Abstract
The delafossite metals PdCoO$_{2}$, PtCoO$_{2}$ and PdCrO$_{2}$ are among the highest conductivity materials known, with low temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure, or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross-section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately $0.001\%$. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths, and highlights how unusual these delafossite metals are in comparison with the vast majority of other multi-component oxides and alloys. We discuss the implications of our findings for future materials research.
Highlights
The rate of resistivity increase is higher in PtCoO2 than in PdCoO2 and PdCrO2, which show the same rate of increase
These observations indicate that the resistivity is dominated by the defects in the conductive Pt=Pd planes, as expected in such twodimensional systems: Pd defects are created at the same rate in PdCoO2 and PdCrO2, while the rate of defect introduction is higher in PtCoO2, because the larger nuclear charge of the Pt atoms leads to a stronger interaction with the incoming electrons
The experimental data and analysis presented in this paper provide very strong evidence in favor of a remarkable experimental fact: the extremely long low-temperature mean free paths of metallic delafossites are mainly due to an exceptional level of crystalline perfection of the Pd or Pt planes in which the conduction takes place
Summary
Throughout the early evolution of the physics of metals, it was assumed that extremely high metallic state electrical conductivity would be restricted to elemental metals, which could be purified and annealed to remove dislocations and other structural defects, resulting in long electron mean. Scattering is predicted to be suppressed by a factor of order 2–4 from that in a material with a trivial Fermi surface of the same size This aspect of the physics of the delafossite metals is important, the predicted scattering suppression is nowhere near sufficient to account for the observed lowtemperature mean free paths. Consistent with this finding, the ratio between the resistive and dHvA mean free paths is a much more modest factor of 10–20 [14,15] than the 104 reported in the Dirac and Weyl materials.
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