V. A. Stephanovich

Domain-enhanced interlayer coupling in ferroelectric/paraelectric superlattices.

We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional, we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength Lambda and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.

The depolarization field effect on the thin ferroelectric films properties

Abstract The calculation of the spontaneous polarization (Ps), dielectric susceptibility (χ) and pyroelectric coefficient (Π) of the ferroelectric films has been performed in the phenomenological theory framework. Euler–Lagrange equation was solved analytically under the boundary conditions with different extrapolation lengths at two surfaces, respectively. The depolarization field contribution was taken into account in the model of short-circuited mono domain ferroelectric film, treated as perfect insulator. The detailed analysis of the aforementioned quantities’ space distribution and their average values in two cases with and without depolarization field was carried out. It was shown that the depolarization field shifts critical temperature to smaller value and critical thickness to bigger value in comparison to those obtained without accounting for the depolarization field, this shift being larger the smaller the extrapolation length values. This phenomenon is related to the different thickness dependence of the critical temperature, namely Tc−Tcl∼1/l or Tc−Tcl∼1/l2 with and without depolarization field contribution correspondingly. Meanwhile average values of Ps, χ and Π are similar in both cases. The depolarization field was shown to flatten Ps, χ and Π space distributions, which have the peculiarities otherwise (e.g. small maxima in χ and Π coordinate profiles near the film surfaces). It was shown that depolarization field influence in a short-circuited film could be neglected when the film thickness or the extrapolation lengths in the boundary conditions are larger than correlation length value.

Random fields and their influence on the phase transitions in disordered ferroelectrics

Calculations of the phase transition temperature and the critical concentration of electric dipoles which induce in the host crystal lattice the phase transition with the appearance of long-range order are carried out. These calculations are made by considering the additional static random electric fields owing to defects of other types: elastic dipoles and point electric charges. The distribution function of the joint action of the aforementioned random fields is also calculated. It is shown that the presence of the defects of other types inhibits phase transition so that, for the occurrence of long-range order, in this case larger concentrations of electric dipoles are necessary. The theoretical results obtained are found to be important for a description of the experimental situation in K1-xLixTaO3, PbMg13/Nb23/O3 and other disordered ferroelectrics.

Ferroelectric thin film properties—Depolarization field and renormalization of a “bulk” free energy coefficients

We suggest an approach to calculate the physical properties of ferroelectric thin films. Our approach takes into account the depolarization field and spatial inhomogeneity of the order parameter inherent to thin films. In or approach, we consider the thin film to be short circuited with the perfectly conducting electrodes and monodomain. The solution of the corresponding Euler–Lagrange equation by the direct variational method shows that the initial free-energy functional of a thin film can be reduced to some effective functional which does not include spatial derivatives of the order parameter. The coefficients of this functional, however, are renormalized as compared to those of the initial free-energy functional. We present the detailed calculation of the spontaneous polarization, pyrocoefficient, and dielectric susceptibility. We analyze in detail the spatial profile of the depolarization field with special attention to its properties near size-driven phase transition in a film.

Dynamic properties of relaxor ferroelectrics

We calculate the dynamic order parameter and dielectric susceptibility for the relaxor ferroelectrics like PbMg1/3Nb2/3O3, PbSc1/2Ta1/2O3 and Pb1−xLaxZryTi1−yO3. Calculations of temperature and frequency dependence of dielectric susceptibility show the existence of low- and high-temperature group of maxima obeyed to Vogel–Fulcher (VF) and Arrhenius laws, respectively. The VF law and non-Debye relaxation behavior as well as the stretched exponential law of susceptibility time decay were shown to be the consequence of random fields distribution which transforms single dipole Arrhenius and Debye laws into VF and non-Debye, stretched exponential laws, respectively. Comparison of observed and calculated characteristics of the relaxors is carried out.

Phenomenological theory of Bloch point relaxation

Abstract In the framework of a generalized magnetic relaxation phenomenological theory, which includes the exchange relaxational term, the Bloch point (BP) dissipative dynamics was investigated. It was shown, that consideration of exchange relaxation can explain the extremely small BP mobility value, observed in the experiment, contrary to conventional relaxation theory, based on relativistic terms.

Nature of ferroelectricity in nonperovskite semiconductors like ZnO:Li

We show that in nontypical ferroelectric substances (having nonperovskite crystalline structure and hence no soft phonon mode) such as ZnO:Li, Be, Mg, the ferroelectricity might appear due to indirect interaction of dipoles, formed by Li, Be, or Mg off-center impurities, via free charge carriers. Our estimations show that the typical semiconducting concentration of the carriers like 1017 cm−3 suffices for effect to occur. We have also shown that the properties of impurity-generated ferroelectricity depend on the difference in the ionic radii of the impurity and host lattice ion as well as on their concentrations. Namely, the growing amount of Li and Be promotes ferroelectricity, while the same for Mg inhibits it. Our calculations of spontaneous polarization and ferroelectric phase transition temperature in the above nontypical ferroelectrics as the functions of concentrations of impurity dipoles and carriers capture well the main peculiarities of all available experimental data.

Magnetic vortices The microscopic analogs of magnetic bubbles

Abstract In the magnet models, which provide the small-radius static topological solitons, the existence of magnetic vortices is considered. The latter vortices can be represented as microscopical magnetic bubbles with radius less than the Bloch wall thickness in the magnet under consideration. The vortices dynamics is investigated and proof of their stability is given.

The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner–Wohlfarth nanoparticles

The influence of interparticle interaction on the processes of magnetization reversal is considered for an ensemble of oriented Stoner–Wohlfarth nanoparticles. This is done through a solution of a kinetic equation describing the relaxation of the total magnetization to its equilibrium value in an effective mean field which includes a term proportional to the instantaneous value of the magnetization. It is shown that the interparticle interaction influences the temperature dependence of a coercive field. Under certain conditions, the presence of the interparticle interaction can lead to the formation of the so-called superferromagnetic state with the correlated directions of the magnetic moments of the particles. If the system is unable to come to the equilibrium during the time interval necessary to perform measurements, some measured quantities become dependent on the measurement time. It is shown that the blocking temperature Tb and the temperature dependence of coercive field at T

Quasiparticles and quantum phase transition in universal low-temperature properties of heavy-fermion metals

We demonstrate, that the main universal features of the low-temperature experimental H-T phase diagram of CeCoIn5 and other heavy-fermion metals can be well explained using Landau paradigm of quasiparticles. The main point of our theory is that the above quasiparticles form the so-called fermion-condensate state, achieved by a fermion condensation quantum phase transition (FCQPT). When a heavy-fermion liquid undergoes FCQPT, the fluctuations accompanying the above quantum critical point are strongly suppressed and cannot destroy the quasiparticles. The comparison of our theoretical results with experimental data on CeCoIn5 have shown that the electronic system of the above substance provides a unique opportunity to study the relationship between quasiparticles properties and non-Fermi–liquid behavior.