473 research outputs found
Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization
We derive expressions for the jumps in entropy and magnetization
characterizing the first-order melting transition of a flux line lattice. In
our analysis we account for the temperature dependence of the Landau parameters
and make use of the proper shape of the melting line as determined by the
relative importance of electromagnetic and Josephson interactions. The results
agree well with experiments on anisotropic YBaCuO and
layered BiSrCaCuO materials and reaffirm the validity of
the London model.Comment: 4 pages. We have restructured the paper to emphasize that in the
London scaling regime (appropriate for YBCO) our results are essentially
exact. We have also emphasized that a major controversy over the relevance of
the London model to describe VL melting has been settled by this wor
A new broken U(1)-symmetry in extreme type-II superconductors
A phase transition within the molten phase of the Abrikosov vortex system
without disorder in extreme type-II superconductors is found via large-scale
Monte-Carlo simulations. It involves breaking a U(1)-symmetry, and has a
zero-field counterpart, unlike vortex lattice melting. Its hallmark is the loss
of number-conservation of connected vortex paths threading the entire system
{\it in any direction}, driving the vortex line tension to zero. This tension
plays the role of a generalized ``stiffness'' of the vortex liquid, and serves
as a probe of the loss of order at the transition, where a weak specific heat
anomaly is found.Comment: 5 pages, 3 figure
Universal properties for linelike melting of the vortex lattice
Using numerical results obtained within two models describing vortex matter
(interacting elastic lines (Bose model) and uniformly frustrated XY-model) we
establish universal properties of the melting transition within the linelike
regime. These properties, which are captured correctly by both models, include
the scaling of the melting temperature with anisotropy and magnetic field, the
effective line tension of vortices in the liquid regime, the latent heat, the
entropy jump per entanglement length, and relative jump of Josephson energy at
the transition as compared to the latent heat. The universal properties can
serve as experimental fingerprints of the linelike regime of melting.
Comparison of the models allows us to establish boundaries of the linelike
regime in temperature and magnetic field.Comment: Revtex, 12 pages, 2 EPS figure
Flux-line entanglement as the mechanism of melting transition in high-temperature superconductors in a magnetic field
The mechanism of the flux-line-lattice (FLL) melting in anisotropic high-T_c
superconductors in is clarified by Monte Carlo
simulations of the 3D frustrated XY model. The percentage of entangled flux
lines abruptly changes at the melting temperature T_m, while no sharp change
can be found in the number and size distribution of vortex loops around T_m.
Therefore, the origin of this melting transition is the entanglement of flux
lines. Scaling behaviors of physical quantities are consistent with the above
mechanism of the FLL melting. The Lindemann number is also evaluated without
any phenomenological arguments.Comment: 10 pages, 5 Postscript figures, RevTeX; changed content and figures,
Phys. Rev. B Rapid Commun. in pres
Nucleation of Stable Superconductivity in YBCO-Films
By means of the linear dynamic conductivity, inductively measured on
epitaxial films between 30mHz and 30 MHz, the transition line to
generic superconductivity is studied in fields between B=0 and 19T. It follows
closely the melting line described recently in terms of a blowout of
thermal vortex loops in clean materials. The critical exponents of the
correlation length and time near , however, seem to be dominated by
some intrinsic disorder. Columnar defects produced by heavy-ion irradiation up
to field-equivalent-doses of lead to a disappointing reduction
of while for the generic line of the pristine film
is recovered. These novel results are also discussed in terms of a loop-driven
destruction of generic superconductivity.Comment: 11 pages including 7 EPS figures, accepted for publication in the
Proceedings of the Spring Meeting of the German Physical Society, Muenster
1999,Festkoerperprobleme/Advances in Solid State Physics 199
Abrikosov vortex escape from a columnar defect as a topological electronic transition in vortex core
We study microscopic scenario of vortex escape from a columnar defect under
the influence of a transport current. For defect radii smaller than the
superconducting coherence length the depinning process is shown to be a
consequence of two subsequent topological electronic transitions in a trapped
vortex core. The first transition at a critical current is associated
with the opening of Fermi surface segments corresponding to the creation of a
vortex--antivortex pair bound to the defect. The second transition at a certain
current is caused by merging of different Fermi surface segments,
which accompanies the formation of a freely moving vortex.Comment: 5 pages, 4 figure
The scale of population structure in Arabidopsis thaliana
The population structure of an organism reflects its evolutionary history and influences its evolutionary trajectory. It constrains the combination of genetic diversity and reveals patterns of past gene flow. Understanding it is a prerequisite for detecting genomic regions under selection, predicting the effect of population disturbances, or modeling gene flow. This paper examines the detailed global population structure of Arabidopsis thaliana. Using a set of 5,707 plants collected from around the globe and genotyped at 149 SNPs, we show that while A. thaliana as a species self-fertilizes 97% of the time, there is considerable variation among local groups. This level of outcrossing greatly limits observed heterozygosity but is sufficient to generate considerable local haplotypic diversity. We also find that in its native Eurasian range A. thaliana exhibits continuous isolation by distance at every geographic scale without natural breaks corresponding to classical notions of populations. By contrast, in North America, where it exists as an exotic species, A. thaliana exhibits little or no population structure at a continental scale but local isolation by distance that extends hundreds of km. This suggests a pattern for the development of isolation by distance that can establish itself shortly after an organism fills a new habitat range. It also raises questions about the general applicability of many standard population genetics models. Any model based on discrete clusters of interchangeable individuals will be an uneasy fit to organisms like A. thaliana which exhibit continuous isolation by distance on many scales
Low-Field Phase Diagram of Layered Superconductors: The Role of Electromagnetic Coupling
We determine the position and shape of the melting line in a layered
superconductor taking the electromagnetic coupling between layers into account.
In the limit of vanishing Josephson coupling we obtain a new generic reentrant
low-field melting line. Finite Josephson coupling pushes the melting line to
higher temperatures and fields and a new line shape is found. We construct the low-field phase diagram including
melting and decoupling lines and discuss various experiments in the light of
our new results.Comment: 12 pages, 1 figure attached as compressed and uuencoded postscrip
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