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A ferroelectric superlattice with an antiferroelectric interfacial coupling is considered; the same model describes a bilayer with antiferroelectric coupling. By mapping minimum points in the Landau free energy expression and plotting them against the applied electric field, a triple hysteresis loop pattern is obtained. The loop patterns vary between typically ferroelectric and typically antiferroelectric depending on the layer thicknesses and the magnitude of the interfacial-coupling constant. This work suggests the possibility of designing multilayer elements for computer memories with four or more different storage states.

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We present the formalism within which all second and third order nonlinear susceptibility coefficients can be calculated from the Landau expansion for an isotropic paraelectric phase by means of the Landau-Khalatnikov dynamical equations. Both first-order and second-order phase transitions are considered. Detailed results are given for all the second-order coefficients in the ferroelectric phase and for third harmonic generation, intensity-dependent refractive index and d.c. Kerr effect in both paraelectric and ferroelectric phases.