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

TL;DR

This study reveals that the pseudogap temperature T* in high-Tc cuprates marks the onset of a novel magnetic order with broken time-reversal symmetry, indicating a phase transition and a new state of matter in the pseudogap region.

Contribution

It provides direct experimental evidence for a magnetic order emerging at T* in HgBa2CuO4+d, supporting the existence of a phase transition and a universal magnetic state in cuprate superconductors.

Findings

T* marks the onset of a magnetic order in HgBa2CuO4+d.

The magnetic order involves broken time-reversal symmetry.

Results support a universal magnetic state in cuprates.

Abstract

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.