Dispersion of Collective Magnetic Excitons in a Single Crystal of the Actinide Intermetallic UPd3

Abstract

In actinide salts and intermetallic compounds the degree of localization of the 5f electrons determines whether low-lying, well-defined magnetic excitations out of the electronic ground state can be observed. By low-lying we mean in the frequency range commonly accessed in neutron scattering (0 ~ 20 THz) thereby excluding magnetic response on the scale of the Fermi energy. In the uranium pnictides, for example, well-defined excitations are observed in the antimonide1, where the U-U distance is large. Where the U-U distance is small, so that some broadening of the f-band may be presumed, as in uranium nitride, the magnetic response at low frequencies consists at most of a broad, diffuse spectrum with an energy gap2. Photoemission studies at Zurich4 and at Chalk River5 show that the broadening is at most a few tenths of an eV, but that the main part of the 5f electron density lies at the Fermi energy. As well as the rocksalt compounds there are several uranium intermetallic compounds3 (UAl 2 and USn3) which exhibit a broad and featureless spectrum centred on zero frequency.