2 research outputs found
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
Low field vortex dynamics over seven time decades in a Bi_2Sr_2CaCu_2O_{8+\delta} single crystal for temperatures 13 K < T < 83 K
Using a custom made dc-SQUID magnetometer, we have measured the time
relaxation of the remanent magnetization M_rem of a Bi_2Sr_2CaCu_2O_{8+\delta}
single crystal from the fully critical state for temperatures 13 K < T < 83 K.
The measurements cover a time window of seven decades 10^{-2} s < t < 10^5 s,
so that the current density j can be studied from values very close to j_c down
to values considerably smaller than j_c. From the data we have obtained: (i)
the flux creep activation barriers U as a function of current density j, (ii)
the current-voltage characteristics E(j) in a typical range of 10^{-7} V/cm to
10^{-15} V/cm, and (iii) the critical current density j_c(0) at T = 0. Three
different regimes of vortex dynamics are observed: For temperatures T < 20 K
the activation barrier U(j) is logarithmic, no unique functional dependence
U(j) could be found for the intermediate temperature interval 20 K < T < 40 K,
and finally for T > 40 K the activation barrier U(j) follows a power-law
behavior with an exponent mu = 0.6. From the analysis of the data within the
weak collective pinning theory for strongly layered superconductors, it is
argued that for temperatures T < 20 K pancake-vortices are pinned individually,
while for temperatures T > 40 K pinning involves large collectively pinned
vortex bundles. A description of the vortex dynamics in the intermediate
temperature interval 20 K < T < 40 K is given on the basis of a qualitative low
field phase diagram of the vortex state in Bi_2Sr_2CaCu_2O_{8+\delta}. Within
this description a second peak in the magnetization loop should occur for
temperatures between 20 K and 40 K, as it has been observed in several
magnetization measurements in the literature.Comment: 12 pages, 10 figure