The specific heat of Y1-xPrxIr2Zn20 with x = 0.044 has been measured down to 0.08 K in magnetic fields of B ≤ 12 T. The 4f contribution to the specific heat divided by temperature C4f/T diverges as −lnT in B = 0 and 2 T applied along the [100] direction. In B = 4 T, however, C4f(T) exhibits a broad maximum, which shifts to higher temperatures with increasing B. This behavior can be explained by the Zeeman effect which splits the ground state doublet through the first excited triplet. The 4f contribution to the entropy, S4f(T), in B ≥ 4 T is well reproduced by the crystalline electric field calculation, whereas the calculation fails to reproduce the S4f(T) data in B ≤ 2 T. We propose that the on-site interaction of the active quadrupole in the ground state doublet with two-channel conduction bands gives rise to the –lnT variation of C4f/T.
