Quasi-two-dimensional Fermi surfaces in the flat antiferromagnetic Brillouin zone of NpRhGa₅ studied by dHvA experiments and energy band calculations

We succeeded in growing a high-quality single crystal of NpRhGa₅ by the Ga-flux method and observed the de Haas-van Alphen oscillation in the antiferromagnetic state. Four kinds of nearly cylindrical Fermi surfaces, which correspond to main Fermi surfaces, were clearly detected. These quasi-two-dimensional Fermi surfaces are formed in the flat antiferromagnetic Brillouin zone and are well explained on the basis of spin- and orbital-polarized LAPW energy band calculations. The cyclotron masses are moderately enhanced, ranging from 8.1 to 11.7 m₀, which are approximately four times larger than the corresponding band masses. This is the first case where the 5f-itinerant band model is applicable to a neptunium magnetic compound.