ELECTROSTRICTION TENSOR OF THE CUBIC BLUE PHASES - THE ROLE OF AMPLITUDES

The phenomenological theory of electrostriction of the cubic blue phas es is further developed by general lizing the model of rigid helices [ H. Stark and H.-R. Trebin, Phys. Rev. A 44, 2752 (1991)]. In the prese nt approach not only the wave vectors of the cubic structures are dist orted, but also the scalar amplitudes of the order parameter. By consi dering a full spectrum of the distortions exact formulas (within the L andau-deGennes approach) for the components of the electrostriction te nser are derived. Detailed results of the numerical calculations are c ompared with those obtained from the earlier model and with experiment al data. It is shown that the electrostriction coefficients are strong ly affected by the deformations of the amplitudes. Quantitatively, for low temperatures (or high chiralities) the contribution due to the de formed amplitudes exceeds that from the distortions of the wave vector s. The results are in good agreement with experimental data except for the region where the anomalous electrostriction of the blue phase I i s observed. This proves that a correct description of the anomalous el ectrostriction is beyond the standard Landau-deGennes theory of the bl ue phases as suggested in our recent publication [L. Longa, M. Zelazna , H.-R. Trebin, and J. Moscicki, Phys.Rev. E 53, 6067 (1996)].