Cotton-Mouton Effect of Alkyl- and Alkoxy-Cyanobiphenyls in Isotropic Phase

Abstract

The magnetic field induced birefringence Δn has been measured for nCB (n=5–8) and nOCB (n=3, 5–9). Sufficiently above the clearing temperature Tc, all the materials investigated, except 7OCB, satisfy the relation Δn/H2=λKC/(T-Tc*) over a wide temperature range of 20°C or more. Here λ=633 nm is the wavelength used, Tc* a temperature slightly lower than Tc, and KC a constant. The anomaly observed in 7OCB is impurity sensitive. As the alkyl chain length increases, KC drops appreciably; 3OCB has the largest value, KC=2.98×10-10°C cm-1 G-2, while 7CB has the smallest value, KC=0.99×10-10. Near Tc, on the other hand, Δn/H2 becomes larger than the above relation predicts. The two constants Tc-Tc* and KC are not enough to characterize the materials; (Δn/H2)T=Tc is also an important factor. Among the materials investigated, 5OCB has the largest value of (Δn/H2)T=Tc, namely 2.50×10-14 G-2.