Electric properties of (CH_{3}NH_{3})_{5}Bi_{2}Cl_{11} and (CH_{3}NH_{3})_{5}Bi_{2}Br_{11} single crystals in the direction of the crystallographic b-axis were studied. The samples tested were natural crystal seeds in the form of thin plates, whose crystallographic b-axis coincides with the normal to the plate. Measurements of the pyroelectric coefficient were performed near the high temperature phase transition and spontaneous polarization from the paraelectric phase down to helium temperature. The attempt of explanation of the anomalous electric properties of these crystals has been presented.
An attempt was made to investigate analytically the mechanical behaviour of a semi-infinite piezoelectric medium coated with a perfectly conducting thin film under the influence of time-dependent flow of heat at the finite end. The problem involves the interaction of three fields, viz. electrical, mechanical and thermal, and the solution was obtained with the aid of operational calculus. The resultant displacement consists of a time-dependent, as well as a time-independent part and the variation of displacements exhibits an arc of a parabola.
Spontaneous polarization of (CH_{3}NH_{3})_{5}Bi_{2}Br_{11} crystals has been measured and the ferroelectric hysteresis loop has been studied. The observed time changes in the polarization have been ascribed to the change in the internal bias field of the crystal. Measurement results have also been discussed for the lower phase transition (T_{II} = 77 K).
Influence of defects on ferroelectric properties of the (CH_{3}ΝΗ_{3})_{5}Bi_{2}Χ_{11} (Χ = Cl, Br) crystals in the vicinity of ferro-paraelectric phase transition was investigated. γ-irradiation of both crystals insignificantly shifts Tc towards higher temperature. The substitution of chlorine atoms by iodine ones in the (CH_{3}ΝΗ_{3})_{5}Bi_{2}Cl_{11} crystal drastically reduces the magnitude of the dielectric permittivity but does not influence on the temperature behaviour of the spontaneous polarization.
The mechanical response in an N-electrode piezo-quartz bar where two consecutive electrodes are separated by an unelectroded region has been studied under electrical and thermal excitations. The bar is subjected to a periodic polarization gradient and a constant flow of heat. The method of operational calculus and Green's function has been used to solve the problem. For time scale ranging from 0.1 to 1.0 s the response becomes a linear function of time and is of the order of 10^{-8} m.
Unit cell electric arrangement of piezoelectric crystals may be described by various multipole/octupole/dipole electric moments which corresponds to a three/two/one-dimensional intrinsic polarity respectively. Such a latent polar structure is totally self-compensated in piezoelectrics if they are mechanically free, but it is non-compensated in pyroelectrics and ferroelectrics. Uniform but anisotropic partial clamping destroys a total self-compensation of non-central crystal intrinsic polarity. This decompensation allows to observe a polar response that is a "dipole projection" ΔP_{i} from, broken by clamping, spatial polar arrangement of piezoelectrics.
Until now, relaxor ferroelectrics are considered as a class of disordered materials possessing peculiar structures and properties which are not yet generalized into a universal model explaining the significant amount of experimental data available. In this work, we demonstrate that one feature of relaxor ferroelectrics - the extraordinary dielectric response - is well-suited for application in electrocaloric refrigerators. We consider the electrocaloric effect with special attention to relaxor ferroelectrics, the dielectric response in the temperature region of interest, the efficiency and the figure of merit of relaxor ferroelectrics for electrocaloric application.
The "steady-state" non-linear optic polarization arising in lithium sulphate monohydrate crystals under the influence of the short pulse laser irradiation has been investigated in the temperature range of 95-305 K. The reversal of the sign of the components of the generalised non-linear susceptibility tensorχ has been found at about 105 K. The change of the photoresponse kinetics and a drastic temperature dependence of the componentsχ_{ijk} in the temperature range of 95-105 K, as well as the change of the spectral dependence of the photoresponse were observed.
The study of pyroelectric response of monocrystalline TGS cubic specimens to changes of temperature induced by linear and pulse heating of three mutually perpendicular pairs of cube sides demonstrated a complicated structure of signals. We attribute their forms to the activation of various hydrogen bonds between glycine G1, G2, and G3 molecules. In the case of pulse heating the pyroelectric signal is observed also in the paraelectric phase.
Dielectric properties of [N(CH_{3})_{4}]_{3}Bi_{2}X_{9} (X = Cl, Br) single crystals were measured in the frequency range 1 kHz - 1 MHz in the vicinity of structural phase transitions of first-order type at 152 K (Cl) and 183 K (Br). The pyroelectric effect was observed in [N(CH_{3})_{4}]_{3}Bi_{2}[N(CH_{3})_{4}]_{3}Bi_{2}Cl_{9} at structural phase transition (T_{c} = 152 K) with the ΔP_{s} of the order of 10^{-5} C m^{-2}. Polarized microscope observations show appearance of ferroelectric domain structure in [N(CH_{3})_{4}]_{3}Bi_{2}Br_{9} within the low temperature phase.
Two-dimensional hole and electron gases in wurtzite GaN/Al_{x}Ga_{1-x}N/GaN heterostructures areinduced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two-dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness, and doping. We have investigated the structural quality, the carrier concentration profiles, and electrical transport properties by a combination of high resolution X-ray diffraction, Hall effect, and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two-dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail.
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