PARAMAGNETIC MEISSNER EFFECT IN SMALL SUPERCONDUCTORS

A superconductor placed in a magnetic field and cooled down through th e transition temperature expels magnetic flux. This phenomenon, known as the Meissner effect, is arguably the most essential property of sup erconductors and implies zero resistivity. Surprisingly, several recen t experiments have shown that some superconducting samples(1-7) may at tract magnetic field-the so-called paramagnetic Meissner effect. The s carce, if not controversial, experimental evidence for this effect mak es it difficult to identify the origin of this enigmatic phenomenon, a lthough a large number of possible explanations have been advanced(8-1 6). Here we report observations of the paramagnetic Meissner effect wi th a resolution better than one quantum of magnetic flux. The paramagn etic Meissner effect is found to be an oscillating function of the mag netic field (due to flux quantization) and replaces the normal Meissne r effect only above a certain field when several nux quanta are frozen inside a superconductor. The paramagnetic state is found to be metast able and the Meissner state can be restored by external noise. We conc lude that the paramagnetic Meissner effect is related to the surface s uperconductivity and, therefore, represents a general property of supe rconductors: on decreasing temperature, the flux captured at the third (surface) critical field inside the superconducting sheath compresses into a smaller volume, allowing extra flux to penetrate at the surfac e.