Spin Fluctuation, Orbital States and Non-conventional Superconductivity in Actinides Compounds
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Spin Fluctuation, Orbital States and Non-conventional Superconductivity in Actinides Compounds S. Kambe1,2,3, H. Sakai1 and Y. Tokunaga1 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195 JAPAN. 2 SPSMS/INAC, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble FRANCE. 3 Université de Joseph Fourier, BP 53, 38041 Grenoble cedex 9 FRANCE. 1
ABSTRACT In d-wave unconventional superconductors, superconducting Cooper pairs are believed to be formed via magnetic fluctuations. In fact, the superconducting transition temperature Tc roughly correlates with the antiferromagnetic spin fluctuation energy in d-wave unconventional superconductors including high Tc cuprates. In addition to this correlation, the superconducting pairing symmetry and the magnetic anisotropy of the normal state are found empirically to be strongly correlated in f-electron unconventional superconductors having crystallographic symmetry lower than cubic. In antiferromagnetic systems, unconventional superconductivity appears with singlet (d-wave) pairing for cases of XY anisotropy. In contrast, in ferromagnetic systems, unconventional superconductivity with triplet (e.g. p-wave) pairing appears for cases of Ising anisotropy. In this report, the d-wave case is addressed, the origin of XY anisotropy is discussed in terms of the orbital character; and the angular momentum character jz for each piece of Fermi surfaces is determined. INTRODUCTION Unconventional superconducting (SC) states with an anisotropic superconducting gap are found to occur owing to strong electron correlations. In these superconductors, the SC pairing force is considered to be mediated by magnetic fluctuations. Especially, in d-wave unconventional superconductors near an antiferromagnetic instability, superconducting transition temperature Tc has a correlation with antiferromagnetic fluctuation energy. Figure 1 shows the correlation between the antiferromagnetic spin fluctuation energy * and Tc in antiferromagnetic unconventional superconductors. This reminds us of the relation between the Debye frequency ZD and Tc in the BCS formula: Tc=ZD exp( -1/DFV),
eq. 1
where DF is the density of states at the Fermi level, and V is the electron-phonon coupling constant. Of course, however, this formula cannot be applied for unconventional superconductivity in a simple way. In addition to the magnitude of the spin fluctuation energy and the type of magnetic correlations, there are two important factors: 1) Dimensionality of space, especially two dimensionality of a system. 2) Anisotropy of spin (pseudo-spin) i.e. Ising, XY, or isotropic (Heisenberg) type.
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In contrast to high Tc cuprates, which have extremely strong two dimensional character (e.g. resistivity has anisotropy of a103), f-electron heavy fermion systems are considered to be rather three dimensional, as anisotropy of resistivity and magnetic susceptibility reaches 2-3 at the maximum. It should be noted, however, there are different pieces of Fermi surface, and the dimensionality depend
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