2,404 research outputs found
Nonequilibrium steady states of driven magnetic flux lines in disordered type-II superconductors
We investigate driven magnetic flux lines in layered type-II superconductors
subject to various configurations of strong point or columnar pinning centers
by means of a three-dimensional elastic line model and Metropolis Monte Carlo
simulations. We characterize the resulting nonequilibrium steady states by
means of the force-velocity / current-voltage curve, static structure factor,
mean vortex radius of gyration, number of double-kink and half-loop
excitations, and velocity / voltage noise spectrum. We compare the results for
the above observables for randomly distributed point and columnar defects, and
demonstrate that the three-dimensional flux line structures and their
fluctuations lead to a remarkable variety of complex phenomena in the
steady-state transport properties of bulk superconductors.Comment: 23 pages, IOP style, 18 figures include
Role of dynamic Jahn-Teller distortions in Na2C60 and Na2CsC60 studied by NMR
Through 13C NMR spin lattice relaxation (T1) measurements in cubic Na2C60, we
detect a gap in its electronic excitations, similar to that observed in
tetragonal A4C60. This establishes that Jahn-Teller distortions (JTD) and
strong electronic correlations must be considered to understand the behaviour
of even electron systems, regardless of the structure. Furthermore, in metallic
Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR,
implying the occurence of excitations typical of JT distorted C60^{2-} (or
equivalently C60^{4-}). This supports the idea that dynamic JTD can induce
attractive electronic interactions in odd electron systems.Comment: 3 figure
Upper critical field divergence induced by mesoscopic phase separation in the organic superconductor (TMTSF)2ReO4
Due to the competition of two anion orders, (TMTSF)2ReO4, presents a phase
coexistence between semiconducting and metallic (superconducting) regions
(filaments or droplets) in a wide range of pressure. In this regime, the
superconducting upper critical field for H parallel to both c* and b' axes
present a linear part at low fields followed by a divergence above a cross-over
field. This cross-over corresponds to the 3D-2D decoupling transition expected
in filamentary or granular superconductors. The sharpness of the transition
also demonstrates that all filaments are of similar sizes and self organize in
a very ordered way. The distance between the filaments and their cross-section
are estimated.Comment: 4 pages, 4 figure
High Field magnetospectroscopy to probe the 1.4eV Ni color center in diamond
A magneto-optical study of the 1.4 eV Ni color center in boron-free synthetic
diamond, grown at high pressure and high temperature, has been performed in
magnetic fields up to 56 T. The data is interpreted using the effective spin
Hamiltonian of Nazar\'e, Nevers and Davies [Phys. Rev. B 43, 14196 (1991)] for
interstitial Ni with the electronic configuration and effective
spin . Our results unequivocally demonstrate the trigonal symmetry of
the defect which preferentially aligns along the [111] growth direction on the
(111) face, but reveal the shortcomings of the crystal field model for this
particular defect.Comment: 12 pages, 13 figures, submitted to PR
Environmental changes in Syowa Station area of Antarctica during the last 2300 years inferred from organic components in lake sediment cores
rganic components in sediment cores from Namazu Ike(lake)(length 40cm) and O-ike(lake)(length 32cm) from Syowa Station area, Antarctica were studied to clarify their features in relation to paleoenvironmental changes, together with carbon-14 dating by Tandetron accelerator mass spectrometry. Namazu Ike sediment core was mainly composed of algal(mainly cyanobacteria) and aquatic moss debris, whereas O-ike sediment core was comprised of coarse and fine sands with the influence of algal(mainly cyanobacteria) debris. The ages of core bottoms of Namazu Ike and O-ike were estimated to be 1550 and 2330 years before present(yBP), respectively. The sedimentation rates of Namazu Ike and O-ike were calculated to be 30 and 59 years/cm, respectively. Very high total organic carbon(TOC) contents(average 24.5%) of Namazu Ike revealed that the sediment core was mainly composed of organic matter. Dramatic increase of TOC/total nitrogen ratios at a depth of 25cm in Namazu Ike strongly suggests that aquatic moss increased from 1100yBP to the core top. Changes in n-alkanes, n-alkanoic and n-alkenoic acids, and sterol compositions in the O-ike sediment core, suggest that microbial composition changed considerably, but their source organisms are not clear and further studies are required
Applicability of layered sine-Gordon models to layered superconductors: II. The case of magnetic coupling
In this paper, we propose a quantum field theoretical renormalization group
approach to the vortex dynamics of magnetically coupled layered
superconductors, to supplement our earlier investigations on the
Josephson-coupled case. We construct a two-dimensional multi-layer sine-Gordon
type model which we map onto a gas of topological excitations. With a special
choice of the mass matrix for our field theoretical model, vortex dominated
properties of magnetically coupled layered superconductors can be described.
The well known interaction potentials of fractional flux vortices are
consistently obtained from our field-theoretical analysis, and the physical
parameters (vortex fugacity and temperature parameter) are also identified. We
analyse the phase structure of the multi-layer sine--Gordon model by a
differential renormalization group method for the magnetically coupled case
from first principles. The dependence of the transition temperature on the
number of layers is found to be in agreement with known results based on other
methods.Comment: 7 pages, 1 figure, published in J. Phys.: Condens. Matte
Disseminated Adenovirus Disease by Multiple Adenovirus Serotypes following Allogeneic Hematopoietic Stem Cell Transplantation
Comets, historical records and vedic literature
A verse in book I of Rigveda mentions a cosmic tree with rope-like aerial
roots held up in the sky. Such an imagery might have ensued from the appearance
of a comet having `tree stem' like tail, with branched out portions resembling
aerial roots. Interestingly enough, a comet referred to as `heavenly tree' was
seen in 162 BC, as reported by old Chinese records. Because of weak surface
gravity, cometary appendages may possibly assume strange shapes depending on
factors like rotation, structure and composition of the comet as well as solar
wind pattern. Varahamihira and Ballala Sena listed several comets having
strange forms as reported originally by ancient seers such as Parashara,
Vriddha Garga, Narada and Garga.
Mahabharata speaks of a mortal king Nahusha who ruled the heavens when Indra,
king of gods, went into hiding. Nahusha became luminous and egoistic after
absorbing radiance from gods and seers. When he kicked Agastya (southern star
Canopus), the latter cursed him to become a serpent and fall from the sky. We
posit arguments to surmise that this Mahabharata lore is a mythical recounting
of a cometary event wherein a comet crossed Ursa Major, moved southwards with
an elongated tail in the direction of Canopus and eventually went out of sight.
In order to check whether such a conjecture is feasible, a preliminary list of
comets (that could have or did come close to Canopus) drawn from various
historical records is presented and discussed.Comment: This work was presented in the International Conference on Oriental
Astronomy held at IISER, Pune (India) during November, 201
Dynamical percolation on general trees
H\"aggstr\"om, Peres, and Steif (1997) have introduced a dynamical version of
percolation on a graph . When is a tree they derived a necessary and
sufficient condition for percolation to exist at some time . In the case
that is a spherically symmetric tree, H\"aggstr\"om, Peres, and Steif
(1997) derived a necessary and sufficient condition for percolation to exist at
some time in a given target set . The main result of the present paper
is a necessary and sufficient condition for the existence of percolation, at
some time , in the case that the underlying tree is not necessary
spherically symmetric. This answers a question of Yuval Peres (personal
communication). We present also a formula for the Hausdorff dimension of the
set of exceptional times of percolation.Comment: 24 pages; to appear in Probability Theory and Related Field
- âŠ