  The effect of superposed dc and ac applied voltages on two types of spatially
periodic instabilities in nematic liquid crystals, flexoelectric domains (FD)
and electroconvection (EC), was studied. The onset characteristics, threshold
voltages and critical wave vectors, were determined. We found that in general
the superposition of driving with different time symmetries inhibits the
pattern forming mechanisms for FD and EC as well. As a consequence the onset
extends to much higher voltages than the individual dc or ac thresholds. A dc
bias induced reduction of the crossover frequency from the conductive to the
dielectric EC regimes and a peculiar transition between two types of
flexodomains with different wavelengths were detected. Direct measurements of
the change of the electrical conductivity and its anisotropy, induced by the
applied dc voltage component, showed that the dc bias substantially affects
both parameters. Taking into account the experimentally detected variations of
the conductivity in the linear stability analysis of the underlying
nemato-hydrodynamic equations, a qualitative agreement with the experimental
findings on the onset behavior of spatially periodic instabilities was
obtained.
