Devil's staircase and harmless staircase in the smectic-C*alpha phase in an electric field.

The unwinding of the short pitch helical smectic-C*alpha structure in an external electric field is studied within a discrete phenomenological model. It is found that the pitch increases quasicontinuously at low electric fields and is commensurate with the smectic layer thickness at any field. The sequence of stable structures recalls the once popular and then abandoned devil's staircase model. At larger fields the pitch grows discontinuously in steps of one smectic layer, forming a harmless staircase. Taking into account the achiral next-nearest-layer interactions the final transition to the unwound structure is found to be discontinuous.