Shyamal Kumar Kundu

Galgotias University

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B. K. Chaudhuri

Indian Association for the Cultivation of Science

Shin Yagihara

Tokai University

Atsushi Yoshizawa

Hirosaki University

Y. Aoki

Japan Aerospace Exploration Agency

Laure Catala

Institut de Chimie Moléculaire et des Matériaux d'Orsay

Stéphane Méry

Centre National de la Recherche Scientifique

Cooperative Institutions

Tokai University

National Institute of Advanced Industrial Science and Technology

Tokyo University of Science

Centre National de la Recherche Scientifique

Sathyabama Institute of Science and Technology

University of Pisa

The University of Tokyo

Kyushu University

Jodhpur National University

Université du Québec à Trois-Rivières

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Dielectric spectroscopy of a smectic liquid crystal

Liquid Crystals (2007)

Shyamal Kumar Kundu Shin Yagihara Atsushi Yoshizawa

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.

Soft modes

10.1080/02678290701541579

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Soft mode and related behaviour in the SmA and SmC* phases of a ferroelectric liquid crystalline polymer by dielectric spectroscopy

Liquid Crystals (2002)

Shyamal Kumar Kundu B. K. Chaudhuri Laure Catala Stéphane Méry

Anomalous dielectric relaxation behaviour is observed in the ferroelectric liquid crystalline polymer (viz. ferroelectric copolysiloxane (R)-COPS 11-10) around the ferroelectric SmC* to paraelectric SmA phase transition. Measurements have been performed on sample of thickness ∼10 μm in indium-tin-oxide coated cell in the frequency range 10 Hz to 13 MHz. With increase of temperature, a gradual shift of the soft mode frequency towards the higher frequency side was observed, while a decrease in the relaxation strength was seen with the corresponding increase in temperature. The shifts of the soft modes in the SmC* and SmA phase are considered to be due to change in the viscosity of the polymer, as an increase in viscosity increases fluctuations of the coupling between the dipoles in the network even far from the paraelectric-ferroelectric phase transition. Application of a bias field causes a shift of the critical frequency towards the higher frequency side, while the dielectric strength (δε) decreases under the bias field. The Cole-Cole fitting parameters obtained from the best fit of the dielectric constant data are found to be consistent with other similar materials. Another relaxation mode (molecular mode) was also observed which comes into play in both the smectic phases (SmC% and SmA) and contributes to the dielectric permittivity.

Soft modes

10.1080/02678290210133123

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Observation of scaling behaviour of dielectric relaxation in ferroelectric liquid crystals

Liquid Crystals (2005)

Shyamal Kumar Kundu B. K. Chaudhuri

The temperature (25–120°C) and frequency (10 Hz–1 MHz) dependent dielectric relaxation spectra of different ferroelectric liquid crystals are found to show interesting scaling behaviour. Considering relaxation frequency (νmax) as the scaling frequency, the imaginary part of the dielectric constants (ε″) of the ferroelectric liquid crystals are found to follow the universal scaling formula , i.e. when (ε″/ε″max) is plotted as a function of (f/νmax), the scaling behaviour of a ferroelectric liquid crystal can be represented by a single master curve irrespective of film thickness, bias field and temperature; ε″max is the maximum value of ε″ at the frequency νmax and m is a parameter ∼1.

10.1080/02678290412331320601

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Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric–ferrielectric transition

Liquid Crystals (2004)

Shyamal Kumar Kundu Y. Aoki B. K. Chaudhuri

We report the dielectric relaxation behaviour in the antiferroelectric SmCA* and ferrielectric SmCγ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmCA* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmCγ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.

Antiferroelectricity

Goldstone

10.1080/02678290410001666057

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Citations (3)