Evidence of Orbital Ordering in Jahn-Teller Undistorted LaSr$_{2}$Mn$_{2}$O$_{7}$

TL;DR

This study reveals strong orbital ordering in LaSr$_{2}$Mn$_{2}$O$_{7}$ at low temperatures despite minimal Jahn-Teller distortions, using resonant soft x-ray diffraction to explore its magnetic and orbital structure.

Contribution

It provides evidence of orbital ordering in a Jahn-Teller undistorted compound, highlighting the role of orbital phenomena independent of lattice distortions.

Findings

Strong orbital order observed at low temperatures

Orbital ordering persists with minimal Jahn-Teller distortion

Presence of Mn$^{3+}$ and Mn$^{2+}$-like valence fluctuations

Abstract

Resonant soft x-ray diffraction has been used to probe the temperature dependent orbital and magnetic structure of $\mathrm{LaSr_{2}Mn_{2}O_7}$. Previous crystallographic studies have shown that this material has almost no MnO$_{6}$ oxygen displacement due to Jahn-Teller distortions at low temperatures. Within the low-temperature A-type antiferromagnetic phase, we found strong intensity at the $({1/4},{1/4},0)$ orbital and $LaSr_{2}Mn_{2}O_7$ magnetic reflections. This shows that even in the near absence of Jahn-Teller distortion, this compound is strongly orbitally ordered. A fit to the Mn $L$-edge resonance spectra demonstrates the presence of orbital ordering of the Mn$^{3+}$ ions with virtually no Jahn-Teller crystal field in addition to possible Mn$^{3+}$ and Mn$^{2+}$~like valence fluctuations.