Layer-by-layer transitions in liquid crystals

Abstract

Employing our free-standing fihn calc~rilnetric system, we have identified three remarkable layer-by-klycr transiticms in scver+,l liquid crystal COlllpottnds. Each of these Iransiticms can be well described by the simple pc~wer-law fc, rm: L= L,,t '. The layerirtg transitions Ibund near the smectic-A hexatic-B and smectic-A crystaI-B transitions are well characterized by the expcment v m I 3. This vahle is consistent with rnodels bz, sed on a van tier Waals-like dc, minant intermolecuhlr interaction. Another novel layer-by-k, yer thin,ling traqsition has been discovered abc, ve the bulk smectic-A isotropic transition of a perIluorim, tcd liquid cryst~,l compotmd. The value of the exponent obtained. v ~ 3 4. c~,nnot be easily explained using familiar models.KEY WORDS: free-standing film: heat capacity: I~tyer-b.v-layer transiticm: liquid crystal: smeclic-A hexalic-B transiticm.

Structural studies of the bond-orientational order and hexatic–smectic transition in liquid crystals of various compositions

“…Instead, a nearly symmetrical specific-heat anomaly showing divergent behavior has been found at the Sm-A -Hex-B transition in three layer [32] and two layer Hex-B films [11,48,60]. As a rule the crystal-hexatic phase transi-tion in LC hexatic films is of the first order, which preempt a continuous transition from the Hex-B to Sm-A phase [11]. Despite the considerable theoretical effort, there is no full understanding yet regarding the mechanism of these transitions.…”

“…For example, the theory predicts a wide bump in the specific heat above the hexatic-liquid transition point due to gradual unbinding of disclination pairs. Instead, a nearly symmetrical specific-heat anomaly showing divergent behavior has been found at the Sm-A -Hex-B transition in three layer [32] and two layer Hex-B films [11,48,60]. As a rule the crystal-hexatic phase transi-tion in LC hexatic films is of the first order, which preempt a continuous transition from the Hex-B to Sm-A phase [11].…”

“…The 2D hexatic phase is characterized by a sixfold quasi-long-range bondorientational (BO) order, while the positional order is short range. Phases with 2D hexatic order have been found in different systems, such as polymer colloids [3][4][5], electrons at the surface of helium [6], superconducting vortices [7,8] and smectic liquid crystals (LCs) [9][10][11]. In the latter case the hexatic phase was experimentally observed in a three-dimensional (3D) stack of the parallel molecular layers [9].…”

Spatially resolved x-ray studies of liquid crystals with strongly developed bond-orientational order

“…The two-dimensional (2D) hexatic phase shows a sixfold quasi-long range bond-orientational (BO) order, while the positional order is short range [3]. The hexatic phase is a general phenomenon that was observed in a number of systems of various physical nature, such as 2D colloids [4][5][6], electrons at the surface of helium [7], 2D superconducting vortexes [8,9] and, particularly, in liquid crystals [10][11][12][13].The hexatic phase was predicted by Halperin and Nelson [14] as an intermediate state in 2D crystal melting. According to their theory the hexatic phase arises as a consequence of the broken translational symmetry of a 2D crystal induced by dissociation of dislocation pairs.…”

“…In our experiment we have used the LC compound 3(10)OBC (n-propyl-4-n-decyloxybiphenyl-4-carboxylate) [13,26]. The hexatic-smectic phase transition was found at T ≈ 66.3 ± 0.1 • C and the material crystallizes below T≈54 • C. The films of 3(10)OBC sample were drawn across a small circular glass aperture of 2 mm in diameter inside the chamber at 10 • C above the temperature of the hexatic-smectic phase transition.…”