Linear Birefringence Behavior of [NH₃C₃H₇]₂MnCl₄ near the Normal-Incommensurate Phase Transitions

Linear birefringence behavior of [NH₃C₃H₇]₂MnCl₄ was measured near normal-incommensurate phase transitions, i.e., β-γ and δ-∈ transitions. Linear birefringence behaved as Δn_ab ∼ |t|^0.74±0.02 in the critical region of the two phase transitions. These results imply that two normal-incommensurate phase transitions belong to the same universality class of the 3-d XY model. In addition, linear birefringence behavior in the incommensurate γ phase is also discussed while comparing it to the previous theoretical description based on classical Landau theory.