Kondo Effect and Spin Glass Freezing in PdCo_1-xCr_xO₂ Metallic Delafossite

Metallic delafossites (e.g., PdCoO₂, PdCrO₂) are under intense study due to their ultraclean metallic transport. Our recent work established chemical vapor transport (CVT) growth of high-quality single crystals of PdCoO₂ and PdCrO₂, elucidating the sublattice purification mechanism underlying the extraordinary low-temperature conductivity of nonmagnetic PdCoO₂ [1], the optical hyperbolicity of PdCoO₂ and PdCrO₂ [2], and the spin-fluctuation-induced high-temperature anomalous Hall effect in antiferromagnetic PdCrO₂ [3]. Here we study CVT-grown crystals of PdCo_1−x​Cr_x​O₂​ in the dilute limit up to x = 0.05. Structural and chemical characterization supports single-phase, single-crystal materials, with uniform Cr substitution. Magnetometry at temperatures (T) above 2 K confirms weakly interacting paramagnetism, scaling as expected with x, T, and field. Below 2 K, however, clear spin glass freezing occurs, seen in both AC susceptibility and specific heat measurements (down to 50 mK). Low-temperature electronic transport measurements reveal retention of high residual conductivity despite Cr substitution, confirming the sublattice purification mechanism [1]. Nevertheless, a small but clear Kondo effect arises in the resistivity, with a non-trivial temperature dependence due to spin glass freezing. These effects are discussed in terms of the interlayer Kondo interaction between Cr moments in the insulating (Co/Cr)-O layers and highly itinerant electrons in the metallic Pd planes.