Diamagnetic activity above T-c as a precursor to superconductivity in La2-xSrxCuO4 thin films

Superconductors show zero resistance to electric current, and expel magneti c flux (the Meissner effect) below the transition temperature (T-c). In con ventional superconductors, the `Cooper pairs' of electrons that are respons ible for superconductivity form only below T-c. In the unconventional high- T-c superconductors, however, a strong electron correlation is essential fo r pair formation: there is evidence(1-5) that some pairs are formed above T -c in samples that have less than the optimal density of charge carriers (u nderdoped) and an energy gap-the `pseudogap'-appears to be present. Moreove r, excitations that look like the vortices that carry magnetic flux inside the superconducting state have been reported above T-c (refs 6, 7). Althoug h the origin of the pseudogap remains controversial(8-11), phase fluctuatio ns above T-c, leading to some form of local superconductivity or local pair ing, seem essential(9,12-16). Here we report magnetic imaging (scanning SQU ID microscopy) of La2-xSrxCuO4 thin films. Clear quantized vortex patterns are visible below T-c (18-19 K), and we observe inhomogeneous magnetic doma ins that persist up to 80 K. We interpret the data as suggesting the existe nce of diamagnetic regions that are precursors to the Meissner state.