Ferro-quadrupolar ordering in PrTi$_2$Al$_{20}$

TL;DR

This study uses neutron scattering to reveal that PrTi$_2$Al$_{20}$ undergoes a ferro-quadrupolar ordering transition driven by the removal of ground-state degeneracy, marking it as a rare non-magnetic quadrupolar ordered Pr compound.

Contribution

It provides microscopic evidence for ferro-quadrupolar ordering in PrTi$_2$Al$_{20}$ using neutron scattering, clarifying the nature of its non-magnetic phase transition.

Findings

Crystalline-electric-field ground state is a non-magnetic $$ doublet.

Ferro-quadrupolar order confirmed under external magnetic field.

Transition involves cooperative removal of ground-state degeneracy.

Abstract

Origin of the non-magnetic phase transition in PrTi$_2$Al$_{20}$, reported earlier in the macroscopic study, has been asserted microscopically using elastic and inelastic neutron scattering techniques. It has been shown spectroscopically that the crystalline-electric-field ground state is a non-magnetic $\Gamma_3$ doublet, whereas the excited states are two triplets ($\Gamma_4$ and $\Gamma_5$) and a singlet ($\Gamma_1$). The diffraction experiment under external magnetic field shows that the non-magnetic transition is indeed ferro-quadrupolar ordering, which takes place as a consequence of cooperative removal of the ground-state-doublet degeneracy. It is therefore concluded that PrTi$_2$Al$_{20}$ is another rare example of Pr compounds exhibiting non-magnetic quadrupolar order.