Origin of band gaps in 3d perovskite oxides (1901.00829v1)

Abstract: With their broad range of magnetic, electronic and structural properties, transition metal perovskite oxides ABO3 have long served as a platform for testing condensed matter theories. In particular, their insulating character - found in most compounds - is often ascribed to dynamical electronic correlations through the celebrated Mott-Hubbard mechanism where gaping arises from a uniform, symmetry-preserving electron repulsion mechanism. However, structural distortions are ubiquitous in perovskites and their relevance with respect to dynamical correlations in producing this rich array of properties remains an open question. Here, we address the origin of band gap opening in the whole family of 3d perovskite oxides. We show that a single-determinant mean-field approach such as density functional theory (DFT) successfully describes the structural, magnetic and electronic properties of the whole series, at low and high temperatures. We find that insulation occurs via energy-lowering crystal symmetry reduction (octahedral rotations, Jahn-Teller and bond disproportionation effects), as well as intrinsic electronic instabilities, all lifting orbital degeneracies. Our work therefore suggests that whereas ABO3 oxides may be complicated, they are not necessarily strongly correlated. It also opens the way towards systematic investigations of doping and defect physics in perovskites, essential for the full realization of oxide-based electronics.

Origin of Band Gaps in 3d Perovskite Oxides

The paper "Origin of band gaps in 3d perovskite oxides" by Varignon, Bibes, and Zunger offers a thorough investigation into the formation of band gaps in transition metal perovskite oxides ABO3, which have historically served as a testing ground for condensed matter theories due to their versatile magnetic, electronic, and structural properties. The authors challenge the predominant view attributing the insulating nature of these materials to the Mott-Hubbard mechanism, proposing instead that structural factors play a dominant role.

Key Findings

The authors employed density functional theory (DFT), specifically utilizing the meta-GGA SCAN functional, without the Hubbard correction parameter U, across the family of 3d perovskite oxides. Their findings suggest that band gaps form not necessarily due to strong electron correlation, but rather through symmetry-breaking events. These events include structural distortions such as octahedral rotations, Jahn-Teller distortions, and bond disproportionation effects. DFT, a mean-field approach, successfully captured these phenomena and described the band gap formation, electronic properties, and magnetic orders at both low and high temperatures.

Numerical Results

Significant numerical discrepancies were noted between the naïve DFT and more refined DFT models. For example, energies calculated for non-magnetic phases with naïve DFT were significantly higher than those calculated with spin-polarized DFT ground states, with differences observed over several energy scales. Such findings reinforce the notion that approximations lacking symmetry-breaking considerations yield less accurate portrayals of these materials' properties.

Implications

The research implies that the nuanced structural nature of 3d perovskite oxides, rather than extreme electronic correlation effects, governs their insulating properties. This insight extends the role of DFT as a reliable, computationally efficient tool for studying band gap formation, as well as discrepancies previously thought to be beyond its scope. It prompts a reconsideration of theoretical models that ostensibly require dynamic electronic correlation accounts.

Speculations on Future Developments

This paper opens pathways for deeper inquiry into doping, defects, and interfaces in perovskites, with DFT as a foundational tool. This advances the potential for predicting complex transition pathways and engineered properties in oxide-based electronic materials.

In conclusion, Varignon et al. argue that the traditional Mott-Hubbard model does not universally apply to 3d perovskite oxides, proposing instead that symmetry-breaking induced by structural distortions—all captured by DFT—can explain the observed insulating nature across this material class. These findings provoke further exploration into oxide electronics, promising advances in synthetic strategies and device applications.