Microscopic Theory of $Γ_3$ Quadrupole Ordering in Pr Compounds on the Basis of a $j$-$j$ Coupling Scheme

Abstract

Toward the understanding of incommensurate $Γ_3$ quadrupole ordering in PrPb$_3$, we develop a microscopic theory of multipole ordering in $f^2$-electron systems from an itinerant picture on the basis of a $j$-$j$ coupling scheme. For this purpose, we introduce the $Γ_7$-$Γ_8$ Hubbard model on a simple cubic lattice with the effective interactions that induce local $Γ_3$ states. By evaluating multipole susceptibility in a random phase approximation, we find that the hybridization between $Γ_7$ and $Γ_8$ orbitals plays a key role in the emergence of $Γ_3$ quadrupole ordering. We also emphasize that $Γ_3$ quadrupole ordering can be understood from the concept of {\it multipole nesting}, in which the Fermi surface region with large $Γ_8$ orbital density should be nested on the area with a significant $Γ_7$ component when the positions of the Fermi surfaces are shifted by the ordering vector. This concept cab be intuitively understood from the fact that local $Γ_3$ doublets are mainly composed of two singlets between $Γ_8$ and $Γ_7$ orbitals. Finally, we discuss the possible relevance of the present theory to the experimental results of PrPb$_3$ and point out some future problems in this direction of research.