104 research outputs found
Spin paramagnetism in d-wave superconductors
The Ginzburg-Landau equations are derived from the microscopic theory for
clean layered superconductors with pairing symmetry, including
the Pauli paramagnetism effect. The upper critical field parallel to
the axis is calculated. A comparison with the experimental data for YBCO
suggests that, relative to the orbital effect, the Pauli paramagnetism
contribution to is significant. The reversible magnetization in
high magnetic fields is also calculated, showing strong temperature dependence
of the slope , as a consequence of the spin paramagnetism. A simple
expression for the high temperature spin susceptibility is derived, in a good
agreement with the Knight shift measurements on YBCO.Comment: 8 pages, 3 figures; minor changes are included; reference 35 is
added; shorter version will be published in Phys. Rev. B, scheduled issue 01
July 200
Coherent vortex structures and 3D enstrophy cascade
Existence of 2D enstrophy cascade in a suitable mathematical setting, and
under suitable conditions compatible with 2D turbulence phenomenology, is known
both in the Fourier and in the physical scales. The goal of this paper is to
show that the same geometric condition preventing the formation of
singularities - 1/2-H\"older coherence of the vorticity direction - coupled
with a suitable condition on a modified Kraichnan scale, and under a certain
modulation assumption on evolution of the vorticity, leads to existence of 3D
enstrophy cascade in physical scales of the flow.Comment: 15 pp; final version -- to appear in CM
Quasiparticle energy spectrum in ferromagnetic Josephson weak links
The quasiparticles energy spectrum in clean ferromagnetic weak links between
conventional superconductors is calculated. Large peaks in DOS, due to a
special case of Andreev reflection at the ferromagnetic barrier, correspond to
spin-splitt bound states. Their energies are obtained as a function of the
barrier thickness, exchange field strength, and of the macroscopic phase
difference at the link, related to the Josephson current. In the ground
state, can be 0 or , depending on the ferromagnetic barrier
influence. Conditions for the appearence of the zero-energy bound states (ZES)
and for the spin polarized ground state (SPGS) are obtained analytically. It is
shown that ZES appear only outside the weak link ground state.Comment: 11 pages, 6 figure
Fine properties of self-similar solutions of the Navier-Stokes equations
We study the solutions of the nonstationary incompressible Navier--Stokes
equations in , , of self-similar form , obtained from small and homogeneous initial
data . We construct an explicit asymptotic formula relating the
self-similar profile of the velocity field to its corresponding initial
datum
Bloggers Behavior and Emergent Communities in Blog Space
Interactions between users in cyberspace may lead to phenomena different from
those observed in common social networks. Here we analyse large data sets about
users and Blogs which they write and comment, mapped onto a bipartite graph. In
such enlarged Blog space we trace user activity over time, which results in
robust temporal patterns of user--Blog behavior and the emergence of
communities. With the spectral methods applied to the projection on weighted
user network we detect clusters of users related to their common interests and
habits. Our results suggest that different mechanisms may play the role in the
case of very popular Blogs. Our analysis makes a suitable basis for theoretical
modeling of the evolution of cyber communities and for practical study of the
data, in particular for an efficient search of interesting Blog clusters and
further retrieval of their contents by text analysis
Phase equilibria investigation and characterization of the Au-In-Sb system
The Au-In-Sb system is of great interest recently, because its alloys belong to the group of potential lead-free solder materials to be used in electronics, while as a part of Au-ln-Sb-Te system, it presents a suitable phase change material in optical recording media. The results of phase equilibria investigation and characterization of the alloys in the AuIn-Sb section of the Au-In-Sb system are presented in this paper. The investigations were performed using different experimental methods - thermal analysis, optical microscopy and SEM-EDX, hardness and electrical conductivity measurements, and also, using ThermoCalc software based on adequate thermodynamic calculation
Energy cascades and flux locality in physical scales of the 3D Navier-Stokes equations
Rigorous estimates for the total - (kinetic) energy plus pressure - flux in
R^3 are obtained from the three dimensional Navier-Stokes equations. The bounds
are used to establish a condition - involving Taylor length scale and the size
of the domain - sufficient for existence of the inertial range and the energy
cascade in decaying turbulence (zero driving force, non-increasing global
energy). Several manifestations of the locality of the flux under this
condition are obtained. All the scales involved are actual physical scales in
R^3 and no regularity or homogeneity/scaling assumptions are made.Comment: 21 pages, 2 figures; accepted to Comm. Math. Phy
Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures
We study a simple microscopic model for thin, ferromagnetic, metallic layers
on semi-infinite bulk superconductor. We find that for certain values of the
exchange spliting, on the ferromagnetic side, the ground states of such
structures feature spontaneously induced spin polarized currents. Using a
mean-field theory, which is selfconsistent with respect to the pairing
amplitude , spin polarization and the spontaneous current
, we show that not only there are Andreev bound states in the
ferromagnet but when their energies are near zero they support
spontaneous currents parallel to the ferromagnetic-superconducting interface.
