The properties of ultrapure delafossite metals

Abstract

Although they were first synthesized in chemistry laboratories nearly fifty years ago, the physical properties of the metals PdCoO, PtCoO and PdCrO have only more recently been studied in detail. The delafossite structure contains triangular co-ordinated atomic layers, and electrical transport in the delafossite metals is strongly 2D. Their most notable feature is their in-plane conductivity, which is amazingly high for oxide metals. At room temperature, the conductivity of non-magnetic PdCoO and PtCoO is higher per carrier than those of any alkali metal and even the most conductive elements, copper and silver. At low temperatures the best crystals have resistivities of a few nΩ cm, corresponding to mean free paths of tens of microns. PdCrO is a frustrated antiferromagnetic metal, with magnetic scattering contributing to the resistivity at high temperatures and small gaps opening in the Fermi surface below the Néel temperature. There is good evidence that electronic correlations are weak in the Pd/Pt layers but strong in the Co/Cr layers; indeed the Cr layer in PdCrO is thought to be a Mott insulator. The delafossite metals therefore act like natural heterostructures between strongly correlated and nearly free electron sub-systems. Combined with the extremely high conductivity, they provide many opportunities to study electrical transport and other physical properties in new regimes. The purpose of this review is to describe current knowledge of these fascinating materials and set the scene for what is likely to be a considerable amount of future research.

Hydrodynamic Electron Flow and Hall Viscosity

“…One needs a large separation of scales between momentum-relaxing and momentum-conserving scattering in order to see these effects. This was recently achieved in graphene [23,24] and PdCoO 2 [25,26].Interest in such a hydrodynamic regime also emanated from a conjectured bound on diffusion constants for the hydrodynamics of strongly interacting quantum systems [27,28]. Even though the physics described in this work is semiclassical and probably still quite far from these quantum-mechanical bounds, the observations that we hope to stimulate would constitute an important first step towards the understanding of emergent hydrodynamical regimes in electronic systems.…”

Copper Delafossites under High Pressure—A Brief Review of XRD and Raman Spectroscopic Studies

“…They show a wide range of conductivity from insulating to metallic [2][3][4]. Most of the Cu and Ag based delafossites are semiconductors whereas Pt and Pd based compounds exhibit good metallic conductivity; with their room temperature in-plane conductivity reaching about a few µΩ cm, which is comparable to that of metallic elemental copper [5]. Along with good electrical conductivity, many of the delafossites show good transparency to optical photons and the compounds exhibiting the combination of these two properties are termed as transparent conducting oxides (TCO) [6][7][8].…”

In Situ Modification of a Delafossite-Type PdCoO2 Bulk Single Crystal for Reversible Hydrogen Sorption and Fast Hydrogen Evolution

“…The most striking characteristic is their remarkably low electrical resistivity, lower than those of all normal state oxides yet reported. [1][2][3] Pd 4d states contribute to the density of states at the Fermi level with sp band-like fast dispersion. [4][5] This leads to an ultra-high carrier velocity of ~ 4.96 × 10 5 m s ⁄ , which even approaches that of graphene with a velocity of the order of ~ 10 6 m s ⁄ .…”

“…[4][5] This leads to an ultra-high carrier velocity of ~ 4.96 × 10 5 m s ⁄ , which even approaches that of graphene with a velocity of the order of ~ 10 6 m s ⁄ . 1,[6][7][8] Of particular interest is the observation of hydrodynamic electron flow in bulk PdCoO2 single crystals, suggesting the anomalous suppression of momentum relaxation resulting from electron-impurity, electron-electron, and electronphonon. 9 In addition, PdCoO2 exhibits a strong quasi-two-dimensional nature, characterized by higher in-plane conductivity than along the normal direction.…”