On the observation of critical indices of primary and secondary order parameters using birefringence

Abstract

The forms of the birefringence change at phase transitions are discussed for four types of phase transition: (i) ferrodistortive, (ii) cubic systems with an n-dimensional soft mode and order parameter, (iii) cubic systems with a one-dimensional soft mode occurring at n q₀ points, and (iv) tetragonal magnetic systems (in which there is no crystallographic symmetry change at the phase transition). The birefringence is shown to vary as ( pi ) for case (i); as the secondary parameter ( pi _alpha pi _alpha )-(1/n) sigma _alpha ( pi _alpha pi _alpha ) for case (ii); as ( pi )² for case (iii); and as the energy for case (iv). The advantage of birefringence is that it is not sensitive to fluctuations, while the disadvantages are that one must work with a single-domain single crystal of a transparent substance.