Change in the Ground State of Y_1−xPr_xIr₂Zn₂₀ due to a Quadrupolar Kondo Effect with Pr Concentration x

We measured the elastic constants (C₁₁ − C₁₂) /2 and C₄₄ of the non-Kramers system Y_0.92Pr_0.08Ir₂Zn₂₀ (Pr-8% system) by means of ultrasound to check how the single-site quadrupolar Kondo effect is modulated by increasing the Pr concentration. In the Pr-8% system, a temperature dependence proportional to √T is observed in (C₁₁ − C₁₂) /2 below ∼ 0.5 K. This behavior rather corresponds to the theoretical prediction of the quadrupolar Kondo virtual “lattice” model, unlike that of a Pr-3.4% system, which shows a logarithmic temperature dependence based on the “single-site” quadrupolar Kondo theory. Such temperature dependence proportional to √T is also observed in a Pr-37% system below 0.15 K, which suggests that a crossover of the quantum ground state from the “single-site” model to the “lattice” model occurs as the localized quadrupolar interaction cannot be ignored with increasing Pr concentration.