Torque curves and rotational hysteresis in a smectic C liquid crystal

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Abstract
Static torque curves in 4,4′‐terephthal‐bis (butylaniline) (TBBA) as a function of temperature are reported. In the smectic A phase (between 173 and 199 °C) a simple sinusoidal torque curve is observed as expected. In the smectic C phase (between 144 and 173 °C) the torque curves are no longer sinusoidal. The torque maximum shifts from π/4 measured from the easy axis to larger angles as the temperature is decreased. Also, rotational hysteresis appears and minor loops are observed. As in the smectic A phase the shape of the torque curves is essentially independent of field, with the torque scaling as the square of the magnetic field. The observed magnetic properties can be understood in terms of the multidomain model for the smectic C phase. Comparison of the calculation with the experimental data allows a determination of the temperature‐dependent cone angle from the torque curves. Assuming a rotational model, the calculated hysteresis is found to be larger than the experimental value.