We review experimental attempts using polarized neutron scattering technique to reveal the existence in high temperature superconductors of a long-range ordered state characterized by the spontaneous appearance of current loops. We draw a particular attention to our recent results (B. Fauqu\'e { \it et al.}, { \it Phys. Rev. Lett.} { \bf 96}, 197001 (2006)) that, up to now, can be explained only by the theory of circulating current proposed by C.M. Varma.
Categories: cond-mat.supr-con cond-mat.str-el