# Magnetic and electronic properties of La$_3M$O$_7$ and possible polaron formation in hole-doped La$_3M$O$_7$ ($M$=Ru and Os)

## TL;DR

This study uses first-principles calculations to analyze the electronic and magnetic properties of La$_3$M$O$_7$ ($M$=Ru, Os), revealing their Mott insulating nature, effects of doping, and potential polaron formation.

## Contribution

It provides new insights into the electronic structure, magnetic order, and doping effects in La$_3$M$O$_7$ oxides, highlighting polaron formation and induced ferromagnetism.

## Key findings

- Both La$_3$OsO$_7$ and La$_3$RuO$_7$ are Mott insulators with antiferromagnetic order.
- Strong $p-d$ hybridization reduces magnetic moments, independent of spin-orbit coupling.
- Ca-doping can induce ferromagnetism in La$_3$RuO$_7$ and promote polaron formation.

## Abstract

Oxides with $4d$/$5d$ transition metal ions are physically interesting for their particular crystalline structures as well as the spin-orbit coupled electronic structures. Recent experiments revealed a series of $4d$/$5d$ transition metal oxides $R_3M$O$_7$ ($R$: rare earth; $M$: $4d$/$5d$ transition metal) with unique quasi-one-dimensional $M$ chains. Here first-principles calculations have been performed to study the electronic structures of La$_3$OsO$_7$ and La$_3$RuO$_7$. Our study confirm both of them to be Mott insulating antiferromagnets with identical magnetic order. The reduced magnetic moments, which are much smaller than the expected value for ideal high-spin state ($3$ $t_{2g}$ orbitals occupied), are attributed to the strong $p-d$ hybridization with oxygen ions, instead of the spin-orbit coupling. The Ca-doping to La$_3$OsO$_7$ and La$_3$RuO$_7$ can not only modulate the nominal carrier density but also affect the orbital order as well as the local distortions. The Coulombic attraction and particular orbital order would prefer to form polarons, which might explain the puzzling insulating behavior of doped $5d$ transition metal oxides. In addition, our calculation predict that the Ca-doping can trigger ferromagnetism in La$_3$RuO$_7$ but not in La$_3$OsO$_7$.