Complete breakdown of magnons in the square lattice iridate Sr₂IrO₄

Quantum (spin-1/2) Heisenberg antiferromagnet on a square lattice is a main building block of cuprate high-temperature superconductors, and its magnetism can mostly be explained by the semi-classical spin-wave theory. However, a pronounced deviation from the spin-wave theory is observed at a certain momentum—also known as (π, 0) anomaly—in the magnetic excitation spectra of cuprates. The single magnon spectral weight is significantly reduced and transferred to an isotropic high-energy continuum, which has been interpreted as a precursor of spinon deconfinement transition. Here, we revisit the magnetic excitation spectra of the single-layer iridate Sr₂IrO₄ using resonant inelastic x-ray scattering. With a judicious selection of a scattering geometry and a small domain aligning magnetic field, we resolve the transverse and longitudinal responses, and reveal a completely isotropic magnetic spectrum at (π,0), indicating a zero spectral weight for a single-magnon pole. This result correlates with a much steeper spin-wave dispersion along the magnetic zone boundary and thus suggests that a large ring exchange interaction is responsible for the complete breakdown of magnons, signaling a nearby quantum critical point.