Anisotropic Kondo-coherence temperatures in Ni-doped CeCo₂Al₈

A family of rare-earth (R) based materials with RCo₂Al₈ (R=La-Sm) chemical composition adopt an orthorhombic crystal structure (space group 55) and have been attracting renewed interest in the past years. Recent investigations of these materials in single crystal form have revealed anisotropic magnetic and electronic properties, as well as quantum critical behavior.

CeCo₂Al₈, a member of this family, exhibits intriguing behavior in resistivity measurements as a function of temperature (T). When the current is applied parallel to the c-axis (ρ_c), a broad peak appears around 66–67 K, which is interpreted as the Kondo coherence temperature (T*_K). In measurements with the current along the ab plane (ρ_ab), a similar peak is found at about ~ 34 – 36 K. When this system is doped with Ni, T*_K, determined from a maximum in ρ_c, initially increases along the Ce(Co_1-xNi_x)₂Al₈ series before dropping to T*_K = 46 K for x =0.19. To further investigate this unusual trend, we examined ρ_ab for the x =0.19, ρ_ab, which turn out to display a maximum at 18.4 K The intricate evolution of the T*_K along the series is not followed by any significant change in heat capacity, suggesting that the changes in T*_K are likely related by some fine tuning of the local hybridizations between Ce and conduction electrons caused by the Ni substitution.