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. 
Takanori Taniguchi, Makoto Yoshida, Hikaru Takeda, Masashi Takigawa,   Masaki Tsujimoto, Akito Sakai, Yosuke Matsumoto, and ...
cond-mat.str-el