Cooperative flux pinning in single crystals of YBa2Cu3O7-delta

We have investigated thermally activated magnetic flux creep in single crystals of superconducting YBa2Cu3O7-delta in the low-field, low-temperature regime of the H,T phase diagram where flux motion is expected to occur via individual fluxoid jumps. Between 5 and 15 K the creep data are accurately described by a current-dependent activation barrier of the form U(j)(proportional to)j(-mu), with mu = 1.20 +/- 0.12 The functional form for U(j) can be understood within the framework of cooperative pinning models that ascribe fluxoid pinning to the collective effects of a large number of weak-pinning defects. We argue that the appropriate extension of scaling arguments to the ''individual fluxoid'' regime implies mu approximate to 1.0, as measured. Some successful features of the model include (1) a natural interpretation for the measured ''four-volume'' VX, the volume of the moving flux bundle times the mean distance to the pinning barrier (this interpretation leads to a value for the length of a fluxoid segment that can move in a single activated jump; the shortest length observed in our experiments is approximately 75 nm); (2) an explicit correction for the temperature dependence of U(j) that agrees with experiment; (3) a physically plausible value for the ''flux velocity'' upsilon(f) approximate to 1000 m/sec; and (4) a satisfactory description of the temperature dependence of the ''critical current'' j(c) between 5 and 15 K. For self-consistency the concentration of pinning defects must be greater than 3 x 10(19) cm(-3), and oxygen vacancies are the likely candidates. The results strongly confirm that a simple collective pinning model of individual fluxoids accurately describes the basic physics of the creep process in YBa(2)Cu(3)0(7-delta) in the region of phase space investigated here.