Magnetic-Field Dependence of the Order Parameter in a Nematic Single Crystal

Poggi, Y.; Filippini, J.C.

doi:10.1103/physrevlett.39.150

Cited by 40 publications

(16 citation statements)

References 9 publications

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“…14 τ LdG increases dramatically with decreasing temperature and diverges as the temperature approaches T*. LdG theory describing the long time orientational dynamic of nematogens in the isotropic phase has been confirmed many times experimentally using techniques such as the optical Kerr effect, 1,2,7 depolarized light scattering, 3 dynamical light scattering, 15 magnetic 16 and electric birefringence, 17 and dielectric relaxation. 4,5 In contrast to the highly temperature-dependent long time scale dynamics, the short time scale (less than a few ns) power law decays have temperature-independent exponents and are not treated by LdG theory.…”

Section: Introductionmentioning

confidence: 95%

Ultrafast to Slow Orientational Dynamics of a Homeotropically Aligned Nematic Liquid Crystal

Wang

Fayer

2004

J. Phys. Chem. B

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The orientational dynamics of a homeotropically aligned nematic liquid crystal, 4′-pentyl-4-biphenylcarbonitrile (5-CB), is studied over more than six decades of time (500 fs to 2 µs) using optical heterodyne detected optical Kerr effect experiments. In contrast to the dynamics of nematogens in the isotropic phase, the data do not decay as a highly temperature-dependent exponential on the longest time scale, but rather, a temperatureindependent power law spanning more than two decades of time, the final power law, is observed. On short time scales (∼3 ps to ∼1 ns) another power law, the intermediate power law, is observed that is temperature dependent. The power law exponent of the correlation function associated with the intermediate power law displays a linear dependence on the change in the nematic order parameter with temperature. Between the intermediate power law and the final power law, there is a crossover region that displays an inflection point. The temperature-dependent orientational dynamics in the nematic phase are shown to be very different than those observed in the isotropic phase.

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Section: Introductionmentioning

confidence: 95%

Ultrafast to Slow Orientational Dynamics of a Homeotropically Aligned Nematic Liquid Crystal

Wang

Fayer

2004

J. Phys. Chem. B

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“…Magnetic-field effects in the nematic phase have been studied in the last two decades [14,15]. Scale fields to change Landau parameters at the NA transition are large; the mean-field effects of such large fields have been studied by Lelidis and Durand [16].…”

Section: Anand Yethiraj and John Bechhoefermentioning

confidence: 99%

Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic–Smectic-APhase Transition

Yethiraj

Bechhoefer

2000

Phys. Rev. Lett.

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We have conducted two quantitative tests of predictions based on the Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced first-order phase transitions. First, we explore the effect of an external magnetic field on the nematic-smectic-A transition in a liquid crystal. Second, we examine the dependence of the first-order discontinuity as a function of mixture concentration in pure 8CB and three 8CB-10CB mixtures. We find the first quantitative evidence for deviations from the HLM theory. PACS numbers: 64.70.Md, 61.30.Gd, 64.60.Fr One of the most important advances made in our understanding of continuous phase transitions has been the modification of critical exponents, due to thermal fluctuations, from the values predicted by mean-field theory [1]. But thermal fluctuations have another effect, one that is less well understood theoretically and studied only to a limited extent experimentally: when two order parameters (or an order parameter and a gauge field) are simultaneously present and interact with each other, the fluctuations of one may drive the phase transition of the other first order. In high-energy physics, for example, an analogous situation occurs in the Higgs mechanism [1]. In condensed-matter physics, over two decades ago, Halperin, Lubensky, and Ma (HLM) [2] predicted that this could occur in two settings: the normal-superconducting phase transition in type-1 superconductors and the nematic-smectic-A (NA) transition in liquid crystals. At the NA transition [3-6], the first fluctuating field is the nematic order parameter Q ij S ° 3n i n j 2 d ij ¢ 2, while the other order parameter is the two-component smectic-A order parameter c. In principle, a complete theory must account for thermal fluctuations in the nematic order parameter magnitude S, the nematic directorˆndirectorˆ directorˆn, and the smectic-A order parameter c. The de Gennes-McMillan mechanism [3] takes into account S fluctuations but is mean field inˆninˆ inˆn and c. The HLM theory, in addition, considersˆnconsidersˆ considersˆn fluctuations but is mean field in c. Deviations from the de Gennes-McMillan mechanism have been quantitatively established by Anisimov et al. [7] and Cladis et al. [8]. Although their conclusions are consistent with the HLM mechanism, the experiments are at their resolution limits and the full implications of the HLM theory remain to be tested. In this paper, we conduct the first two quantitative tests of the HLM theory, both leading to significant discrepancies from HLM. (i) Mukhopadhyay et al. [9] have made a detailed prediction for the external-field dependence of the HLM effect; we look for this field effect experimentally in the cyanobiphenyl liquid crystal 8CB, as well as in mixtures of 8CB and 10CB. (ii) The first-order discon-tinuity implied by the HLM theory (which can be characterized by the number t 0 T NA 2 T T , where T NA is the transition temperature and T is the spinodal point) should also be sensitive to the mixture concentration x in the 8CB-10CB system. Here, we make a quantitative study o...

