Periodic ground state in the nematic phase of DIO due to an intrinsic surface electric field

Nematic layers of DIO held in planar aligning cells are found to exist in a periodic ground state, involving polar and azimuthal director deviations, even in the absence of any external perturbing field. The stripe instability appears definitively in thin cells, over a few °C above the antiferroelectric smectic Z_A onset point, weakening progressively with increasing temperature. The pattern wave vector lies practically along the normal to the rubbing direction. In 90°-twist cells, two sets of stripes form with their wave vectors dependent on the substrate rubbing directions, evidencing the instability as a surface phenomenon. We propose for its origin a new mechanism that involves only the usual Frank elasticity and the surface electric field generated by adsorption of ions. The elastic energy of the proposed periodic director deformation field is zero, and it results from the gain in dielectric energy due to the surface electric field overcoming the cost of weakened anchoring energy.