* APS March Meeting 1999 * Abstact Draft

Field Dependence of Diamagnetic Shielding Fraction with Carrier
Concentration in High Temperature Superconductors

P. M. Grant, W. Y. Lee, A. Nazzal and M. E. Lopez-Morales

We have measured the ac susceptibility as a function of probe field
strength and temperature in (Pr, Ca)-doped YBCO and Sr-doped La2CuO4.
For the former, the sample doping range covered the entire
superconducting phase; for the latter, La2-xSrxCuO4, both underdoped (x
< 0.15) and overdoped (x > 0.15) samples were examined.  The probe field
ranged from 0.1-10 Oe rms.  Only samples with "optimal" carrier
concentration and highest Tc displayed bulk-level diamagnetic shielding
fractions independent of field strength.  In all other samples, a
considerable decrease in saturation shielding fraction was observed as
the probe field was increased to 10 Oe.  We interpret this behavior as
evidence for micro-granularity arising either from structural or
electronic inhomogeneity, possibly from formation of stripe domains, in
all but "optimally" doped high temperature superconductors.