914 research outputs found
Vortex Entanglement and Broken Symmetry
Based on the London approximation, we investigate numerically the stability
of the elementary configurations of entanglement, the twisted-pair and the
twisted-triplet, in the vortex-lattice and -liquid phases. We find that, except
for the dilute limit, the twisted-pair is unstable and hence irrelevant in the
discussion of entanglement. In the lattice phase the twisted-triplet
constitutes a metastable, confined configuration of high energy. Loss of
lattice symmetry upon melting leads to deconfinement and the twisted-triplet
turns into a low-energy helical configuration.Comment: 4 pages, RevTex, 2 figures on reques
Scaling of the microwave magneto-impedance in TlBaCaCuO thin films
We present measurements of the magnetic field-induced microwave complex
resistivity changes at 47 GHz in TlBaCaCuO (TBCCO)
thin films, in the ranges 58 K and 00.8 T. The large
imaginary part points to strong elastic response, but the
data are not easily reconciled with a rigid vortex model. We find that, over a
wide range of temperatures, all the pairs of curves and
can be collapsed on a pair of scaling curves
, , with the same
scaling field . We argue that is related to the loss of
vortex rigidity due to a vortex transformation.Comment: Two printed pages, Proceedings of M2S (Dresden, 2006), to appear in
Physica
Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals
The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined
from magnetization curves for different field sweep rates dH/dt and
temperatures. The obtained results are consistent with theoretical reports in
the literature about vortex creep over surface and geometrical barriers. The
frequently observed low-temperature upturn of H_p is shown to be related to
metastable configurations due to barriers for vortex entry. Data of the true
lower critical field H_c1 are presented. The low-temperature dependence of H_c1
is consistent with a superconducting state with nodes in the gap function.
[PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs
Equilibrium First-Order Melting and Second-Order Glass Transitions of the Vortex Matter in BiSrCaCuO
The thermodynamic phase diagram of BiSrCaCuO was mapped
by measuring local \emph{equilibrium} magnetization in presence of
vortex `shaking'. Two equally sharp first-order magnetization steps are
revealed in a single temperature sweep, manifesting a liquid-solid-liquid
sequence. In addition, a second-order glass transition line is revealed by a
sharp break in the equilibrium slope. The first- and second-order lines
intersect at intermediate temperatures, suggesting the existence of four
phases: Bragg glass and vortex crystal at low fields, glass and liquid at
higher fields.Comment: 5 pages, 4 figures. To be published in Phys. Rev. Let
Thermal Suppression of Strong Pinning
We study vortex pinning in layered type-II superconductors in the presence of
uncorrelated disorder for decoupled layers. Introducing the new concept of
variable-range thermal smoothing, we describe the interplay between strong
pinning and thermal fluctuations. We discuss the appearance and analyze the
evolution in temperature of two distinct non-linear features in the
current-voltage characteristics. We show how the combination of layering and
electromagnetic interactions leads to a sharp jump in the critical current for
the onset of glassy response as a function of temperature.Comment: LaTeX 2.09, 4 pages, 2 figures, submitted to Phys. Rev. Let
Dissociation of vortex stacks into fractional-flux vortices
We discuss the zero field superconducting phase transition in a finite system
of magnetically coupled superconducting layers. Transverse screening is
modified by the presence of other layers resulting in topological excitations
with fractional flux. Vortex stacks trapping a full flux and present at any
finite temperature undergo an evaporation transition which corresponds to the
depairing of fractional-flux vortices in individual layers. We propose an
experiment with a bi-layer system allowing us to identify the dissociation of
bound vortex molecules.Comment: 4 pages, 1 figure; revised version, to appear in Phys. Rev. Let
Microwave vortex dynamics in Tl-2212 thin films
We present measurements of the effective surface impedance changes due to a
static magnetic field, \Delta Z(H,T)=\Delta R(H,T)+\rmi \Delta X(H,T), in a
Tl-2212 thin film with 103 K, grown on a CeO buffered sapphire
substrate. Measurements were performed through a dielectric resonator operating
at 47.7 GHz, for temperatures 60 K and magnetic fields T.
We observe exceptionally large field induced variations and pronounced
super-linear field dependencies in both and with
in almost the whole range explored. A careful
analysis of the data allows for an interpretation of these results as dominated
by vortex dynamics. In the intermediate-high field range we extract the main
vortex parameters by resorting to standard high frequency model and by taking
into proper account the creep contribution. The pinning constant shows a marked
decrease with the field which can be interpreted in terms of flux lines
softening associated to an incipient layer decoupling. Small vortex viscosity,
by an order of magnitude lower than in Y-123 are found. Some speculations about
these findings are provided.Comment: pdfTeX, 4 pages, 3 figures, VORTEX 2007 proceedings, to appear in
Physica
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