Evidence for Intra-Unit-Cell magnetic order in the pseudo-gap state of high-T_c cuprates

The existence of the mysterious pseudo-gap state in the phase diagram of copper oxide superconductors and its interplay with unconventional d-wave superconductivity has been a long standing issue for more than a decade. There is now a growing number of experimental indications that the pseudo-gap phase actually corresponds to a symmetry breaking state. In his theory for cuprates, C. M. Varma proposes that the pseudo-gap is a new state of matter associated with the spontaneous appearance of circulating current loops within CuO₂ unit cell. This intra-unit-cell order breaks time reversal symmetry, but preserves lattice translation invariance. Polarized elastic neutron scattering measurements provide evidence for an intra-unit-cell magnetic order inside the pseudo-gap state. This order could be produced by the orbital-like magnetic moments induced by the circulating current loops. The magnetic order displays the same characteristic features in HgBa₂CuO_4+δ, YBa₂Cu₃O_6+x and Bi₂Sr₂CaCu₂O_8+δ demonstrating that this genuine phase is ubiquitous of the pseudo-gap of high temperature copper oxide materials. We review the main properties characterizing this intra-unit-cell magnetic order and discuss its interplay or competition with other spin and charge instabilities.