173 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
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.
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.
The Effect of Splayed Pins on Vortex Creep and Critical Currents
We study the effects of splayed columnar pins on the vortex motion using
realistic London Langevin simulations. At low currents vortex creep is strongly
suppressed, whereas the critical current j_c is enhanced only moderately.
Splaying the pins generates an increasing energy barrier against vortex
hopping, and leads to the forced entanglement of vortices, both of which
suppress creep efficiently. On the other hand splaying enhances kink nucleation
and introduces intersecting pins, which cut off the energy barriers. Thus the
j_c enhancement is strongly parameter sensitive. We also characterize the angle
dependence of j_c, and the effect of different splaying geometries.Comment: 4 figure
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
ΠΠΎΠ²ΡΡΠ΅Π½ΠΈΠ΅ ΡΡΡΠ΅ΠΊΡΠΈΠ²Π½ΠΎΡΡΠΈ Π²Π·Π°ΠΈΠΌΠΎΠ΄Π΅ΠΉΡΡΠ²ΠΈΡ ΠΏΡΠΎΠ΅ΠΊΡΠΈΡΠΎΠ²ΡΠΈΠΊΠΎΠ² Π±ΠΎΡΡΠΎΠ²ΠΎΠΉ ΡΠ°Π΄ΠΈΠΎΡΠ»Π΅ΠΊΡΡΠΎΠ½Π½ΠΎΠΉ Π°ΠΏΠΏΠ°ΡΠ°ΡΡΡΡ ΠΊΠΎΡΠΌΠΈΡΠ΅ΡΠΊΠΈΡ Π°ΠΏΠΏΠ°ΡΠ°ΡΠΎΠ² Π½Π° Π±Π°Π·Π΅ ΠΈΠ½ΡΠ΅Π³ΡΠ°ΡΠΈΠΈ ΠΈΠ½ΡΠΎΡΠΌΠ°ΡΠΈΠΎΠ½Π½ΡΡ ΡΠΈΡΡΠ΅ΠΌ
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