L. M. Blinov

Two-dimensional ferroelectric films

Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films,, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 Å in diameter, in perovskite films 100 Å thick and in crystalline ferroelectric polymers as thin as 25 Å (refs 7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 Å (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

Thermotropic liquid crystalline polymers

SummaryLiquid crystalline comb-like polymers with spontaneous polarization are described.

Ferroelectric Langmuir-Blodgett films

Abstract The ferroelectric Langmuir-Blodgett films are prepared and investigated for the first time The films are prepared from the ferroelectric copolymer of vinylidene fluoride with trifluorethylene The films with the thickness off 150 A show the pyroelectric effect being measured along the polar C-axis perpendicular to the substrate surface, partial remnant polarization switching and ferroelectric phase transition of the first order, characterized by the temperature hysteresis

Piezoelectric and pyroelectric properties of ferroelectric Langmuir-Blodgett polymer films

The piezoelectric and pyroelectric responses of ferroelectric Langmuir–Blodgett polymer films are less than the largest values measured with bulk films of the same composition. The films of the crystalline copolymer poly(vinylidene fluoride trifluoroethylene) fabricated by the Langmuir–Blodgett technique are 30 ML thick (15 nm) and are highly crystalline and oriented with polarization perpendicular to the film. Both piezoelectric and pyroelectric measurements show reversible ferroelectric switching. The films are suitable for use in pyroelectric infrared imaging and in piezoelectric acoustic transducers.

Ferroelectric liquid crystals

Abstract The idea either to reveal or to construct a liquid ferroelectric was never abandoned by researchers since the discovery of ferroelectricity in solid crystals at the beginning of this century. In principle, nature does not forbid the existence of ferroelectric liquids. For instance, a centro-symmetric liquid can change its point symmetry from the K h , to C∝v , group with the formation of a uniform ferroelectric phase. A liquid without the inversion center can have a phase transition K → C∝, into the inhomogeneous, helical structure. In both these hypothetical cases the spontaneous polarization, P s , would play the role of the true order parameter. However, such liquid ferroelectric phases (proper ferroelectrics) have not been discovered as yet.

Novel switching phenomena in ferroelectric Langmuir-Blodgett films

We have established ferroelectric switching and observed a novel conductance switching phenomenon in ferroelectric polymer films fabricated by the Langmuir–Blodgett technique. The films consist of 10–30 monolayers (ML) of a copolymer of vinylidene fluoride (70%) and trifluoroethylene (30%), exhibiting a first‐order ferroelectric phase transition at 70 °C and nearly rectangular ferroelectric and conductance hysteresis at 24 °C. Ferroelectric switching is accompanied by switching of the conductance by three orders of magnitude. A model describing this switching phenomenon is proposed.

Location of charge transfer states in α-sexithienyl determined by the electroabsorption technique

Abstract Absorption and electroabsorption measurements were carried out at 300 and 4 K on evaporated thin films of α-sexithienyl, which is a chemically well-defined prototype of conjugated polymers (i.e. polythiophene). The results allowed for the determination of the energy for the optical charge pairs formation in this material. The polarizabilities of the excited Frenkel exciton states have also been estimated. The singlet exciton binding energy is in the order of 0.4 eV.

Domain mode—A novel relaxation process in ferroelectric liquid crystals

Abstract New relaxation processes are observed in ferroelectric liquid crystals with considerable high spontaneous polarization. These processes are attributed to the formation of ferroelectric domains in FLC cells. The domains have different characteristic periodicities and relaxation times in the bulk of FLC cell and near the surfaces of the electrodes. Characteristic relaxation processes are found in the dielectric response at frequencies VDB and VDS corresponding to both cases. The behaviour of dielectric strengths δϵv;DB and δϵDS versus d.c. field, spontaneous polarization and pitch of the helix is investigated.

Antiferroelectric behaviour of achiral mesogenic polymer mixtures

Abstract X-ray structure determination and pyroelectric and piezoelectric measurements have been carried out on mesogenic mixtures composed of achiral side chain polymers and their monomers. Certain mixtures show antiferroelectric polarization hysteresis loops in the mesophase which was shown to be a bilayered smectic C, while the two components of the mixtures taken alone show no antiferroelectricity. The results obtained on the polymer-monomer mixtures are interpreted in terms of the smectic C structure with alternating tilt which, from a symmetry viewpoint, is allowed to be antiferroelectric. The antiferroelectric mixtures, on being cooled to the glassy state under a d.c. electric field applied, reveal high pyroelectric coefficients with prospects for application as IR detectors.

Permanent polarization gratings in photosensitive Langmuir–Blodgett films

An investigation was carried out on thin permanent phase polarization gratings recorded in Langmuir–Blodgett films using two orthogonal circularly polarized Ar-ion laser beams. The films are composed of amphiphilic azo-dye molecules and manifest an extremely large value of photoinduced optical anisotropy, Δn≈0.36. The experimental results are in excellent accord with the theoretical model based on the Jones matrix representation of the polarization pattern. The gratings are stable at least, for half a year.