# Microscopic Theory of ${\Gamma}_3$ Quadrupole Ordering in Pr Compounds on the Basis of a $j$-$j$ Coupling Scheme

## TL;DR

This paper develops a microscopic theory for incommensurate $$ quadrupole ordering in Pr-based compounds using a $j$-$j$ coupling scheme, highlighting the role of orbital hybridization and Fermi surface nesting.

## Contribution

It introduces a Hubbard model with effective interactions to explain $$ quadrupole ordering, emphasizing the importance of orbital hybridization and multipole nesting in Pr compounds.

## Key findings

- Hybridization between $$ and $$ orbitals is crucial for $$ quadrupole order.
- Fermi surface nesting with orbital density differences explains the ordering.
- The theory aligns with experimental observations in PrPb$_3$.

## Abstract

Toward the understanding of incommensurate $\Gamma_3$ 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 $\Gamma_7$-$\Gamma_8$ Hubbard model on a simple cubic lattice with the effective interactions that induce local $\Gamma_3$ states. By evaluating multipole susceptibility in a random phase approximation, we find that the hybridization between $\Gamma_7$ and $\Gamma_8$ orbitals plays a key role in the emergence of $\Gamma_3$ quadrupole ordering. We also emphasize that $\Gamma_3$ quadrupole ordering can be understood from the concept of {\it multipole nesting}, in which the Fermi surface region with large $\Gamma_8$ orbital density should be nested on the area with a significant $\Gamma_7$ 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 $\Gamma_3$ doublets are mainly composed of two singlets between $\Gamma_8$ and $\Gamma_7$ 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.