Fig. 7-1 Np metal This sample was prepared using a hydrometallurgical method with a mercury cathode.

Fig. 7-2 Large, high-quality single crystal ingot of NpCoGa5 This crystal was obtained using the flux method.

Fig. 7-3 dHvA oscillations and two cylindrical Fermi surfaces of NpCoGa5 The dHvA frequency is proportional to the cross-sectional area of the Fermi surface. Electronic properties such as the electrical resistivity can be understood via Fermi surfaces properties. The effective mass of the conduction electrons can also be determined from the temperature dependence of the dHvA amplitude.

Neptunium (Np) is located just to the right of uranium in the periodic table, and is an artificial radioactive element called a transuranium element. Because of its high radioactivity, details about the electronic properties of Np compounds are not well known. We have grown high-quality single crystals of Np compounds, and have succeeded in observing the de Haas-van Alphen (dHvA) effect in these compounds, thus characterizing the Fermi-surface electronic states of the 5f electrons. Np metal was prepared using a hydrometallurgical method, starting from NpO2 (Fig. 7-1). A small amount, ca. 1 g, of high-quality, pure Np metal has been prepared by this unique method. Using this Np metal, we have grown high-quality single crystals of two Np compounds, NpNiGa5 and NpCoGa5 (Fig. 7-2). The 5f electrons of the element Np play an important role in the electronic properties of Np compounds. The dHvA effect is a powerful method for investigating the electronic properties of 5f electrons in these compounds. This effect measures small oscillations in the magnetic susceptibility of the crystal as a function of the external magnetic field, which provide information on the Fermi surface properties of the conducting electrons. The Fermi surface is called "the face of a metal." One can also determine the effective mass of the conduction electrons from dHvA oscillations. We have succeeded in observing dHvA oscillations in the Np compounds, NpNiGa5 and NpCoGa5, the latter being shown in Fig. 7-3. This figure also shows the corresponding Fermi surfaces of NpCoGa5, with two cylindrical Fermi surfaces labeled alpha and beta, demonstrating that the 5f electrons occupy quasi-two dimensional electronic states in this compound. We have also found that the effective mass of the conduction electrons is ten times large as the free-electron mass of an electron; thus, the conduction electrons move through these crystals slowly, like clouds.

Reference D. Aoki et al., First Observation of the de Haas-van Alphen Effect in NpNiGa5, J. Phys Soc. Jpn., 73(3), 519 (2004).

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