Search

It is shown that in the frame of the ferroelectric state model with electron-phonon coupling the temperature dependence of critical vibration and of order parameter can be explained by an electron-hole pair scattering on phonons. Such consideration does not permit phonon unharmonicity (independently from energy gap value) and considering the system instability and the system stabilization as a result of the same interaction. The connection of conduction-band and valence-band extreme mutual disposition with a phase transition type (ferroelectric, antiferroelectric, superstructural) is discussed.

A relation between absorption band frequencies connected with free carrier absorption the IR-region, mobility values, and phase transition temperatures in titanates, tantalates, and niobates is discovered. The existence of a single mechanism determining both phase transition and carrier kinetics in the conduction band is supposed. Es wird eine Beziehung zwischen den Frequenzen der Absorptionsbanden der freien Trager im infraroten Gebiet, der Grose der Tragerbeweglichkeiten und den Temperaturen der Phasenubergange in Titanaten, Niobaten und Tantalaten festgestellt. Das erlaubt zu vermuten. das ein einhcitlicher Mechanismus den Phasenubergang und die Kinetik der Trager im Leitungsband bestimmt.

A theory is developed of the bulk photovoltaic effect in ferroelectrics with strong interband interaction and of the coherent current state, connected with the interband absorption of electromagnetic field. The systematisation of possible nonlinear responses called by exciting of polar systems is suggested.

The behaviour of a ferroelectric in an intensive external electro-magnetic field is discussed. It is shown that, in fields with the amplitude E 0 > 105 V/cm, the phase transition temperature becomes higher, the static permittivity and Curie-Weiss constant increase. The optical absorption coefficient decrease changes to its increase. There is a threshold field, passing by which the system divides into layers of periodical structure of para-and ferroelectric phases. A difference for damping and refraction index was studied, while the field propagation was directed along and against the spontaneous polarization vector.

A number of phenomena are discussed which appear in the polar phase of nonequilibrium ferroelectric (systems without an inversion center): bulk photogalvanic effect, polar thermo-EMP and thermoconductivity and some others, in particular, these phenomena have anomalous and different values along and against to the spontaneous polarization velocity of distribution and damping of the electromagnetic and sound waves. All of them are connected with the specific character of the electronic quantum transitions in the system with off-diagonal long range order (ODLRO) and they can be united under one term “coherent effects”.

On an example of a two-band system with the electron-phonon interaction we show that a homogeneous stationary spontaneous current may arise when a phase transition in the presence of an external electromagnetic field occurs. The current is proportional to the imaginary part of the vector order parameter. In particular, in an illuminated homogeneous perfect ferroelectric this current appears simultaneously with the spontaneous polarization. The contribution of the spontaneous current into the photogalvanic (photovoltaic) effect is traced.

The light absorption and concentration dependence of the thermopower in BaTiO3 conducting single crystals were investigated. The absorption peak at Λ = 2 μm is interpreted as due to small polaron absorption. A quantitative comparison is made between theory and experiment. [Russian Text Ignored].

Splitting of the mobility tensor component at the transition of BaTiO3 crystal from the cubic state to the tetragonal state is explained within the limits of the small-radius polaron model by the change of the overlapping integral magnitude. A quantitative comparison is made between theory and experiment and for this purpose the data obtained by Berglund and Baer, and Murakami were reproduced. From the same point of view Wemple's data about the dependence of mobility on hydrostatic pressure and a great difference of mobilities in SrTiO3 are considered.

The certain connection between Curie temperature and parameters of electronic spectrum (such as the energy gap, drift mobility, the position of the maximum of the intraband absorption of light) is discovered. The results are being discussed within the limits of the “pure interband” theory of ferroelectricity.

The anisotropy and the temperature dependence of the frequency and absorption value were measured in conducting BaTiO3 for the line connected with the polaron absorption (δ ∽ 2 μm). It is shown that the results can be explained within the limits of the small-radius polaron model.