Lattice Distortion and Octupole Ordering Model in CexLa1-xB6

TL;DR

This paper proposes a b5u-type octupole order as the cause of lattice distortion in CexLa1-xB6, linking it to specific crystalline electric field level splitting and quadrupole interactions, explaining observed phase transition features.

Contribution

It introduces a novel octupole ordering model with lattice coupling to explain phase IV in CexLa1-xB6, connecting lattice distortion and magnetic properties.

Findings

Octupole order induces lattice distortion along [111]

Model reproduces magnetization cusp at phase transition

Predicted internal magnetic field is smaller than observed

Abstract

Possible order parameters of the phase IV in CexLa1-xB6 are discussed with special attention to the lattice distortion recently observed. A \Gamma_{5u}-type octupole order with finite wave number is proposed as the origin of the distortion along the [111] direction. The \Gamma_8 crystalline electric field (CEF) level splits into three levels by a mean field with the \Gamma_{5u} symmetry. The ground and highest singlets have the same quadrupole moment, while the intermediate doublet has an opposite sign. It is shown that any collinear order of \Gamma_{5u}-type octupole moment accompanies the \Gamma_{5g}-type ferro-quadrupole order, and the coupling of the quadrupole moment with the lattice induces the distortion. The cusp in the magnetization at the phase transition is reproduced, but the internal magnetic field due to the octupole moment is smaller than the observed one by an order of magnitude.