NMR Observation of Ferro-quadrupole Order in PrTi2Al20

TL;DR

This study uses 27Al-NMR to detect ferro-quadrupole order in PrTi2Al20, revealing symmetry breaking and phase transition characteristics associated with quadrupole degrees of freedom in a nonmagnetic doublet ground state.

Contribution

First microscopic evidence of ferro-quadrupole order in PrTi2Al20 via NMR, demonstrating symmetry breaking and phase transition behavior.

Findings

NMR line splitting indicates ferro-quadrupole order

Symmetry breaking observed under magnetic field along <111>

Continuous line splitting suggests a first-order transition

Abstract

We report the results of 27Al-NMR measurements on a single crystal of PrTi2Al20. This compound shows a phase transition near 2 K associated with the quadrupole degree of freedom of Pr ions, for which the ground state of the crystalline electric field is a nonmagnetic doublet. When a magnetic field is applied along the <111> direction, the NMR lines split upon entering the low-temperature phase, indicating the breaking of the three fold rotation symmetry due to the field-induced magnetic dipole perpendicular to the field. This provides microscopic evidence for a ferro order of the O20 quadrupole. Although a first-order transition is expected theoretically, the line splitting evolves continuously within the experimental resolution.