SOFT-MODE DIELECTRIC ANOMALY OF A CHIRAL SMECTIC-B LIQUID-CRYSTAL WITH SIGN REVERSAL IN THE ELECTROCLINIC RESPONSE

The static dielectric permittivity of the smectic-B* phase is studied as a function of temperature. The field-induced tilt angle, which is due to the electroclinic response, is measured as a function of temperature. The investigated phase precedes the chiral smectic-A* phase upon heating. In the A* phase, the soft-mode dielectric contribution increases with decreasing temperature but does not diverge at the transition, a behavior similar to that of a first-order phase transition. Below the A*-B* transition temperature T(c), the dielectric permittivity shows a sharp drop to a value considerably lower than that of the adjacent A* phase. On further cooling one would expect a monotonic decrease of the soft-mode contribution. However, in this compound, at a certain temperature T(rev), in the B* phase, the dielectric permittivity starts to increase again with decreasing temperature. This anomaly in the soft-mode dielectric response is attributed to a change of sign in the electroclinic coefficient, which has been observed by an electro-optic experiment. An attempt to fit the measured dielectric permittivity to a modified expression for the dielectric function that contains a temperature-dependent coupling coefficient is presented.