Vicinity Of Phase Transition Point Research Articles

Correlations between electrocaloric effect and dielectric diffuseness of lead-free Ba(Ti1-Sn )O3 ceramics

This paper reported the correlations between electrocaloric effect (ECE) and dielectric diffuseness of Ba(Ti1-xSnx)O3 (abbreviated as BT-100xSn) ceramics. The BT-100xSn with pure perovskite structure has been synthesized by conventional solid-state reaction method. The temperature dependence of dielectric permittivity indicates that all the BT-100xSn ceramics are normal ferroelectric with diffused phase transition, and the dielectric diffuseness is strengthened as the Sn contents increase. The maximum ECE takes place in the vicinity of phase transition point, which is the characteristic of normal ferroelectrics. The BT-8Sn indicates the largest ∆T = 0.46 K and ∆T/∆E = 0.30 × 10−6 K m/V. The substitutions of Sn contents adjust the maximum ECE to different temperatures, and the good temperature-modulation of ECE is beneficial for the realization of ECE application. Considering the correlations between ECE and dielectric diffuseness, the experimental results demonstrate that the stronger dielectric diffuseness leads to the strengthened temperature stabilities of ECE but accompanied with decreased maximum ECE.

Influence of pressure and temperature on the attenuation of ultrasound in the nematic phase of the liquid crystal working medium of technical devices in the vicinity of phase transition point

The dependence of the attenuation coefficient of ultrasound in the nematic phase on pressure and temperature, which characterizes the parameters of electronic devices with a liquid-crystal working medium in the vicinity of phase transition point, is studied. Liquid crystals and their mixtures with a wide temperature range of the nematic phase were researched. The analysis is performed taking into account the normal relaxation process associated with conformational transitions of the end chains, and the critical relaxation process due to the nature of this phase transition. The nature of the dependence on the pressure and temperature of the normal, critical and classical components of the attenuation coefficient of ultrasound is established.

Raman opalescence and central peak scattering near phase transition point in crystals

Comparison of isotemperature and isofrequency temperature dependences was used for revealing the soft mode behavior near phase transition points in ferroelectric and ferro-structural phase transitions in crystals. Sharp increase of the Raman scattering spectral intensity maximum near phase transition points was found and classified as soft mode Raman opalescence effect. Such phenomenon was observed in ferroelectric crystals LiNbO3, LiTaO3, and Pb5Ge3O11, and crystalline quartz. Presence of the high intensive and narrow central peak in low frequency area of light scattering spectra in vicinity of phase transition point was explained on the base of two coupled oscillator model.

Topological phases of higher Chern numbers in Kitaev–Heisenberg ferromagnet with further-neighbor interactions

Emergence of multiple topological phases with a series of Chern numbers, 1, 1, 2, 2, 3 and 4, is found in a ferromagnetic Kitaev–Heisenberg-spin-anisotropic model on honeycomb lattice with next-next-nearest-neighbor interactions in the presence of an external magnetic field. Magnon Chern insulating dispersions of this two-band model are studied by using linear spin-wave theory formulated on the exact ferromagnetic ground state. Magnon edge states are obtained for the respective topological phases along with density of states. A topological phase diagram of this model is presented. Behavior of thermal Hall conductivity for those phases is studied. Sharp jumps of thermal hall conductivity is noted near the vicinity of phase transition points. Topological phases of the Kitaev spin-liquid compounds, -RuCl3, X2IrO3, X = (Na, Li) and CrY3, Y = (Cl, Br, I) are characterized based on this theoretical findings.

Nonequilibrium dynamics of a spin-3/2 Blume-Capel model with quenched random crystal field

The relaxation and complex magnetic susceptibility treatments of a spin-3/2 Blume-Capel model with quenched random crystal field on a two-dimensional square lattice are investigated by a method combining the statistical equilibrium theory and the thermodynamics of linear irreversible processes. Generalized force and flux are defined in irreversible thermodynamics limit. The kinetic equation for the magnetization is obtained by using linear response theory. Temperature and also crystal field dependencies of the relaxation time are obtained in the vicinity of phase transition points. We found that the relaxation time exhibits divergent treatment near the order–disorder phase transition point as well as near the isolated critical point whereas it displays cusp behavior near the first-order phase transition point. In addition, much effort has been devoted to investigation of complex magnetic susceptibility response of the system to changing applied field frequencies and it is observed that the considered disordered magnetic system exhibits unusual and interesting behavior. Furthermore, dynamical mean field critical exponents for the relaxation time and complex magnetic susceptibility are calculated in order to formulate the critical behavior of the system. Finally, a comparison of our observations with those of recently published studies is represented and it is shown that there exists a qualitatively good agreement.

