title: Unusual magnetic order in the pseudogap region of the superconductor HgBa2CuO4+delta.
abstract: 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. 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(2)Cu(3)O(6+delta) (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(2)Sr(2)CaCu(2)O(8+delta)(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa(2)CuO(4+delta) (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.
authors.affiliation: Department of Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Li
authors.forename: V
authors.initials: V
authors.lastname: Balédent
authors.forename: N
authors.initials: N
authors.lastname: Barisić
authors.forename: Y
authors.initials: Y
authors.lastname: Cho
authors.forename: B
authors.initials: B
authors.lastname: Fauqué
authors.forename: Y
authors.initials: Y
authors.lastname: Sidis
authors.forename: G
authors.initials: G
authors.lastname: Yu
authors.forename: X
authors.initials: X
authors.lastname: Zhao
authors.forename: P
authors.initials: P
authors.lastname: Bourges
authors.forename: M
authors.initials: M
authors.lastname: Greven