Unusual magnetic order in the pseudogap region of the superconductor HgBa2CuO4+δ

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high- transition- temperature (high-T-c) superconductivity, characterized by anomalous physical properties(1,2). There are open questions about the number of distinct phases and the possible presence of a quantum- critical point underneath the superconducting dome(3-5). The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa2Cu3O6+delta (YBCO) resulted in contradictory claims of no(6,7) and weak(8,9) magnetic order, and the interpretation of muon spin relaxation measurements on YBCO10,11 and of circularly polarized photoemission experiments on Bi2Sr2CaCu2O8+delta ( refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa2CuO4+delta (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO14,15, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature(16-18) rather than a phase transition temperature(19-21). Instead, they are consistent with a variant of previously proposed charge- current- loop order(19,20) that involves apical oxygen orbitals(22), and with the notion that many of the unusual properties arise from the presence of a quantum- critical point(3-5,19).