Dynamic Nonlinear Response of Diglycine Nitrate Crystals in the Vicinity of Phase Transition Point

Nonlinear dielectric response of diglycine nitrate (DGN) crystal was investigated via measurements of linear, nonlinear dielectric susceptibilities and ferroelectric hysteresis loops. The behaviour of the third order susceptibility agrees with predictions of the thermodynamic theory of the continuous ferroelectric phase transitions. The second order susceptibility and hysteresis loops points at presence of the internal electric bias field. DGN has appeared to be electrically very “soft” within the ferroelectric phase, but it hardens at the phase transition point and remains hard above it.

Cross Correlations in Scaling Analyses of Phase Transitions

Thermal or finite-size scaling analyses of importance sampling Monte Carlo computer simulations in the vicinity of phase transition points often combine different estimates for the same quantity, such as a critical exponent, with the intent to reduce statistical fluctuations. We point out that the origin of such estimates in the same time series often results in pronounced cross correlations which are usually ignored even in high-precision studies, generically leading to significant underestimation of statistical fluctuations. We suggest to use a simple extension of the conventional analysis taking correlation effects into account, which leads to improved estimators with often substantially reduced statistical fluctuations at almost no extra cost in terms of computation time.

Band structure and optical spectra of ferroelectric triglycine sulphate

First-principal calculations of the band structure, density of electronic states, and dielectric functions of the triglycine sulphate crystal (TGS) together with the corresponding experimental dielectric functions in the range of 4–10 eV have been presented. Calculations have been performed in the frame of density functional theory using the CASTEP code. The experimental dielectric functions ε′(ω) and ε″(ω) have been obtained for different temperatures by ellipsometry using the synchrotron radiation source BESSY II. The theoretical dielectric functions ε′(ω) and ε″(ω) of TGS crystal agree satisfactorily with corresponding experimental dependencies. The character and magnitude of temperature changes of dielectric functions in the vicinity of phase transition point depend on the crystal orientation and photon energy. The greatest maximum-like temperature changes of dielectric functions observed at the phase transition point are detected for the photon energy ω ≈ 9.5 eV.

High-pressure study of ternary mercury chalcogenides: phase transitions, mechanical and electrical properties

In this paper investigations of electrical and mechanical properties of ternary mercury chalcogenides Hg1−xCdxSe, HgSe1−xSx, HgTe1−xSx were performed subjected to hydrostatic pressure treatment. The influence of cation and anion substitutions was tested on the electrical resistivity dependence of P and microhardness H values before and after applying pressure P up to 2.3 GPa. The variations of both the start pressure Pt of sphalerite B3 to cinnabar B9 structure transformation and also H as a function of chemical composition were obtained. A macroscopic bending of the samples found after reversible phase transformations suggests the increase of plasticity in the vicinity of phase transition point. The measurements performed have confirmed the sharp variation in both the electrical and volumetric properties of samples at the beginning of B3→B9 phase transition in contrary to slow structural transformations obtained by x-ray and synchrotron investigations.

Dynamical Property of Domain Walls of Triglycine Sulfate in the Vicinity of T c

The dynamical behavior of domain walls of triglycine sulfate was studied in the vicinity of phase transition point T c, by dielectric measurement over the frequency range from 1 MHz to 1 GHz. Above T c, the dielectric dispersion appears in the form of one Cole-Cole arc which is related to the critical slowing down. Below T c, the dielectric dispersion separates into two Cole-Cole arcs. One is ascribed to the contribution of critical slowing down as that above T c ; the other one is caused by domain wall motion. The dispersion is well described by two Debye-type relaxations. The phenomenon that characteristic frequency f D of domain wall motion changes steeply in the vicinity of T c, which was found in earlier studies, has been confirmed again. It has been identified in both the cooling process and the heating process. j l D shows the maximum at a temperature about 0.2°C lower than T c in the cooling process. The isothermal experiment reveals that j l D decreases monotonically with time and f D keeps unchanged after the initial small increase. Discussion about the factors that lead to the phenomenon is given.

Dielectric and Electrooptic Properties of the DMAMeS (Me = Al, Ga) Crystals in the Region of Low Temperature Phases

This paper presents the results of investigations of the dielectric, birefringent and electrooptic properties of NH 2 (CH 3 ) 2 Me(SO 4 ) 2 .6H 2 O (Me = Al, Ga) crystals. The data obtained are compared with the predictions of a phenomenological theory. In particular, the temperature behaviour of the dielectric susceptibility components and their anomalies in the vicinity of phase transition points were investigated in details. The specific behaviour of the studied parameters in the low temperature region was related to the coexistence of ferroelectric and antiferroelectric ordering of the dimethylammonium groups carrying a dipole moment.

