Quantified Degeneracy, Entropy and Metal-Insulator Transition in Complex Transition-Metal Oxides

TL;DR

This paper introduces a new entropy-based measure of effective degeneracy to better understand phase transitions in complex transition-metal oxides, providing insights into their electronic behaviors and a novel degeneracy-controlled metal-insulator transition.

Contribution

It proposes a quantitative method to measure effective degeneracy using entropy-like terms, applicable to complex oxides with intricate band structures.

Findings

Effective degeneracy correlates with metal-insulator transitions.

The method applies to titanates, ruthenates, and iridates.

A degeneracy control transition is proposed for LaTiO₃/LaAlO₃ superlattice.

Abstract

Understanding complex correlated oxides and their phase transitions has long been a challenge. The difficulty largely arises from the intriguing interplay between multiple degrees of freedoms. While degeneracy can play an important role in determining material characteristics, there is no well-defined way to quantify and to unveil its role in real materials having complicated band structures. Here we suggest a way to quantify the `effective degeneracy' relevant to metal-insulator transition by introducing entropy-like terms. This new quantity well describes the electronic behaviors of transition-metal oxides as a function of external and internal parameters. With $3d$ titanates, $4d$ ruthenates, and $5d$ iridates as our examples, we show that this new effective quantity provides useful insights to understand these systems and their phase transitions. For LaTiO$_3$/LaAlO$_3$ superlattice, we suggest a novel `degeneracy control' metal-insulator transition.