Polarized Neutron Diffraction to discover symmetry breaking in pseudogap region of Y(123)-Cuprate

\vspace {0.5 cm} One of the leading issues in high-$T_C$ superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YBa$_2$Cu$_3$O$_{6+x}$ system$^*$. The observed magnetic order preserves translational symmetry as proposed for orbital moments in the circulating current theory of the pseudogap state (see C.M. Varma, at http://fr.arxiv.org/abs/cond-mat/0507214). To date, it is the first direct evidence of an hidden order parameter characterizing the pseudogap phase in high-$T_C$ cuprates. \\ $^*$ B. Fauqu\'e, Y.~Sidis, V.~Hinkov, S.~Pailh\`{e}s, C.T. Lin, X. Chaud and P.~Bourges, at http://fr.arxiv.org/abs/cond- mat/0509210.