Order and magnetic excitations in the pseudogap phase of superconducting cuprates : existence of current loops ?

One of the major problems limiting our understanding of superconductivity at high critical temperatures is the existence in the phase diagram of superconducting copper oxides of a so-called pseudogap phase in which all electronic properties are anomalous. Several theoretical models associate this pseudogap phase with a "hidden order parameter".} To test these theoretical approaches, we used polarized neutron diffraction. This revealed a magnetic order present only in the pseudogap phase. We have reproduced this result in several families of superconducting cuprates. More recently, we have identified a novel magnetic excitation in the same doping and temperature regime using inelastic polarized neutron scattering measurements. All these results will be discussed in the light of a theoretical model that explains the pseudogap phase by nano-loops of currents inside the copper-oxygen planes, thus producing orbital magnetic moments