Cholesteric pitch-transitions induced by a magnetic field in a sample containing incomplete number of pitches
Ioannis Lelidis, Giovanni Barbero
, and Antonio Scarfone
1 Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, Zografos, Athens, 157 84, Greece
2 Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italia
3 ISC-CNR, Istituto dei Sistemi Complessi — Consiglio Nazionale delle Ricerche, c/o Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italia
We investigate the pitch transitions induced by an external bulk field in a Cholesteric Liquid Crystal slab of finite thickness ℓ that contains an incomplete number of π-twists. The analysis is performed for a magnetic field that is (i) perpendicular to the helical axis, and (ii) tilted with respect to one of the easy directions imposed by planar and rigid boundary conditions. For finite ℓ we obtain a cascade of transitions, where the bulk expels a half-pitch at a time with increasing field to avoid divergences in the elastic energy. The dependence of the threshold magnetic field inducing the expulsion on the easy axes twist angle δ is investigated for all the cascade of pitch transitions and in particular for the final one, corresponding to the Cholesteric-Nematic transition. In the ℓ → ∞ limit this dependence disappears and we reobtain the results of de Gennes for an infinite sample.