Spontaneous currents and paramagnetic Meissner effect (the Wohlleben Effect) in high-Tc superconductors

Tile survey is given of the Wohlleben Effect (or Paramagnetic Meissner Effect)-the paramagnetic, instead of diamagnetic, response of certain samples of HTSC in a field-cooled regime at very low fields (less than or similar to 1 Oe). The main experimental facts are shortly presented, and theoretical interpretation is given. The effect is attributed to the formation of spontaneous currents and magnetic moments in a granular system where some of the contacts between grains have inverse Josephson coupling. One of the possible microscopic reasons for such a behaviour is an unconventional (e.g. d-wave) pairing. In this connection some other recent experiments are shortly discussed.