Moreover, we demonstrate that the spin-polarization of these currents depends
sensitively on the band filling.Comment: 4 pages, 5 Postscript figures (included
Hibdridni Nd-Fe-B/barijum-ferit magnetni materijali sa epoksi matricom
Lately, there has been great attention paid to the research of bonded hybrid composites with improved dynamic mechanical capacities capable of replacing bonded Nd-Fe-B magnetic materials, by using cheaper (ferrite) materials instead of the Nd-Fe-B powder while retaining the satisfying values of the maximum magnetic energy. The objective of this study is to assess how different contents of Nd-Fe-B and/or barium ferrite particles can affect morphological, dynamic mechanical and magnetic properties of bonded composite materials. The interactions between employed magnetic powders and the interactions between magnetic powders and polymer binder are considered. For the examination of the magnetic behaviour, a vibrating sample magnetometer (VSM) is used. Different shapes and sizes of the obtained hysteresis loops are used for comparison and prediction of the polymer bonded materials properties. The homogeneous distribution of the magnetic particles in the polymer matrix is validated using scanning electron microscopy (SEM). The elastic and damping behavior examined by dynamic mechanical analysis (DMA) show improved properties for hybrid composite materials.IstraživaÄki trend u oblasti polimerom vezanih (bonded) Nd-Fe-B magnetnih materijala se reflektuje kroz razvoj bonded hibridnih kompozita sa poboljÅ”anim dinamiÄko-mehaniÄkim svojstvima i primetno nižoj ceni zbog zamene skupe Nd- Fe-B legure jeftinijim magnetnim materijalima (ferit) uz postizanje zadovoljavajuÄih vrednosti maksimalne magnetne energije. Cilj ovog rada je da ukaže i predvidi uticaj razliÄitog sadržaja Nd-Fe-B i/ili barijum feritnih Äestica na morfoloÅ”ka, dinamiÄko mehaniÄka i magnetna svojstava bonded magneta. Posmatrane su i analizirane interakcije izmeÄu magnetnih prahova, kao i interakcije izmeÄu magnetnih prahova i polimernog veziva i njihov uticaj na finalna svojstva bonded magneta. Za ispitivanje magnetnog ponaÅ”anja koriÅ”Äen je vibraconi magnetometar (VSM). RazliÄite veliÄine i oblici dobijenih histerezisnih krivih su poreÄeni i koriÅ”Äeni za predviÄanje svojstava ispitivanih kompozitnih materijala. Homogena raspodela magnetnih Äestica u polimernoj matrici je potvrÄena koriÅ”Äenjem skenirajuÄeg elektronskog mikroskopa (SEM). ElastiÄna svojstva i svojstvo priguÅ”enja oscilacija ispitivano je koriÅ”Äenjem dinamiÄko mehaniÄke analize (DMA) koja ukazuje na poboljÅ”ana svojstva kod hibridnih kompozita
Empirical analysis of web-based user-object bipartite networks
Understanding the structure and evolution of web-based user-object networks
is a significant task since they play a crucial role in e-commerce nowadays.
This Letter reports the empirical analysis on two large-scale web sites,
audioscrobbler.com and del.icio.us, where users are connected with music groups
and bookmarks, respectively. The degree distributions and degree-degree
correlations for both users and objects are reported. We propose a new index,
named collaborative clustering coefficient, to quantify the clustering behavior
based on the collaborative selection. Accordingly, the clustering properties
and clustering-degree correlations are investigated. We report some novel
phenomena well characterizing the selection mechanism of web users and outline
the relevance of these phenomena to the information recommendation problem.Comment: 6 pages, 7 figures and 1 tabl
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