# Low-energy excitation spectra in the excitonic phase of cobalt oxides

## TL;DR

This paper investigates the low-energy excitation spectra of the excitonic phase in cobalt oxides using a multi-orbital Hubbard model, revealing collective excitations like Goldstone and Higgs modes relevant for experiments.

## Contribution

It introduces a detailed theoretical analysis of excitonic condensation and collective excitations in cobalt oxides using RPA and mean-field methods.

## Key findings

- Identification of gapless spin-transverse (Goldstone) mode.
- Detection of gapful spin-longitudinal (Higgs) mode.
- Prediction of excitonic instability in cobalt oxides.

## Abstract

We study the excitonic phase and low-energy excitation spectra of perovskite cobalt oxides. Constructing the five-orbital Hubbard model defined on the three-dimensional cubic lattice for the $3d$ bands of Pr$_{0.5}$Ca$_{0.5}$CoO$_3$, we calculate the excitonic susceptibility in the normal state in the random-phase approximation (RPA) to show the presence of the instability toward excitonic condensation. Based on the excitonic ground state with a magnetic multipole obtained in the mean-field approximation, we calculate the dynamical susceptibility of the excitonic phase in the RPA and find that there appear the gapless collective excitation in the spin-transverse mode (Goldstone mode) and the gapful collective excitation in the spin-longitudinal mode (Higgs mode). Experimental relevance is discussed.