# Magnetization density distribution of Sr$_2$IrO$_4$: Deviation from a local $j_\text{eff}=1/2$ picture

## TL;DR

This study reveals that the magnetization density in Sr$_2$IrO$_4$ deviates significantly from the expected local $j_ ext{eff}=1/2$ model, highlighting a dominant $xy$ orbital contribution and challenging previous assumptions about its electronic structure.

## Contribution

The paper provides direct experimental evidence of deviation from the local $j_ ext{eff}=1/2$ picture in Sr$_2$IrO$_4$, using polarized neutron diffraction and advanced density reconstruction techniques.

## Key findings

- Magnetization density is anisotropic and aspherical.
- $xy$ orbital contribution dominates the magnetization density.
- Deviation from the local $j_ ext{eff}=1/2$ model is significant.

## Abstract

$5d$ iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr$_2$IrO$_4$ is particularly in the spotlight because of the so-called $j_\text{eff}=1/2$ state consisting of a quantum superposition of the three local $t_{2g}$ orbitals with -- in its most simple version -- nearly equal population, which stabilizes an unconventional Mott insulating state. Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5~T at 4~K, which strongly deviates from a local \jeffHalf picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows cross-shaped positive four lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the $xy$ orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.