Depolarization of light by critical fluctuations in RbAg4I5

The cubic-to-rhombohedral phase transition in the superionic conductor Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$ is characterized by strong birefringence in the rhombohedral phase. Precursive fluctuations in the cubic phase lead to complete depolarization of transmitted light at $T_c=208\mathrm{}$ K. We show that the depolarization increases exponentially as a power of reduced temperature. For the case of a linear relationship between the order parameter and the birefringence, as in Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$, the power of temperature in the exponential is the critical exponent $\gamma$ which governs the susceptibility-like response function. An experimental value $\gamma =1.25\mathrm{}\pm 0.1$ is obtained which agrees well with Ising-like values, and scales with other critical exponents. We believe this to be the first observation of a critical depolarization effect.