Reentrant low-temperature phase transition in an “orbital nematic”

The influence of an external magnetic field on the Jahn–Teller phase transition at Tc ≈ 14 K was studied in a KDy(MoO4)2 single crystal. At temperatures below Tc a significant change in the velocity and attenuation of the acoustic modes of the crystal was observed for the magnetic field oriented along the [110] direction. This behavior of the elastic characteristics occurs due to the destruction of the antiferrodistortive ordering, which is, in turn, coupled with the ordering of the quadrupole orbital moments, characteristic for an “orbital nematic.” The low-temperature region of the H–T phase diagram for this direction of the field was reconstructed. It was found that the magnetic field dependence of the critical temperature is nonmonotonic, i.e., the phase transition has the reentrant character. Theoretical description of the observed anomalies of the temperature and magnetic field behavior of the acoustic characteristics was given.