Ultrasound Investigations of Orbital Quadrupolar Ordering in UPd_3

TL;DR

This study investigates the elastic and ultrasonic properties of UPd_3, revealing multiple phase transitions including orbital and quadrupolar ordering, and details symmetry changes and order parameters associated with these transitions.

Contribution

It provides detailed ultrasonic and elastic constant measurements of UPd_3, identifying multiple phase transitions and their symmetry changes, including orbital ordering at T_0.

Findings

Orbital ordering occurs at T_0=7.6K with a symmetry change from hexagonal to orthorhombic.

The phase transition at T_1 splits into two distinct transitions: second-order at 6.9K and first-order at 6.7K.

Multiple quadrupolar order parameters are specified for the identified phase transitions.

Abstract

For a high-quality single crystal of UPd_3 we present the relevant elastic constants and ultrasonic attenuation data. In addition to the magnetic phase transition at T_2=4.4 +/- 0.1K and the quadrupolar transition at T_1~6.8K, we find orbital ordering at T_0=7.6 +/- 0.1K concomitant with a symmetry change from hexagonal to orthorhombic. A striking feature is the splitting of the phase transition at T_1 into a second-order transition at T_{+1}=6.9 +/- 0.05K and a first-order transition at T_{-1}=6.7 +/- 0.05K. For the four phase transitions, the quadrupolar order parameters and the respective symmetry changes are specified.