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“…(8) predicts that the induced order is a modular function of the field strength, and experiments by Poggi and FilippiniX using magnetic fields have confirmed a linear relationship between magnetic field-induced birefringence and magnetic field strength for the nematic phases of two materials. In this paper we report measurements of the electric field-induced birefringence in 44'n-pentylcyanobiphenyl (5CB) and 44'n-octylcyanobiphenyl (8CB).…”

Section: Introductionmentioning

confidence: 81%

“…(8). The longest wave length fluctuation mode that can be sustained in sample of thickness D has a wave-vector qmin = 2 d D , and the short wavelength cut off qmax = 2 d l must be related to an average intermolecular distance 1.…”

Section: Introductionmentioning

confidence: 99%

Electric Field Induced Birefringence in Nematic Liquid Crystal Films: Evidence for Wall Quenching of Director Fluctuations

Dunmur

Waterworth

Palffy‐Muhoray

1985

Molecular Crystals and Liquid Crystals

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The effect of an electric field on the measured order parameter of a liquid crystal is considered. It is pointed out that an external field may influence the apparent degree of order by changing the microscopic order parameter. and by quenching director fluctuations. In nematic liquid crystals the latter dominates, but for smectic phases the effects of field-induced molecular order may he observable.Measurements are reported of the electric field-induced birefringence in nematic 44'-n-pentylcyanohiphenyl (5CB) and the nematic and smectic phases of 44'-n-octylcyanohiphenyi (8CB). The modular dependence of induced order on electric field strength predicted for nematics is experimentally confirmed by Fourier analysis of the electro-optic response. At low field strengths, and in the smectic phase, the induced order varies as the square of the field strength, and the dependence on cell thickness is not as predicted by continuum theory. By introducing two scaling parameters into the theory it is possible to fit quantitatively the experimental results for the nematic phase, and it is suggested that these parameters may describe the effect of the cell wails on the fluctuation mode spectrum in the liquid crystal. The birefringence induced in the smectic phase by an electric field is measured to he 100 times smaller than in the nematic phase, and this can be explained in terms of field-induced microscopic order.

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Magnetic-Field Dependence of the Order Parameter in a Nematic Single Crystal

Cited by 40 publications

References 9 publications

Ultrafast to Slow Orientational Dynamics of a Homeotropically Aligned Nematic Liquid Crystal

Ultrafast to Slow Orientational Dynamics of a Homeotropically Aligned Nematic Liquid Crystal

Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic–Smectic-APhase Transition

Electric Field Induced Birefringence in Nematic Liquid Crystal Films: Evidence for Wall Quenching of Director Fluctuations

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