---
abstract: 'We derive an $S=1$ spin polaron model which describes the motion of a single hole introduced into the $S=1$ spin antiferromagnetic ground state of Ca$_2$RuO$_4$. We solve the model using the self-consistent Born approximation and show that its hole spectral function qualitatively agrees with the experimentally observed high-binding energy part of the Ca$_2$RuO$_4$ photoemission spectrum. We explain the observed peculiarities of the photoemission spectrum by linking them to two anisotropies present in the employed model—the spin anisotropy and the hopping anisotropy. We verify that these anisotropies, and *not* the possible differences between the ruthenate ($S=1$) and the cuprate ($S=1/2$) spin polaron models, are responsible for the strong qualitative differences between the photoemission spectrum of Ca$_2$RuO$_4$ and of the undoped cuprates.'
author:
- 'Adam Kł‚osiń„ski'
- 'Dmitri V. Efremov'
- Jeroen van den Brink
- Krzysztof Wohlfeld
date:
- 
- 
title: |
    Photoemission Spectrum of Ca$_2$RuO$_4$:\
    Spin Polaron Physics in an $S=1$ Antiferromagnet with Anisotropies
---

Introduction. {#sec:introduction}
=============

Understanding the strongly correlated physics of the transition metal oxides constitutes a nontrivial task [@Dagotto1994; @Imada1998; @Lee2006; @Khomskii2010; @Khomskii2014]. On one hand, this is due to the fact that even the simplest, but still realistic, effective models may have to 