title: Magnetic-octupole order in neptunium dioxide?
abstract: The phase transition occurring at 25 K in NpO2, discovered almost 50 years ago, is the most long-lasting mystery in the physics of actinide compounds. Theories based on magnetic or electric-quadrupole order lead inevitably to fundamental, qualitative inconsistencies with observations. We show that the phenomenology of NpO2 can be understood if the order parameter is assumed to be a magnetic octupole of gamma(2) ( xyz) symmetry. NpO2 is the first compound for which indications of an octupolar phase transition have been found.
authors.affiliation: Oxford Physics, Clarendon Laboratory, Oxford OX1 3PU, United Kingdom.
authors.initials: P
authors.lastname: Santini
authors.initials: G
authors.lastname: Amoretti