Two Ising-like magnetic excitations in a single-layer cuprate superconductor

TL;DR

This paper reports the discovery of two distinct magnetic excitations in a single-layer cuprate superconductor, supporting theories of orbital current order and quantum criticality in high-Tc superconductors.

Contribution

It provides experimental evidence for a second magnetic excitation in HgBa2CuO4+d, aligning with theoretical predictions of orbital current order below the pseudogap temperature.

Findings

Observation of a second magnetic excitation with local character

Evidence of mixing between magnetic excitations and antiferromagnetic fluctuations

Supports a unifying multi-band magnetic model in cuprates

Abstract

There exists increasing evidence that the phase diagram of the high-transition temperature (Tc) cuprate superconductors is controlled by a quantum critical point. One distinct theoretical proposal is that, with decreasing hole-carrier concentration, a transition occurs to an ordered state with two circulating orbital currents per CuO2 square. Below the 'pseudogap' temperature T* (T* > Tc), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds that should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa2CuO4+d the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points toward a unifying picture of magnetism in the cuprates that will likely require a multi-band description.