The upper critical field of superconducting (SN)x

The upper superconducting critical magnetic field of the polymer (SN) x was investigated as a function of temperature and crystallographic orientation, over the range 40 mK-1K using accurately steerable magnetic fields up to 11 kG. The values are slightly sample dependent and at T=0 the authors estimate that for the best sample H c2 approximately=8.5 kG for H//b, the fibre axis, with H c2 approximately=900 G for H//a* and H c2 approximately=1100 G for H//c. The variations between samples are ascribed to the (100) twinning shown by all samples. This takes place on a microscopic scale, as is shown by X-ray measurements, and is probably associated with the fibrous structure. The X-ray measurements give a mean fibre size of about 50 AA. The temperature dependence H c2 //(T) agrees with the predictions of the da Silva-Gyorffy model (1979) for an independent anisotropic fibre, but H c2 perpendicular to (T) is quite different from that predicted for this model.