NMR observation of ferro-quadrupole order in PrTi₂Al₂₀

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. (author)