Insulating band gaps both below and above the Néel temperature in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>d</mml:mi></mml:math>-electron <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>LaTi</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>LaV</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>SrMn</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>LaMn</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> perovskites as a symmetry-breaking event

Abstract

Metal $d$-electron oxides having an odd number of electrons per cell should exhibit band degeneracy at the Fermi energy, making them, in band theory, formally metallic. In many cases, however, these are false metals, as evidenced by the observation that many $\mathrm{AB}{\mathrm{O}}_{3}$ oxide perovskites with a magnetic $3d$ B atom are observed to be insulators both below and above the N\'eel temperature. These inconsistencies between experimental observation and expectation have historically been resolved by invoking degeneracy-breaking physics, based largely on pure electron effects, such as strong interelectronic correlation for $d$-electron compounds (the Mott mechanism). Such explanations generally consider the microscopic lattice or magnetic degrees of freedom (m-DOFs) as largely passive spectators, not causes of the formal metal being an insulator. However, it has long been known that $\mathrm{AB}{\mathrm{O}}_{3}$ perovskites can manifest an arrangement of m-DOFs in the form of octahedral tilting, bond dimerization, Jahn-Teller distortions, and ordering of local magnetic moments. It appears reasonable that such structural and magnetic local degrees of freedom need to be allowed to compete with purely electronic strong correlation. To answer this question, we explored a range of $d$-electron oxide perovskites exemplified by the archetypes $\mathrm{LaTi}{\mathrm{O}}_{3}$, $\mathrm{LaV}{\mathrm{O}}_{3}$, $\mathrm{SrMn}{\mathrm{O}}_{3}$, and $\mathrm{LaMn}{\mathrm{O}}_{3}$ with 1, 2, 3, or 4 $d$ electrons, respectively. Using a mean-field-like electronic structure method (here, density functional theory), we find that a combination of magnetic symmetry breaking (SB) with structural distortions can account for insulating band gaps in this series while correctly predicting for the control case, an intrinsic paramagnetic metal in $\mathrm{SrV}{\mathrm{O}}_{3}$, as SB is insufficiently strong to remove the degeneracy. This indicates that calculating quantitatively local magnetic and positional SB motifs in unit cells that avoid averaging at the outset over the low-symmetry motifs can provide consistent trends in a Mott transition without Mott U.

Topics & Concepts