Critical dynamics in LiCuVO₄

Without an external magnetic field the 1D spin chain compound LiCuVO₄ has a phase transition into a cycloidal spin ordered phase below T_N = 2.3 K where it simultaneously is antiferromagnetic and ferroelectric. The transition temperature of this phase transition can be lowered with increasing magnetic field. Ferroelectric phase transitions are of continuous type and are accompanied by a symmetry lowering that yields soft modes. Near the critical point the dynamics show a ''critical slowing down'' scenario. We studied the magnetic field and temperature dependence of ε(ν) in LiCuVO₄ in the vicinity of the multiferroic phase transition by means of broadband dielectric spectroscopy. While we find the expected relaxational behavior close to T_N, below 0.4 K a nearly gapless excitation is observed as was theoretically predicted in terms of chiral solitons.