ONSET OF ELECTROCONVECTION IN NEMATIC LIQUID-CRYSTALS WITH PARALLEL MAGNETIC-FIELD

We report experiments measuring the threshold electric field for elect rohydrodynamic instabilities in nematic liquid crystals as a function of magnetic field applied parallel to the electric field. At low magne tic fields, the threshold voltage decreases in quantitative agreement with theoretical predictions. However, at higher magnetic fields, a cr ossover is seen and the threshold voltage increases linearly with the magnetic field. In this region, the bifurcation from conduction to con vection is subcritical, with the discontinuity increasing linearly wit h magnetic field. The wavelength exhibited by the roll structure at on set also increases dramatically with the applied magnetic held.