The magnetic ground state of Sr2IrO4 and implications for second-harmonic generation

TL;DR

This paper investigates the magnetic ground state of Sr2IrO4, proposing explanations for recent second-harmonic generation experiments without invoking orbital currents, and discusses implications for magnetic symmetry and experimental verification.

Contribution

It demonstrates that certain magnetic configurations can explain SHG results, challenging the previously accepted inversion-symmetric ground state and suggesting new interpretations of experimental data.

Findings

Magnetoelectric -+-+ state explains SHG data

Non-magnetoelectric ++++ state also consistent with observations

Surface effects may contribute to SHG signals

Abstract

The currently accepted magnetic ground state of Sr2IrO4 (the -++- state) preserves inversion symmetry. This is at odds, though, with recent experiments that indicate a magnetoelectric ground state, leading to the speculation that orbital currents or more exotic magnetic multipoles might exist in this material. Here, we analyze various magnetic configurations and demonstrate that two of them, the magnetoelectric -+-+ state and the non-magnetoelectric ++++ state, can explain these recent second-harmonic generation (SHG) experiments, obviating the need to invoke orbital currents. The SHG-probed magnetic order parameter has the symmetry of a parity-breaking multipole in the -+-+ state and of a parity-preserving multipole in the ++++ state. We speculate that either might have been created by the laser pump used in the experiments. An alternative is that the observed magnetic SHG signal is a surface effect. We suggest experiments that could be performed to test these various possibilities, and also address the important issue of the suppression of the RXS intensity at the L2 edge.