To clarify the origin of the nontrivial thermoelectric properties observed in the delafossite oxide CuRhO 2 polycrystals, we have performed the systematic transport measurements on the single-crystalline CuRhO 2 samples. In the parent compound, we find a pronounced peak structure due to a phonon-drag effect in the temperature dependence of the Seebeck coefficient, which is also confirmed by the size effect experiments. In the Mg-substituted crystals, in contrast to the results of the polycrystals, both the resistivity and the Seebeck coefficient decrease with increasing Mg content y. In particular, the coefficient A for the T 2 term of the resistivity and the T -linear coefficient for the Seebeck coefficient at low temperatures are well described within a simple relationship expected for metals, which is also applicable to the correlated materials with low carrier densities.

In order to clarify the origin of unusual thermoelectric transport properties of CuRhO 2 polycrystals at low temperature, we synthesized the hole-doped CuRhO 2 single crystals by using self-flux method and measured the temperature dependence of in-plane transport coefficients. In contrast to the results of polycrystals, the Seebeck coefficients of CuRh 1−y Mg y O 2 single crystals systematically decrease with increasing Mg content y. We have considered the carrier density dependence of the coefficient A for the T 2 term of the Fermi-liquid resistivity and the T-linear coefficient for the Seebeck coefficient at low temperature, and find that the scaling in A and (S /T) 2 may hold in a wide range of materials including low-carrier density materials except for some organics. This scaling can be described with the relationship in the simple metals, and experimental data of CuRh 1−y Mg y O 2 and other materials are consistent with this scaling.

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