316 research outputs found
Vortex pinning in high-Tc materials via randomly oriented columnar defects, created by GeV proton-induced fission fragments
Extensive work has shown that irradiation with 0.8 GeV protons can produce
randomly oriented columnar defects (CD's) in a large number of HTS materials,
specifically those cuprates containing Hg, Tl, Pb, Bi, and similar heavy
elements. Absorbing the incident proton causes the nucleus of these species to
fission, and the recoiling fission fragments create amorphous tracks, i.e.,
CD's. The superconductive transition temperature Tc decreases linearly with
proton fluence and we analyze how the rate depends on the family of
superconductors. In a study of Tl-2212 materials, adding defects decreases the
equilibrium magnetization Meq(H) significantly in magnitude and changes its
field dependence; this result is modeled in terms of vortex pinning. Analysis
of the irreversible magnetization and its time dependence shows marked
increases in the persistent current density and effective pinning energy, and
leads to an estimate for the elementary attempt time for vortex hopping, tau ~
4x10^(-9) s.Comment: Submitted to Physica C; presentation at ISS-2001. PDF file only, 13
pp. tota
Universal linear relations between susceptibility and Tc in cuprates
We developed an experimental method for measuring the intrinsic
susceptibility \chi of powder of cuprate superconductors in the zero field
limit using a DC-magnetometer. The method is tested with lead spheres. Using
this method we determine \chi for a number of cuprate families as a function of
doping. A universal linear (and not proportionality) relation between Tc and
\chi is found. We suggest possible explanations for this phenomenon.Comment: Accepted for publication in PR
The Role of Splayed Disorder and Channel Flow on the Dynamics of Driven 3D Vortices
We present the results of three-dimensional molecular dynamics simulations of
vortices which indicate that, for B greater than the matching field, the
enhanced pinning effectiveness of splayed columnar defects relative to vertical
columnar defects can be explained in terms of the existence or absence of
channels through which the vortices can flow without encountering defects.Comment: 2 Pages, 3 Figures, Proceedings from Houston M2S-HTSC-VI References
altered, minor correction
Zeeman and Orbital Limiting Fields: Separated Spin and Charge Degrees of Freedom in Cuprate Superconductors
Recent in-plane thermal (Nernst) and interlayer (tunnelling) transport
experiments in BiSrCaCuO high temperature superconductors
report hugely different limiting magnetic fields. Based on pairing (and the
uncertainty principle) combined with the definitions of the Zeeman energy and
the magnetic length, we show that in the underdoped regime both fields convert
to the same (normal state) pseudogap energy scale upon transformation as
orbital and spin (Zeeman) critical fields, respectively. We reconcile these
seemingly disparate findings invoking separated spin and charge degrees of
freedom residing in different regions of a truncated Fermi surface.Comment: 4 pages, 3 figures; accepted for publication in Phys. Rev. B (Rapid
Comm.
Singular robust room-temperature spin response from topological Dirac fermions
Topological insulators are a class of solids in which the nontrivial inverted
bulk band structure gives rise to metallic surface states that are robust
against impurity scattering. In three-dimensional (3D) topological insulators,
however, the surface Dirac fermions intermix with the conducting bulk, thereby
complicating access to the low energy (Dirac point) charge transport or
magnetic response. Here we use differential magnetometry to probe spin rotation
in the 3D topological material family (BiSe, BiTe, and
SbTe). We report a paramagnetic singularity in the magnetic
susceptibility at low magnetic fields which persists up to room temperature,
and which we demonstrate to arise from the surfaces of the samples. The
singularity is universal to the entire family, largely independent of the bulk
carrier density, and consistent with the existence of electronic states near
the spin-degenerate Dirac point of the 2D helical metal. The exceptional
thermal stability of the signal points to an intrinsic surface cooling process,
likely of thermoelectric origin, and establishes a sustainable platform for the
singular field-tunable Dirac spin response.Comment: 20 pages, 14 figure
Fractal geometry of normal phase clusters and magnetic flux trapping in high-Tc superconductors
The effect of geometry and morphology of superconducting structure on
magnetic flux trapping is considered. It is found that the clusters of normal
phase, which act as pinning centers, have significant fractal properties. The
fractal dimension of the boundary of these clusters is estimated using a simple
area-perimeter relation. A superconductor is treated as a percolation system.
It is revealed that the fractality intensifies the magnetic flux trapping and
thereby enhances the critical current value.Comment: 5 pages with 1 table and 2 figures, revtex, published in Phys.Lett.A
267 (2000) 66 with more complicated figure
Suppression of matching field effects by splay and pinning energy dispersion in YBa_2Cu_3O_7 with columnar defects
We report measurements of the irreversible magnetization M_i of a large
number of YBa_2Cu_3O_7 single crystals with columnar defects (CD). Some of them
exhibit a maximum in M_i when the density of vortices equals the density of
tracks, at temperatures above 40K. We show that the observation of these
matching field effects is constrained to those crystals where the orientational
and pinning energy dispersion of the CD system lies below a certain threshold.
The amount of such dispersion is determined by the mass and energy of the
irradiation ions, and by the crystal thickness. Time relaxation measurements
show that the matching effects are associated with a reduction of the creep
rate, and occur deep into the collective pinning regime.Comment: 7 pages, 5 figures, submitted to Phys. Rev.
Entanglement of Solid Vortex Matter: A Boomerang Shaped Reduction Forced by Disorder in Interlayer Phase Coherence in Bi2Sr2CaCu2O8+y
We present evidence for entangled solid vortex matter in a glassy state in a
layered superconductor BiSrCaCuO containing randomly
splayed linear defects. The interlayer phase coherence--probed by the Josephson
plasma resonance--is enhanced at high temperatures, reflecting the recoupling
of vortex liquid by the defects. At low temperatures in the vortex solid state,
the interlayer coherence follows a boomerang-shaped reentrant temperature path
with an unusual low field decrease in coherence, indicative of meandering
vortices. We uncover a distinct temperature scaling between in-plane and
out-of-plane critical currents with opposing dependencies on field and time,
consistent with the theoretically proposed "splayed-glass" state.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Diminished equilibrium magnetization in Hg-1223 and Tl-2212 superconductors with fission-generated columnar defects
- …