Attenuation of finite amplitude sound near the ferroelectric phase transition

The temperature dependences of the effective sound attenuation near the phase transition in the triglycine sulfate crystal are calculated for powerful acoustic waves with a strain of an order of 10^?5. It is demonstrated that the generation of harmonics and their absorption can appreciably contribute to the measured absorption coefficient at the basic frequency owing to a critical increase in the third-order elastic moduli. The calculated data are compared with the experiment.     

Localized charge inhomogeneities and phase separation near a second-order phase transition

Localized charge inhomogeneities and phase separation are described in the framework of the phenomenological theory of phase transitions. It is shown that Coulomb interaction determines the charge distribution and the characteristic size of the emerging inhomogeneities. Phase separation associated with charge segregation becomes possible because of a high dielectric constant and a low excess charge density in the localization region. The phase diagram of the system is calculated, and estimates are obtained for the gain in energy associated with the emerging state. The role of Coulomb interaction is exposed, and corresponding estimates are given.     

Self consistent theory of fluctuations near D-dimensional superconducting phase transitions

Nonlinear fluctuation contributions in the vicinity of the transition temperature for D-dimensional superconductors (D = 0…3) are treated in self-consistent Hartree approximation to the Ginzburg-Landau free energy. The results concern specific heat, correlation length, and the effect of a magnetic field on the specific heat (for D = 2).     

Nonequilibrium phase transitions in coordinated biological motion: critical fluctuations

We report the results of experiments on biological motion demonstrating the presence of critical order parameter fluctuations as the system evolves from one coordinated state to another at a critical control parameter value. This is a key feature of nonequilibrium phase transitions.     

Smectic A order parameter measurements and critical scattering near the smectic A↔nematic phase transition in CBOOA

The temperature dependence of the order parameter for the smectic A phase of CBOOA is determined by measuring, with elastic coherent neutron scattering, the intensity of the Bragg reflection from the smectic layers. Within the experimental accuracy, the transition smectic A? nematic appears continuous in this system. However, a fit with (T_c-T)^2β yields β = 0.18 ± 0.01. Critical scattering is also observed in both phases. It can be described with an anisotropic Ornstein-Zernike law. The correlation length perpendicular to the smectic layers behaves like (T-T_c)^?v, with an exponent v = 0.47 ± 0.06. In contrast, the lateral extension of the layers seems to increase uncritically.     

Paramagnetic resonance study of fluctuations near phase transition in triglycine sulphate

The EPR line widths of the high and the low field resonances of Cr³⁺ in triglycine sulphate (TGS) show two distinct anomalous broadenings near T_c. One is associated with a change in the motional frequency of the hindered rotation of the NH₃ groups and the other is related to the fluctuation of the polarization. Just below T_c there is a two fold splitting of the resonances which has a$\text{(}\text{T}{}_{\mathrm{c}}\text{?}\text{T}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ temperature dependence indicating the local geometry about the Cr³⁺ ion is determined by mean field theory.     

Scattering of surface magnetic polaritons rom order-parameter fluctuations near second-order magnetic phase transitions

A study is reported of the scattering of surface magnetic polaritons in the vicinity of second-order spin-reorientational phase transitions, which is caused by spatial fluctuations of the order parameter. A four-sublattice antiferromagnet α-Fe₂O₃ is used as an example to derive expressions for the vacuum-scattered radiation near exchange magnetic-vibration frequencies. The scattering of surface magnetic polaritons is shown to be resonantly enhanced in characteristic regions of their spectrum. Estimates made for α-Fe₂O₃ indicate a possibility of experimental observation of surface magnetic-polariton scattering from fluctuations of the refractive index near second-order spin-reorientational phase transitions.     

Dynamic critical scattering near the smectic-A—nematic phase transition in CBOOA

High resolution neutron quasielastic scattering has been applied to a study on the critical dynamics near the smectic-A—nematic phase transition in CBOOA. A line narrowing is observed for a certain value of the momentum transfer. We interpret this narrowing as a critical slowing down of the smectic order parameter fluctuations near the transition temperature T_SN. The relaxation times vary between 10^-8 and 10^-7 sec for a temperature interval of 2 K below T_SN.     

Magnetocaloric effect near the first-order phase transitions in compounds of rare earth and transition metals

The results of the calculations and the experimental values of the magnetocaloric effect (MCE) in a group of compounds that are promising for use in magnetic coolers are reported. In the materials studied, MCE reaches the largest values near the magnetic first-order phase transitions and is mainly determined by the evolving latent heat of the transition. Contributions of different processes, occurring at the first-order phase transitions, to the observed MCE value have been separated on the basis of the thermodynamic model.     

