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

• Published in: Nature (London) 2008-09, Vol.455 (7211), p.372-375
• Main Authors: Li, Y., Balédent, V., Barišić, N., Cho, Y., Fauqué, B., Sidis, Y., Yu, G., Zhao, X., Bourges, P., Greven, M.
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Humanities and Social Sciences; letter; Magnetic properties; multidisciplinary; Physics; Properties of type I and type II superconductors; Science; Science (multidisciplinary); Superconductivity
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature07251

Pseudogaps: secret of superconductivity The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high transition-temperature superconductivity. Li et al . report polarized neutron diffraction data that demonstrate that the characteristic temperature, T * , marks the onset of an unusual magnetic order, and hence a novel state of matter with broken time-reversal symmetry, for the model superconductor HgBa 2 CuO 4+ δ (Hg1201). 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 the notion that many of the unusual properties arise from the presence of a quantum-critical point. 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 , 4 , 5 . 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+ δ (YBCO) resulted in contradictory claims of no 6 , 7 and weak 8 , 9 magnetic order, and the interpretation of muon spin relaxation measurements on YBCO 10 , 11 and of circularly polarized photoemission experiments on Bi 2 Sr 2 CaCu 2 O 8+ δ (refs 12 , 13 ) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa 2 CuO 4+ δ (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO 14 , 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 , 17 , 18 rather than a phase transition temperature 19 , 20 , 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 , 4 , 5 , 19 .