Impact of Local Lattice Disorder on Spin and Orbital Orders in Ca$_{2-x}$Sr$_x$RuO$_4$

TL;DR

This study investigates how local lattice distortions influence spin and orbital orders in Ca$_{2-x}$Sr$_x$RuO$_4$, revealing that local octahedral elongation or compression can induce significant changes in magnetic and electronic states.

Contribution

The paper introduces a computational model linking local lattice distortions to global magnetic and orbital orders in Ca$_{2-x}$Sr$_x$RuO$_4$, providing new insights into disorder effects.

Findings

Local elongation narrows the Mott gap in Ca$_2$RuO$_4$.

Local compression induces magnetic order in Sr$_2$RuO$_4$.

Results align with recent $bc$SR experimental findings.

Abstract

We have studied relationship between local lattice disorder, Ru 4$d$ spin-orbit interaction, and global spin/orbital orders of Ca$_{2-x}$Sr$_x$RuO$_4$ in the Sr-rich and Ca-rich regions of its phase diagram by using unrestricted Hartree-Fock calculation on a $8\times8$ RuO$_4$ lattice model. The calculations show that the local elongation of RuO$_6$ octahedron in an antiferromagnetic insulator Ca$_2$RuO$_4$ induces local orbital change with making the Mott gap narrower. On the other hand, local compression of RuO$_6$ octahedron in a paramagnetic metal Sr$_2$RuO$_4$ induces global antiferromagnetic or ferromagnetic states. This result is consistent with a recent systematic $\mu$SR study by Carlo {\it et al.} which has revealed the static antiferromagnetic order at low temperature in the Sr-rich region.