Critical slowing of local fluctuations near the paraelectric-incommensurate phase transition in Rb2ZnCl4

A study is reported of the anomalous broadening of Mn²⁺ EPR lines on the high-temperature side of the paraelectric-incommensurate phase transition in Rb₂ZnCl₄ crystals. It is shown that the resonant-line broadening is inhomogeneous and due to the contribution of low-frequency fluctuations corresponding to the central peak in the elementary-excitation spectrum. The data obtained have permitted us to obtain the critical correlation-length exponent ν=0.64±0.02 corresponding to the 3d XY model of Heisenberg. Fiz. Tverd. Tela (St. Petersburg) 41, 122–125 (January 1999)     

Temperature anomalies in the dispersion and absorption of sound near phase transition points at low temperatures

We examined the attenuation and dispersion of sound during phase transitions with an overdamped soft mode at low temperatures. The obtained temperature and frequency dependences differ from results known at the high-temperature asymptote. The temperature anomalies are sharper. At low frequencies, the attenuation behaves like (T – T_c)^–5/2 for a critical phonon spectrum isotropic in k, and like (T–T_c)^–2 for uniaxial ferroelectrics. The presence of a temperature anomaly is characteristic also for high-frequency attenuation, behaving like (T–T_c)^–1/2 and ln(T–T_c)., respectively. We discuss briefly the results obtained from an experimental test.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 70–74, January, 1989.     

Attenuation of longitudinal ultrasonic wave near the diffuse phase transition in CdF2

The attenuation of a longitudinal ultrasonic wave propagating in the [111] direction in CdF₂ is studied as function of temperature from 300K to 1030K. An ultrasonic attenuation peak has been observed for the first time near 983K. This peak is used to define the diffuse transition temperature (T_c = 983K) in CdF₂ which is well below its melting temperature of 1372K. The Arrhenius activation energy of anion motion above T_c was obtained from the temperature dependence of the attenuation and the theory of the dynamics of the coupled crystalline-cage-charged-liquid fluctuations. The elastic constant, $\text{C}{}_{11}\text{+2}\text{C}{}_{12}\text{+4}\text{C}{}_{44}\text{)}\text{3}$, measured simulataneously with the ultrasonic attenuation displays a large decrease near 983K in addition to the nearly linear decrease in the elastic constant with temperature.     

Density fluctuations and specific heat near the critical point

The density dependence of the proton magnetic shielding constants in liquid methanol and ethanol has been accurately measured at a temperature of 29.2°C and at external pressures between 1 and 2500 bar. For comparison, a measurement in the vapor coexisting with the liquid at 29.2°C has also been made. For the carbonyl protons the results appear to be in qualitative agreement with previous measurements of the density dependence of the shielding constants in methane and ethylene. The results are discussed theoretically on basis of the Buckingham electric field model.     

Criticality and phase transition in stock-price fluctuations

We analyze the behavior of the U.S. S&P 500 index from 1984 to 1995, and characterize the non-Gaussian probability density functions (PDF) of the log returns. The temporal dependence of fat tails in the PDF of a ten-minute log return shows a gradual, systematic increase in the probability of the appearance of large increments on approaching black Monday in October 1987, reminiscent of parameter tuning towards criticality. On the occurrence of the black Monday crash, this culminates in an abrupt transition of the scale dependence of the non-Gaussian PDF towards scale-invariance characteristic of critical behavior. These facts suggest the need for revisiting the turbulent cascade paradigm recently proposed for modeling the underlying dynamics of the financial index, to account for time varying-phase transitionlike and scale invariant-critical-like behavior.     

Magnetoelastic quantum fluctuations and phase transitions in the iron superconductors

We examine the relevance of magnetoelastic coupling to describe the complex magnetic and structural behavior of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact with the distortions of the underlying square lattice. Going beyond the mean field, we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention: first, why iron telluride orders magnetically at a non-nesting wave vector (π/2,π/2) and not at the nesting wave vector (π,0) as in the iron arsenides, even though the nominal band structures of both these systems are similar, and second, why the (π,0) magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magnetoelastic interaction.     

Field theory of fluctuations during phase transitions

The evolution of the most probable amplitudes of the hydrodynamic mode is investigated in the self-consistent scheme. Fluctuations of the amplitude of the conjugate force are also investigated. Fiz. Tverd. Tela (St. Petersburg) 41, 1689–1692 (September 1999)     

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x _thr, 1, (2) lead to a superconducting state for x _thr, 1 < x < x _thr, 2, and (3) decay as x > x _thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.     

Behavior of sound velocities in the compounds La1−x SrxMnO3 near magnetic and structural phase transitions

The temperature dependences of the transverse V _t and longitudinal V _l sound velocities in single crystals of the perovskites La_1−x Sr_xMnO₃ (x=0.1, 0.175, 0.2, 0.25) in the temperature interval T=70–350 K are investigated by the resonance method. Anomalies — small minima and kinks in the temperature dependences V _l (T) and V _t (T) — are observed at the Curie points. A strong jumplike increase (by up to 30%) in both the longitudinal and transverse sound velocities, which attests to sizable hardening of the acoustic branches of the phonon spectrum, is observed near the temperatures of the structural transitions between the rhombohedral and orthorhombic phases (for x=0.175, 0.2, 0.25) and at the phase transition to the polaron-ordered state (for x=0.1). Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 141–146 (25 July 1998)