This work presents a new magnetic order in the pseudo-gap phase of superconducting cuprates. The study of the three families YBa2Cu3O6+δ, HgBa2CuO4+δ and La1.92Sr0.08CuO4 reveals a magnetic order parameter developping under the temperature of the opening pseudo-gap. For the first time, we report the existence of collective modes associated to this order in the system HgBa2CuO4+δ. All the results converge to the conclusion that the pseudo-gap is linked to a true phase transition, with an order parameter and a symmetry breaking : the time reversal symmetry. It is worth noticing that the translation symmetry of the lattice is preserved, so that this order can be qualified as Q=0 order. Moreover, our study of incommensurate spin fluctuations near the antiferromagnetic wave vector (QAF) has enabled us to demonstrate the underlying competition between two magnetic instabilities : the Q=0 order and the QAF spin fluctuations in both YBa2Cu3O6+δ, and La1.92Sr0.08CuO4 . All these results could be a key toward the understanding of the puzzling high TC superconductivity.
