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Previously electric-field-induced transitions from helical to planar structures in bulk liquid crystals were considered to take place with unwinding of helix. We show that the transition to the planar structure in polar smectic with frustrating interlayer interaction can occur without unwinding but through the formation of a periodic modulated structure with successive left-hand and right-hand sense of rotation in every half period. With field the amplitude of the left-right rotation decreases and a planar structure is formed. This mechanism of transition can take place in the short-pitch SmC(α)(*) phase formed as a result of frustrating interaction.

Recently Wang et al. [Phys. Rev. Lett. 104, 027801 (2010)] reported the discovery of a novel multilayer phase in polar liquid crystals. The phase was unambiguously assigned to a six-layer antiferroelectric structure (Sm-C(d6)(*)) by resonant x-ray diffraction. This discovery lead to essential progress in understanding the nature of polar phases. However, more recently, Chandani et al. [Liq. Cryst. 38, 663 (2011)] in the same material clearly identified the novel phase as a ferrielectric five-layer structure (Sm-C(d5)(*)) by the electric-field-induced birefringence. This contradiction seemed to be a mystery. In this paper we show that the two experiments are in agreement. Phenomenological Landau theory of the phase transitions shows that both phases (Sm-C(d6)(*) and Sm-C(d5)(*)) exist and transform into each other in a relatively low electric field.

We revisit a theoretical approach based on the discrete Landau model of polar smectic liquid crystals. Treating equilibrium structures on many length scales, we have analyzed different periodically modulated polar smectic phases. Besides already known smectic structures, we have obtained a number of other phases which are stable in a narrow range (that is why the phases can be termed as microphases) of model parameters and thermodynamic conditions. The sequence of microphases represents a so-called "harmless staircase" of structures with oscillating periods. We anticipate that the range of stabilities for various microphases can be extended (and therefore the microphases can be easier to detect experimentally) by applying external electric fields or/and investigating freestanding smectic films.

Recently Wang [Phys. Rev. Lett. 104, 027801 (2010)] discovered a novel smectic-C* liquid-crystal commensurate structure with six-layer period. Challenged by this discovery, we show that the observed novel structure and the unusual sequence of polar phases can be explained in the framework of the discrete Landau model of phase transitions with a two-component order parameter. Peculiarities of the six-layer phase and the influence of short-range and long-range interactions on the formation of different phases and phase sequences are discussed.

Using the discrete phenomenological model with interlayer anticlinic and chiral interaction, the unwinding of the antiferroelectric helix in external electric field was studied. The dependence of the helical pitch on the electric field differs essentially in antiferroelectric and ferroelectric phases. It was found that in antiferroelectric the critical electric field E h of the transition into the unwound structure is proportional to the value of chiral interaction and does not depend on the anticlinic interaction between nearest layers. This behavior differs significantly from the case of ferroelectric in which E h increases as the square of the chiral interaction and is inversely related to the synclinic interaction between layers. Peculiarities of the unwinding process and the dependence of the critical field E h on model parameters are discussed. Based on our calculations, we propose an analytical equation for the critical unwinding field in the antiferroelectric.

High-quality films of copper phthalocyanine (CuPc) prepared in situ were used as a model to characterize unoccupied states of organic molecular semiconductors. We demonstrate that a combination of high-resolution near-edge x-ray absorption together with first-principles calculations constitutes a reliable tool for the detection and identification of particular molecular orbitals.

The transformation of a chiral antiferroelectric liquid crystal in an electric field was calculated. Minimization of the free energy was performed with respect to both the phase and the modulus of the two-component order parameter. Competition between the electric field, which favors a planar structure, and chirality and the anticlinic ordering leads to frustration and sequential formation of a distorted helix, a soliton state, transition to an unwound distorted antiferroelectric, and finally the planar synclinic state. In the soliton and distorted antiferroelectric states an anomalous electroclinic effect (the decrease of the tilt angle and layer polarization) was found. The antiferroelectric soliton can be removed by a large electric field only via the transition to the synclinic state.

The electronic structure of the organic semiconductor copper-phthalocyanine (CuPc) has been determined by a combination of conventional and resonant photoemission, near-edge x-ray absorption, as well as by the first-principles calculations. The experimentally obtained electronic valence band structure of CuPc is in very good agreement with the calculated density of states results, allowing the derivation of detailed site specific information.

We report an inelastic light scattering study of the cyclotron spin-flip mode in the two-dimensional electron system at filling nu=1. The energy of this mode can serve as a probe of the many-body exchange interaction on short length scales. Its magnetic field dependence is compared with predictions based on Hartree-Fock theory. They agree well when including the nonzero width of the electron system. From the measured energies, the exchange enhanced g factor is extracted. It diverges at small fields and differs largely from g factors obtained via transport activation studies.

A discrete phenomenological model of antiferroelectric liquid crystals is used to study the structures and phase transitions in bulk samples and thin films. An important ingredient of our investigations is minimization of the free energy with respect to the phase and modulus of the order parameter. A simple version of the free energy, which contains only the nearest-neighbor and the next-nearest-neighbor layer interactions gives a complete phase diagram with all the observed smectic-C* (SmC*) variant phases. In thin free-standing films, surface ordering may lead to suppression of the bulk SmC(*)(alpha) helix and to formation of planar structures. Transitions between these structures are accompanied by the 90 degrees reorientation of the polarization direction. We also discuss the influence of chirality on subphase structures.