Tricritical Exponents for the Isotropic-nematic Transition: An Experimental Verification

The gap exponent for the isotropic-nematic phase transition in MBBA (N-[$p$-methoxybenzylidine]-$p$-butylaniline) has been determined from measurements of the induced birefringence in very strong magnetic fields. The experimental value, $\Delta =1.26\mathrm{}\pm 0.10$, is consistent with the tricritical exponent, $\Delta =\frac{5}{4}$, but is clearly inconsistent with the exponent $\Delta =2\mathrm{}$ predicted by the mean-field theory. It is also shown that some of the observed effects are more than two orders of magnitude larger than the values predicted by the mean-field theory.