EFFECT OF A STRONG ELECTRIC-FIELD ON A NEMATOGEN - EVIDENCE FOR POLARSHORT-RANGE ORDER

We present experimental studies on the effect of strong electric field s on the nematogen p-cyanophenyl p-n-heptyl benzoate which has the str ongly polar cyano end group and a large positive dielectric anisotropy . We use a local temperature measurement to take into account heating effects and an electrical impedance analysis to determine both the die lectric constant (epsilon(parallel to)) and the resistance (R) of the sample. We also measure the higher harmonic responses of the medium. T he new results obtained in this study are: (i) a detailed temperature dependence of the terms which describe (a) the quenching of macroscopi c thermal fluctuations of the nematic director and (b) the enhancement of the orientational order parameter due to Kerr effect: (ii) clear e vidence for the critical divergence of susceptibility as reflected in the third harmonic signal, (iii) an unusual enhancement of the conduct ivity which shows a large peak just below the critical point T-c, (iv) a significant peak in the second harmonic signal at T-c and (v) evide nce for a field induced nematic-nematic transition well inside the nem atic range. Mie argue that the results (iii)-(v) indicate the presence of polar short range order in the medium and hence support a molecula r model in which such an order has been proposed.