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Résumé:

1. Introduction
Spontaneously distorted nematic states have been theoretically predicted long ago for bent-shaped mesogens [1-3]. Recent experiments [4] confirmed the existence of the twist-bend nematic phase, NTB, with short-pitch heliconical precession of the director n. Another predicted state [3], the splay-bend nematic, NSB, with n oscillating in a plane, is still elusive. However, a recent birefringence study [5] suggested NSB structure of the defect wall separating two NTB domains with opposite chiralities.
2. Twist-bend to splay-bend transition under strong electric field
Here we investigate the NTB state of the bent-shaped dimer CB7CB under strong a.c. electric field, (E ~10 V/µm) parallel to the helix axis. This in-plane field is applied in a 2 µm cell using two ITO electrodes (Fig.1). Crossing the N- NTB transition under field, we observe in the interelectrode region slightly lower transition temperature and much stronger birefringence than that reported in the NTB phase [5], suggesting an electric-field induced NTB – NSB transition.
3. Conclusion
A strong electric field applied to the NTB phase induces a transition to the splay-bend nematic phase, instead to the usual uniform nematic N. We explain this in the framework of the elastic-instability model [3] of the NTB and NSB phases by taking into account the strong biaxiality of the bent-shape nematic.
References
[1]R. B. Meyer, in Molecular Fluids, edited by R. Balian and G. Weill (Gordon and Breach, New York 1976), pp. 273
[2]V. L. Lorman and B. Mettout, Phys. Rev. Lett. 82, 940 (1999).
[3]I. Dozov, EPL (Europhysics Letters) 56, 247 (2001).
[4]M. Cestari, S. Diez-Berart, D. A. Dunmur, A. Ferrarini, M. R. de la Fuente, D. J. B. Jackson, D. O. Lopez, G. R. Luckhurst, M. A. Perez-Jubindo, R. M. Richardson, J. Salud, B. A. Timimi and H. Zimmermann, Physical Review E 84, 031704 (2011).
[5]C. Meyer, G. R. Luckhurst and I. Dozov, Journal of Materials Chemistry C 3, 318 (2015).