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

TLDR: P polarized neutron diffraction is used to demonstrate for the model superconductor HgBa2CuO4+δ (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order, a demonstration of the universal existence of such a state.

Abstract: 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 HgBa2CuO4+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-Tc) superconductivity, characterized by anomalous physical properties1,2. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome3,4,5. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa2Cu3O6+δ (YBCO) resulted in contradictory claims of no6,7 and weak8,9 magnetic order, and the interpretation of muon spin relaxation measurements on YBCO10,11 and of circularly polarized photoemission experiments on Bi2Sr2CaCu2O8+δ(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa2CuO4+δ (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 temperature16,17,18 rather than a phase transition temperature19,20,21. Instead, they are consistent with a variant of previously proposed charge-current-loop order19,20 that involves apical oxygen orbitals22, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point3,4,5,19.