Quantum spin nematic phase in a square-lattice iridate

TL;DR

This paper reports the experimental discovery of a quantum spin nematic phase in a square-lattice iridate, revealing complex magnetic order and quantum entanglement that could be related to high-temperature superconductivity.

Contribution

It provides the first unambiguous experimental evidence of a spin nematic phase in a square-lattice iridate, using Raman spectra, resonant x-ray diffraction, and inelastic x-ray scattering.

Findings

Identification of a spin nematic phase at T_C ≈ 263 K

Observation of quadrupolar order persisting below T_N

Breakdown of coherent magnon excitations at short wavelengths

Abstract

Spin nematic (SN) is a magnetic analog of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid. Particularly intriguing is a valence-bond SN, in which spins are quantum entangled to form a multi-polar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here, we establish a SN phase in the square-lattice iridate Sr$_2$IrO$_4$, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet (AF) in the strong spin-orbit coupling limit. Upon cooling, the transition into the SN phase at T$_C$ $\approx$ 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra, and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic (AF) phase below T$_N$ $\approx$ 230 K, and becomes directly observable through its interference with the AF order in resonant x-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic x-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a resonating-valence-bond-like quantum entanglement in the AF state. Taken together, our results reveal a quantum order underlying the N\'eel AF that is widely believed to be intimately connected to the mechanism of high temperature superconductivity (HTSC).