# Orbitally selective breakdown of Fermi liquid quasiparticles in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$

## TL;DR

This study uses angle-resolved photoemission spectroscopy to reveal that in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, some orbitals exhibit Fermi liquid behavior while others show non-Fermi-liquid properties, indicating an orbitally selective breakdown of quasiparticles.

## Contribution

It provides the first detailed orbital-resolved analysis showing orbitally selective Fermi liquid breakdown in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ using combined experimental and theoretical methods.

## Key findings

- $d_{xz}, d_{yz}$ bands show Fermi liquid behavior with mass renormalization.
- $d_{xy}$ band exhibits non-Fermi-liquid behavior with heavy quasiparticles.
- Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ is a hybrid metal with orbitally selective quasiparticle breakdown.

## Abstract

We present a comprehensive angle-resolved photoemission spectroscopy study of Ca$_{1.8}$Sr$_{0.2}$RuO$_4$. Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to $d_{xz}, d_{yz}$ orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions - associated with a predominantly $d_{xy}$ band character - are shown to display non-Fermi-liquid behavior. We thus demonstrate that Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.