Lanthanum oxide doping effect on dc electrical conductivity of ferroelectrics sodium vanadate and rubidium vanadate

The temperature dependence of dc electrical conductivity was measured by two-probe technique in the vicinity of phase transition point for ferroelectrics sodium vanadate and rubidium vanadate doped with different concentrations of La2O3. These materials show a sharp change in conductivity at their phase transition temperatures. The results were found to obey the conventional exponential law and the activation energies were calculated for ferroelectric and paraelectric states. It was found that activation energy in ferroelectric phase is smaller than in the paraelectric phase. The activation energy increases slowly with increase in doping concentration of La2O3 up to 0·1 mol%, however, it decreases with further increase in doping concentration, in both ferro and para states. The dc electrical conductivity below the Curie temperature is of mixed type (ionic-electronic) while it is electronic type above the Curie temperature.

Nematic Phase Transition in Systems of Polymer Chains with Mesogenic Groups

Abstract Nematic phase transition in melts of mesogenic macromolecules is studied. Each polymer chain is regarded as a sequence of rigid and flexible segments. Mean field approximation is used to calculate free energy of the system. Equilibrium temperature dependences of order parameters are obtained both for rigid and flexible fraction, and phase transition temperature is calculated. It is examined how lengths of different chain segments and values of anisotropic interaction between them can influence various properties of phase transition in the system. In a set of cases more often realized in experiments (when flexible segments are shorter or at least comparable with rigid ones) ordering of flexible component proves to be extremely weak in the vicinity of phase transition point. But there is a range of values of flexible spacer relative length and of anisotropic interaction between different segments when flexible fraction turns out to be even more ordered than rigid one in the vicinity of phase transi...

Effect of rare-earth ion on electrical conductivity of ferroelectrics KVO3and LiVO3

Abstract The dc steady state electrical conductivity was measured by two terminal methods in the vicinity of phase transition point for pure and doped ferroelectric KVO3 and LiVO3. An exponential law is obeyed in both ferroelectric and paraelectric regions, with a change in activation energy at Tc . It was observed that the activation energy increases slowly with increasing dopant concentration up to 0.5 mol% Nd2O3, however decreases for higher concentrations, in both ferro and para states. The activation energy in the paraelectric phase is greater than that in the ferroelectric phase.

The Hyperfine Field Distribution in Thin Films of Binary Diluted Alloys

AbstractIn amorphous ferromagnets spatially fluctuating exchange interaction give rise to a critical behaviour substantially different from crystalline magnets. Within the framework of a molecular field theoretical approach, a non‐local Landau‐Ginzburg equation is applied for thin amorphous film in the vicinity of phase transition point by means of a correlated molecular field, which can only vary on larger scale than the spin correlation length. Therefore, the manner how the structural disorder affects the critical behaviour changes with temperature. The calculation of the hyperfine field distribution measured in the Mössbauer experiment is done for the case of a binary alloy with simple cubic lattice. Numerical results for various surface parameters and temperatures are obtained.

Mössbauer lineshape in the case of diluted ferromagnetic alloy thin films

AbstractThe theory of Mössbauer line shape in fluctuating environment is applied to the case of thin film of a diluted alloy. The distribution of the spin autocorrelation time characterizing the line shape in the vicinity of phase transition point across the sample is calculated. It is shown that the change of concentration of magnetic atoms in diluted alloys leads to a variety of Mössbauer spectra at the same sample temperature. All the results obtained are calculated for the particular case of S = 1/2 and for a film with b.c.c. lattice, although another crystallographic lattices may be consider as well.

Non-linear physical characteristics of ferroelectric-semiconductor solid solutions in the vicinity of phase transition point

The main physical properties of ferroelectric-semiconductor solid solutions (FSSS) in the vicinity of phase transitions of first (PT I) and second order (PT II) have been analysed as function of admixture concentration, hydrostatic pressure and constant external electric field. An analysis has been carried out of the main features in the behavior of each particular parameter: polarization, dielectric permeability, nonlinearity, electro-caloric effect etc.

Scattering of surface polaritons in the vicinity of phase transition points

In the region of phase transitions the intensity of dielectric permeability fluctuations increases due to fluctuations of the order parameter. This can lead to the decrease in the length of propagation of surface polaritons and also it can influence the angular distribution of light reflected from the medium surface. In this paper the scattering of surface polaritons by the order parameter fluctuations is considered in conditions when the surface polariton is propagating along the surface of the metal coated with a thin dielectric film near the phase transition point in this film and for the case when the film is absent and the phase transition occurs in the whole substrate volume.