  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. Together with prior results for YBa2Cu3O6+d
(YBCO), this observation constitutes an essential and decisive demonstration of
the universal existence of such a state. The new findings appear to rule out a
large class of 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.
