Ultraclean Transport and Frustrated Magnetism in the Metallic Delafossites PdCoO2 and PdCrO2

Metallic delafossites (e.g., PdCoO2, PdCrO2) have drawn much attention since the realization that they are the most conductive oxides known [1]. PtCoO2 has room-temperature resistivity comparable to Au, while PdCoO2 attains low-temperature resistivities as low as ~8 nΩcm, corresponding to ~20 μm mean-free-path [1]. This is achieved despite their complex oxidic nature, their layered structure (conduction is confined to 2D Pd/Pt sheets between insulating Co-O octahedral layers), and their relatively crude syntheses. This presentation first focuses on PdCoO2, describing a new crystal growth by chemical vapor transport that results in >500-fold gains in crystal size, and the highest quality yet reported (reaching residual resistivity ratio (RRR) >440) [2]. Despite this performance, detailed purity analyses reveal relatively high impurity concentrations. This paradox is resolved through a crystal-chemical analysis showing that impurities in metallic delafossites are forced to incorporate in insulating B-O octahedral layers, leaving ultrapure metallic Pd/Pt sheets [2]. This “sublattice purification” provides the first full explanation for the ultraclean transport in these materials. The presentation will then shift to PdCo1-xCrxO2 [3] and PdCrO2 [4], introducing geometrically frustrated local magnetic moments into these ultraclean metals. PdCrO2 is shown to display a host of striking phenomena, including large negative thermal expansion, large positive magnetoresistance of magnetic origin, and an unconventional anomalous Hall effect [4]. Surprisingly, these phenomena do not vanish at the 38-K Néel temperature (TN), instead persisting to above room temperature, ~10-times TN [4]. These effects are shown to derive from a wide regime of strong spin fluctuations, as directly probed by inelastic neutron spectroscopy and magnetic pair distribution function methods [4].