• S Chandrasekhar

      Articles written in Pramana – Journal of Physics

    • Magnetic and electric birefringence in the isotropic phase of nematic liquid crystals

      N V Madhusudana S Chandrasekhar

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      A theory is developed of the anomalous magnetic and electric birefringence in the isotropic phase of nematic liquid crystals taking into account orientational correlations between neighbouring molecules. Use is made of a modification of Bethe’s method due to Krieger and James, and the properties of the system are derived in terms of a single parameter, viz., the two-particle interaction constant. The expressions for the magnetic and electric birefringence are similar in form to those given by the phenomenological model of de Gennes. Theoretical curves forp-azoxyanisole reproduce the trends in the observed data. A calculation of the nematic-isotropic transition point confirms that this treatment is an improvement over the mean field approximation in describing pre-transition phenomena in the isotropic phase.

    • Flow of cholesteric liquid crystals—I: Flow along the helical axis

      U D Kini G S Ranganath S Chandrasekhar

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      It is shown that the essential features of Helfrich’s permeation model for flow along the helical axis of a cholesteric liquid crystal can be derived approximately on the basis of the Ericksen-Leslie theory.

    • Short range orientational order in nematic liquid crystals

      N V Madhusudana K L Savithramma S Chandrasekhar

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      The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.

    • Liquid crystals of disc-like molecules

      S Chandrasekhar B K Sadashiva K A Suresh

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      Thermotropic mesomorphism has been observed in pure compounds consisting of simple disc-like molecules, viz., benzene-hexa-n-alkanoates. Thermodynamic, optical and x-ray studies indicate that the mesophase is a highly ordered lamellar type of liquid crystal. Based on the x-ray data, a structure is proposed in which the discs are stacked one on top of the other in columns that constitute a hexagonal arrangement, but the spacing between the discs in each column is irregular. Thus the structure has translational periodicity in two dimensions and liquid-like disorder in the third.

    • Paramagnetic nematic liquid crystals

      S Chandrasekhar B K Sadashiva S Ramesha B S Srikanta

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      The paper reports new nematic liquid crystals that are paramagnetic.

    • Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

      V S K Balagurusamy S Krishna Prasad S Chandrasekhar Sandeep Kumar M Manickam C V Yelamaggad

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      We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ) to increase by a factor of 107 or more relative to that in undoped samples; σ attains a value of 10−2S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ/σ ≥ 1010, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor.

      We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.

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