Localized 5f2 States in UPd5Al2 and Valence Crossover in the Vicinity of Heavy-Fermion Superconductivity

Inelastic neutron scattering experiments have been carried out to reveal the 5f electron states of UPd5Al2, an isostructural compound of the heavy-fermion superconductor NpPd5Al2. The tetravalent U4+ with localized 5f2 configuration has been revealed from the sharp crystalline electric field (CEF) excitations at 8.5 and 15.5 meV with a similar level scheme to PrPd5Al2. The absence of a quasi-elastic response is evidence for the singlet states only realized in the 5f2 non-Kramers ion. The large splitting and significant broadening at high temperatures are the characteristic of 5f electrons with large orbitals. The magnetic susceptibility is reasonably explained with the mean-field model based on the low-lying levels, where the wave functions have been determined from the neutron spectra. The present study involves the valence crossover in AnPd5Al2 from tetravalent U4+ to trivalent Np3+. This valence crossover can be recognized from an abnormal lattice expansion and large electrical resistivity. Many-body e...