We modify the Sakai-Sugimoto model of chiral symmetry breaking to take into account the open string tachyon which stretches between the flavour D8-branes and D8-branes. There are several reasons of consistency for doing this: (i) Even if it might be reasonable to ignore the tachyon in the ultraviolet where the flavour branes and antibranes are well separated and the tachyon is small, it is likely to condense and acquire large values in the infrared where the branes meet. This takes the system far away from the perturbatively stable minimum of the Sakai-Sugimoto model; (ii) The bifundamental coupling of the tachyon to fermions of opposite chirality makes it a suitable candidate for the quark mass and chiral condensate parameters. We show that the modified Sakai-Sugimoto model with the tachyon present has a classical solution satisfying all the desired consistency properties. In this solution chiral symmetry breaking coincides with tachyon condensation. We identify the parameters corresponding to the quark mass and the chiral condensate and also briefly discuss the mesonic spectra.

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The effects of bias voltages on the field-induced nematic-cholesteric relaxation are investigated both for samples with perpendicular boundaries and for ones with parallel boundaries, and are discussed in relation to the hysteresis behavior of texture. When the applied voltage is reduced from a value above the threshold to a bias voltage below the threshold a relaxation from the field-induced nematic to a Grandjean-like cholesteric state occurs, the relaxation time of which increases as the bias voltage approaches a certain value Vc↓; the ratio of Vc↓ to the threshold voltage Vc for cholesteric-to-nematic transition is about 0.5 irrespective of the boundary conditions. When the bias exceeds Vc↓, the transformation shows a hysteresis behavior such that the nematic state is kept for a long time until gradually replaced by focal conic domains nucleating from some local defect points; the boundary condition is found to play no essential role in the hysteresis behavior. An interpretation of the results is presented.