Pseudo-triplet 5f electron state in the heavy fermion superconductor NpPd₅Al₂

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 Γ_t1 with the main component of |0> and the first excited doublet Γ_t5 dominated by |±1> at ΔE = 49 K. The magnetization curve can be explained from the effective local Hamiltonian for pseudo spin J = 1 with DJ_z² (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 R ln 3 that corresponds to the pseudo-triplet state. The entropy balance in the superconducting state is also satisfactory with this model. The obtained Kondo temperature T_K = 55 K is comparable to the level splitting ΔE = 49 K, indicating the contribution of the excited doublet Γ_t5 to the Kondo effect. (author)