Nature of the enigmatic pseudogap state: novel magnetic order in superconducting HgBa2CuO4+d

Understand

The nature of the enigmatic pseudogap region of the phase diagram is the most important and intriguing unsolved puzzle in the field of high transition-temperature (Tc) superconductivity.

• This region, the temperature range above Tc and below a characteristic temperature T*, is characterized by highly anomalous magnetic, charge transport, thermodynamic and optical properties.
• Associated with the pseudogap puzzle are open questions pertaining to the number of distinct phases and the presence of a quantum-critical point underneath the superconducting dome.
• Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa2CuO4+d (Hg1201) that T* marks the onset of an unusual magnetic order, and hence a novel state of matter with broken time-reversal symmetry.