research outputs found
Josephson current in superconductor-ferromagnet structures with a nonhomogeneous magnetization
We calculate the dc Josephson current for two types of
superconductor-ferromagnet (S/F) Josephson junctions. The junction of the first
type is a S/F/S junction. On the basis of the Eilenberger equation, the
Josephson current is calculated for an arbitrary impurity concentration. If the expression for the Josephson critical current is reduced
to that which can be obtained from the Usadel equation ( is the exchange
energy, is the momentum relaxation time). In the opposite limit
the superconducting condensate oscillates with period and
penetrates into the F region over distances of the order of the mean free path
. For this kind of junctions we also calculate in the case when the F
layer presents a nonhomogeneous (spiral) magnetic structure with the period
. It is shown that for not too low temperatures, the -state which
occurs in the case of a homogeneous magnetization (Q=0) may disappear even at
small values of . In this nonhomogeneous case, the superconducting
condensate has a nonzero triplet component and can penetrate into the F layer
over a long distance of the order of . The junction
of the second type consists of two S/F bilayers separated by a thin insulating
film. It is shown that the critical Josephson current depends on the
relative orientation of the effective exchange field of the bilayers. In
the case of an antiparallel orientation, increases with increasing .
We establish also that in the F film deposited on a superconductor, the
Meissner current created by the internal magnetic field may be both diamagnetic
or paramagnetic.Comment: 13 pages, 11 figures. To be published in Phys. Rev.
Vortex dynamics and second magnetization peak in PrFeAsOF superconductor
We have studied the vortex dynamics in the PrFeAsOF
superconducting sample by dc magnetization and dynamic magnetization-relaxation
rate measurements. The field dependence of the superconducting
irreversible magnetization reveals a second magnetization peak or
fishtail effect. The large value of is an indication of moderate vortex
motion and relatively weak pinning energy. Data analysis based on the
generalized inversion scheme suggests that the vortex dynamics can be described
by the collective pinning model. The temperature dependence of the critical
current is consistent with the pinning due to the spatial variation in the mean
free path near a lattice defect ( pinning). The temperature and field
dependence of indicates a crossover from elastic to plastic vortex creep
with increasing temperature and magnetic field. Finally, we have constructed
the vortex phase diagram based on the present data.Comment: 11 pages, 8 Figures, Accepted for publication in Journal of Applied
Physic
Entropy effects in the collective dynamic behavior of alkyl monolayers tethered to Si(111)
International audienceDynamic properties of n-alkyl monolayers covalently bonded to Si(111) were studied by broadband admittance spectroscopy as a function of the temperature and the applied voltage using rectifying Hg/C 12 H 25 /n-type Si junctions. Partial substitution of methyl end groups by polar (carboxylic acid) moieties was used to enhance the chain end relaxation response. Two thermally activated dissipation mechanisms (B1 and B2, with f B1 < f B2) are evidenced for all reverse bias values. The strong decrease of both relaxation frequencies with increasing reverse dc bias reveals increasing motional constraints, attributed to electrostatic pressure applied to the densely-packed nanometer-thick monolayer. Spectral decomposition of the frequency response shows a power-law dependence of their activation energies on |V DC |. A large reverse bias reversibly increases the B2 response attributed to the distribution of gauche defects, in contrast with the constant strength of the acid dipole loss (B1). A trans–gauche isomerization energy of 50 meV is derived from the temperature dependence of the B2 dipolar strength. For both dissipation mechanisms, the observed linear correlation between activation energy and logarithm of pre-exponential factor is consistent with a multi-excitation entropy model, in which the molecular reorientation path is strongly coupled with a large number of low energy excitations (here the n-alkyl bending vibrational mode) collected from the thermal bath. This collective dynamic behavior of alkyl chains tethered to Si is also confirmed by the asymmetric relaxation peak shape related to many-body interactions in complex systems. 58
Improving Christofides' Algorithm for the s-t Path TSP
We present a deterministic (1+sqrt(5))/2-approximation algorithm for the s-t
path TSP for an arbitrary metric. Given a symmetric metric cost on n vertices
including two prespecified endpoints, the problem is to find a shortest
Hamiltonian path between the two endpoints; Hoogeveen showed that the natural
variant of Christofides' algorithm is a 5/3-approximation algorithm for this
problem, and this asymptotically tight bound in fact has been the best
approximation ratio known until now. We modify this algorithm so that it
chooses the initial spanning tree based on an optimal solution to the Held-Karp
relaxation rather than a minimum spanning tree; we prove this simple but
crucial modification leads to an improved approximation ratio, surpassing the
20-year-old barrier set by the natural Christofides' algorithm variant. Our
algorithm also proves an upper bound of (1+sqrt(5))/2 on the integrality gap of
the path-variant Held-Karp relaxation. The techniques devised in this paper can
be applied to other optimization problems as well: these applications include
improved approximation algorithms and improved LP integrality gap upper bounds
for the prize-collecting s-t path problem and the unit-weight graphical metric
s-t path TSP.Comment: 31 pages, 5 figure
Glassy dynamics in asymmetric binary mixtures of hard-spheres
The binary hard-sphere mixture is one of the simplest representations of a
many-body system with competing time and length scales. This model is relevant
to fundamentally understand both the structural and dynamical properties of
materials, such as metallic melts, colloids, polymers and bio-based composites.
It also allows us to study how different scales influence the physical behavior
of a multicomponent glass-forming liquid; a question that still awaits a
unified description. In this contribution, we report on distinct dynamical
arrest transitions in highly asymmetric binary colloidal mixtures, namely, a
single glass of big particles, in which the small species remains ergodic, and
a double glass with the simultaneous arrest of both components. When the
mixture approaches any glass transition, the relaxation of the collective
dynamics of both species becomes coupled. In the single glass domain, spatial
modulations occur due to the structure of the large spheres, a feature not
observed in the two-glass domain. The relaxation of the \emph{self} dynamics of
small and large particles, in contrast, become decoupled at the boundaries of
both transitions; the large species always displays dynamical arrest, whereas
the small ones appear arrested only in the double glass. Thus, in order to
obtain a complete picture of the distinct glassy states, one needs to take into
account the dynamics of both species
The linearization problem of a binary quadratic problem and its applications
We provide several applications of the linearization problem of a binary
quadratic problem. We propose a new lower bounding strategy, called the
linearization-based scheme, that is based on a simple certificate for a
quadratic function to be non-negative on the feasible set. Each
linearization-based bound requires a set of linearizable matrices as an input.
We prove that the Generalized Gilmore-Lawler bounding scheme for binary
quadratic problems provides linearization-based bounds. Moreover, we show that
the bound obtained from the first level reformulation linearization technique
is also a type of linearization-based bound, which enables us to provide a
comparison among mentioned bounds. However, the strongest linearization-based
bound is the one that uses the full characterization of the set of linearizable
matrices. Finally, we present a polynomial-time algorithm for the linearization
problem of the quadratic shortest path problem on directed acyclic graphs. Our
algorithm gives a complete characterization of the set of linearizable matrices
for the quadratic shortest path problem
- …