Fragile altermagnetism and orbital disorder in Mott insulator LaTiO$_3$

TL;DR

This study reveals that LaTiO3, a Mott insulator, exhibits altermagnetic properties characterized by fully compensated antiferromagnetism and momentum-dependent spin splitting, protected by crystal symmetry and orbital order, with orbital disorder disrupting these features.

Contribution

First demonstration that LaTiO3 can be an altermagnet with symmetry-protected spin splitting, highlighting the role of orbital order and disorder in its magnetic properties.

Findings

LaTiO3 shows altermagnetic features with spin-split bands without spin-orbit coupling.

Orbital disorder weakens the spin splitting and the altermagnetic state.

Spin-orbit coupling can induce orbital disorder and suppress altermagnetism.

Abstract

Based on ab initio calculations, we demonstrate that a Mott insulator LaTiO$_3$ (LTO), not inspected previously as an altermagnetic material, shows the characteristic features of altermagnets, i.e., (i) fully compensated antiferromagnetism and (ii) $\mathbf{k}$-dependent spin-split electron bands in the absence of spin-orbit coupling. The altermagnetic ground state of LTO is protected by the crystal symmetry and specifically ordered $d$-orbitals of Ti ions with the orbital momentum $l=2.$ The altermagnetism occurs when sites of Ti pair in the unit cell are occupied by single electrons with $m=-1,s_{z}=+1/2$ and $m=+1,s_{z}=-1/2$ per site, with $m$ and $s_{z}-$ being the $z-$ component of the orbital momentum and spin, respectively. By further simulating orbital disorder within the Green's function method, we disclose its damaging character on the spin splitting and the resulting altermagnetism. When the single-electron spin-polarized state at each Ti site is contributed almost equally by two or three $t_{2g}$ orbitals, LTO becomes antiferromagnetic. The effect of the spin-orbit coupling, which can cause orbital disorder and suppress altermagnetism, is discussed.