Fyzikální ústav Akademie věd ČR

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Selected results of department 12

Ferroelectric phase transition in tensile-stressed SrTiO3 thin films and heterostructures driven by an electric-field tunable soft phonon mode

Theoretically predicted ferroelectric phase induced by tensile stress was confirmed in SrTiO₃ epitaxial films and SrTiO₃/DyScO₃ heterostructures on DyScO₃ substrates, prepared in top world laboratories. It is driven near 270 K by a soft phonon mode in the THz range which in the whole measured temperature region of 20-300 K couples to an overdamped excitation of order-of-magnitude lower frequency (obviously of strong anharmonic origin) and is tunable by electric field. The full text >>

Magnetoelectric multiferroics

We have investigated the magnetoelectric effect in BiFeO₃ and observed rather a large change of permittivity with magnetic field at high temperatures, where the sample becomes partially conducting due to defects. However, in this case the magnetoelectric effect is not intrinsic, i.e. due to coupling of polarization and magnetization, but due to combination of magnetoresistance and Maxwell-Wagner polarization effect (see S. Kamba et al., Phys. Rev. B 75, 024403 (2007)). The full text >>

Piezoelectric response of nanotwinned BaTiO3

Computer simulations based on Ginzburg-Landau-Devonshire theory are used to investigate piezoelectric response of tetragonal BaTiO₃ crystals. We have shown that piezoelectric response of twinned BaTiO₃ increases with increasing density of 90° domain walls. A considerable enhancement of the longitudinal piezoelectric coefficient is predicted for domain sizes below 50 nm. The full text >>

A nematic-polar columnar phase sequence in new bent-shaped liquid crystals based on a 7-hydroxynaphtalene-2-carboxylic acid core

Series of 15 new liquid crystalline compounds have been synthesized and studied composed of non-symmetrical bent-shaped molecules based on a 7-hydroxynaphtalene-2-carboxylic acid, the central naphtalene core being laterally substituted by chlorine or the methyl group. Studies of textures in planar samples, calorimetric study, X-ray diffraction and dielectric measurements enabled to establish mesomorphic properties (ie. phase transitions and sequences of phases) of these compounds. The studied compounds exhibit the antiferroelectric B2 phase and relatively scarce polar columnar phases. The full text >>

First photoresponsive liquid-crystalline materials with small layer shrinkage at the transition to the ferroelectric phase

A new series of new multifunctional materials with the azo-group in the molecular core has been prepared, which exhibit ferroelectric properties and are photosensitive. With these materials (known as photoferroelectrics) the illumination by visible light can change their electric and optical properties. After the light is switched off their properties are restored to the original state. Such materials offer a series of applications in optoelectronics or photonics. The full text >>

Ultrafast photoconductivity in semiconductors and molecular systems

Nanostructured and organic semiconductors form the basis of a new generation of prospective materials for solar cell fabrication. The efficiency of solar cells crucially depends on the speed of long-range charge carrier transport and the understanding of the transport mechanisms in the mentioned materials is a key knowleged for its improvements. We used time-resolved terahertz spectroscopy as a contact-free probe of ultrafast carrier transport. The technique permits to access easily the complex conductivity in the terahertz spectral range with subpicosecond time resolution. The experiments were carried out in a concert with numerical simulations of the conductivity spectra and this allowed us to elucidate the transport mechanisms in a number of complex materials. The full text >>

Tunable metamaterials with negative permeability for terahertz spectral range

Metamaterials are artificially created composite periodic structures with a unit cell much smaller than the targeted wavelength of the radiation. These materials may exhibit electromagnetic properties not found in nature. It appear that by using a suitable combination of composite constituents it is possible to conceive for example an “invisibility cloak” or plates with a negative refractive index allowing one to overcome the diffraction limit in the optical imaging. However, these properties can be used in a narrow spectral range only restricted by the width of very sharp magnetic resonances (in permeability). For this reason we proposed and experimentally realized a metamaterial with a tunable range of negative effective permeability in the terahertz spectral range (0.2 – 0.36 THz) [H. Němec et al., Phys. Rev. B 79, 241108 (2009)]. The full text >>