Orbital Order Instability and Orbital Excitations in Degenerate Itinerant Electron Systems

TL;DR

This paper develops a theoretical framework for understanding orbital order instability and excitations in degenerate itinerant electron systems, highlighting the spectral features and experimental signatures of orbital ordering.

Contribution

It introduces a criterion for orbital order instability and analyzes the excitation spectra and experimental signatures in orbital-ordered phases.

Findings

Finite gaps in orbital and spin-orbital excitation spectra

Spectral features in ARPES consistent with orbital order

Orbital occupation patterns observed in RXS

Abstract

We present the theory of orbital ordering in orbital-degenerate itinerant electron systems. After proposing the criterion of instability for orbital ordering or orbital density wave ordering, we find that the orbital and the spin-orbital collective excitation spectra in ferro-orbital ordered phase exhibit finite gaps. The anomalous electronic energy spectra manifested in the angle-resolved photoemission spectroscopy (ARPES) and the orbital occupation in the resonant X-ray scattering (RXS) intensities are also presented for the orbital-ordered phase.