Symmetry Preservation and Critical Fluctuations in a Pseudospin Crossover Perovskite LaCoO$_3$

TL;DR

This study uses orbital-resolved NMR spectroscopy to investigate spin states and fluctuations in LaCoO$_3$, revealing symmetry preservation and critical fluctuations during the pseudospin crossover, challenging conventional ligand-field theory.

Contribution

It provides microscopic evidence of symmetry preservation and orbital degeneracy in LaCoO$_3$, introducing a new perspective on spin-state crossover as a statistical critical phenomenon.

Findings

Crystal symmetry remains intact across the crossover.

Orbital degeneracy leads to a pseudospin excited state.

Critical spin-state fluctuations emerge under magnetic field.

Abstract

Spin-state crossover beyond a conventional ligand-field theory has been a fundamental issue in condensed matter physics. Here, we report microscopic observations of spin states and low-energy dynamics through orbital-resolved NMR spectroscopy in the prototype compound LaCoO$_3$. The $^{59}$Co NMR spectrum shows the preserved crystal symmetry across the crossover, inconsistent with $d$ orbital ordering due to the Jahn-Teller distortion. The orbital degeneracy results in a pseudospin ($\tilde{J} = 1$) excited state with an orbital moment observed as $^{59}$Co hyperfine coupling tensors. We found that the population of the excited state evolves above the heart crossover temperature. The crossover involves critical spin-state fluctuations emerging under the magnetic field. These results suggest that the spin-state crossover can be mapped into a statistical problem, analogous to the supercritical liquid in liquid-gas transition.