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

Superconductors show zero resistance to electric current, and expel magnetic flux (the Meissner effect) below the transition temperature (T-c). In conventional superconductors, the `Cooper pairs' of electrons that are responsible for superconductivity form only below T-c. In the unconventional high-T-c superconductors, however, a strong electron correlation is essential for 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 (underdoped) and an energy gap-the `pseudogap'-appears to be present. Moreover, excitations that look like the vortices that carry magnetic flux inside the superconducting state have been reported above T-c (refs 6, 7). Although the origin of the pseudogap remains controversial(8-11), phase fluctuations above T-c, leading to some form of local superconductivity or local pairing, seem essential(9,12-16). Here we report magnetic imaging (scanning SQUID microscopy) of La2-xSrxCuO4 thin films. Clear quantized vortex patterns are visible below T-c (18-19 K), and we observe inhomogeneous magnetic domains that persist up to 80 K. We interpret the data as suggesting the existence of diamagnetic regions that are precursors to the Meissner state.