Nearly-free electrons in a 5d delafossite oxide metal

TL;DR

This paper demonstrates that PtCoO2 is an exceptionally conductive 5d transition-metal oxide with nearly free electron behavior, characterized by a steep conduction band, weak electron-electron scattering, and a simple Fermi surface.

Contribution

It provides the first detailed experimental and theoretical evidence of nearly free electron behavior in a 5d delafossite oxide, expanding understanding of electron correlations in such materials.

Findings

PtCoO2 has the highest known conductivity among oxides.

The Fermi surface is a single, nearly hexagonal cylinder with weak k_z dispersion.

Photoemission shows well-defined spectral features with minimal broadening.

Abstract

Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realisation of numerous exotic phases including spin-orbit assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focussed on the $5d$ iridium-based oxides. Here, we instead consider the Pt-based delafossite oxide PtCoO$_2$. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focussed ion-beam techniques, yield a room-temperature resistivity of only 2.1~$\mu\Omega$cm, establishing PtCoO$_2$ as the most conductive oxide known. From angle-resolved photoemission and density-functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along k$_z$. Despite being predominantly composed of $d$-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14$m_e$. Moreover, the sharp spectral features observed in photoemission remain well-defined with little additional broadening for over 500~meV below E$_F$, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO$_2$ as a model nearly-free electron system in a $5d$ delafossite transition-metal oxide.