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Li, Y.,Balé,dent, V.,Bariš,ix107,, N.,Cho, Y.,Fauqué,, B.,Sidis, Y.,Yu, G.,Zhao, X.,Bourges, P.,Greven, M. Macmillan Publishers Limited. All rights reserved 2008 Nature Vol.455 No.7211
The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-T<SUB>c</SUB>) superconductivity, characterized by anomalous physical properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>6+δ</SUB> (YBCO) resulted in contradictory claims of no and weak magnetic order, and the interpretation of muon spin relaxation measurements on YBCO and of circularly polarized photoemission experiments on Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>8+δ</SUB>(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa<SUB>2</SUB>CuO<SUB>4+δ</SUB> (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO, 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 rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point.