Pseudo-triplet ground state in the heavy-fermion superconductor NpPd5Al2 was proposed from the macroscopic data analyzed by the crystalline electric field (CEF) level scheme deduced from the systematic neutron scattering of the rare-earth and U-based isostructural family. The magnetic susceptibility χ can be reproduced with the singlet ground state Γt1 with the main component of ∣0〉 and the first excited doublet Γt5 dominated by ∣±1〉 at ΔE = 49K. The magnetization curve can be explained from the effective local Hamiltonian for pseudo spin J = 1 with DJz 2 (D = ΔE) equivalent to the quadrupole operator O20. The specific heat in the normal state can be described with the numerical result of a Kondo model, where the model was normalized to give the entropy Rln3 that corresponds to the pseudo-triplet state. The entropy balance in the superconducting state is also satisfactory with this model. The obtained Kondo temperature TK = 55K is comparable to the level splitting ΔE = 49K, indicating the contribution of the excited doublet Γt5 to the Kondo effect.
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