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.     

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).     

Critical surface wave velocity near phase transitions

The temperature dependent Rayleigh wave velocity is discussed for crystals with a surface layer of depth equal to the correlation length, ξ, with special reference to a critical region near phase transitions. For finite qξ the Rayleigh wave velocity reflects the specific critical properties of the surface layer. Under such conditions the temperature dependence of Rayleigh waves cannot be predicted on the temperature dependence of constants. A phenomenological analysis is made for qξ ≦? 1. Experimental results on SrTiO₃ show bulk-dominated critical Rayleigh wave velocities due to the short correlation length in this material.     

Observation of two isotropic-nematic phase transitions near a surface

Using specified conditions, we succeeded in observing the isotropic-nematic (Iso-N) liquid crystal phase transition at surfaces followed by that in bulk for the first time. An additional heat anomaly peak was found at a higher temperature side of a main phase transition peak using highly sensitive differential scanning calorimetry (HS-DSC). The peak is pronounced particularly in the cooling process, since the transition starts at surfaces on cooling. The temperature dependence of retardation allows us to safely conclude that the higher temperature peak that appeared in HS-DSC is attributed to the Iso-N transition at surfaces. These measurements also indicate that the surface transition is of first order. These behaviors were theoretically explained by generalized Maier-Saupe theory.     

Raman scattering in Europium chalcogenides near magnetic phase transitions

The inelastic light scattering spectra of the europium chalcogenides are analyzed in terms of a one phonon-one spin Raman process. Calculations are presented for the temperature dependence of the integrated intensity of the “spin-disorder” spectrum of EuS and compared with experiment. The observed temperature dependence of the Raman spectrum of EuSe in the two-sublattice antiferromagnetic phase is presented and examined in terms of the one phonon-one spin mechanism. Experiments are suggested in which the Raman spectra can be used to measure the magnetic field dependence of the order parameter exponent β in EuTe.     

Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La_2/3(Ca_1−xSr_x)_1/3MnO₃. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.     

Optical-absorption anomalies near magnetic phase transitions in copper oxide

The optical absorption coefficient in the region of a wide charge-transfer band in copper oxide CuO (ℏω=3.19 eV) exhibits sharp and anomalously large (up to 8% (!)) changes near two magnetic phase transition temperatures . It is shown that the wide fundamental absorption bands can be used for optical detection of magnetic phase transitions, the parameters of long-and short-range magnetic order, and the effects of a magnetic-impurity state. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 251–255 (25 February 1996)     

Fracture and second-order phase transitions

Using the global fiber bundle model as a tractable scheme of progressive fracture in heterogeneous materials, we define the branching ratio in avalanches as a suitable order parameter to clarify the order of the phase transition occurring at the collapse of the system. The model is analyzed using a probabilistic approach suited to smooth fluctuations. The branching ratio shows a behavior analogous to the magnetization in known magnetic systems with second-order phase transitions. We obtain a universal critical exponent beta approximately = 0.5 independent of the probability distribution used to assign the strengths of individual fibers.     

The effect of spin fluctuations on phase transitions in magnetic systems

In this paper we study the problem of admitting an exact solution of the effect of spin fluctuations on phase transition when modeling a multiferroic superconducting system in a strong magnetic field. New results are obtained for phase portraits of the system of equations for amplitudes of spin density and temperature waves. The possibility is justified for the transition of the system to a phase in which superconductivity and antiferromagnetic ordering coexist, particularly via slowly fluctuating spin-density waves.     

Entropy and magnetocaloric effects in ferromagnets undergoing first- and second-order magnetic phase transitions

The exchange striction model is invoked to derive an expression for the entropy of ferromagnetic materials undergoing first- and second-order magnetic phase transitions. The magnetocaloric and barocaloric effects are calculated for the ferromagnet La(Fe_0.88Si_0.12)₁₃ undergoing a first-order phase transition. The calculated results are in fair agreement with experimental data. The ferromagnet La(Fe_0.88Si_0.12)₁₃ is used as an example to predict the changes in magnetic and magnetocaloric properties associated with gradual increase in the magnetoelastic coupling constant (i.e., with passage from first- to second-order magnetic transition region). It is shown that stronger magnetoelastic coupling leads to stronger magnetocaloric effects and changes their dependence on magnetic field and pressure. Expressions are obtained for the maximum field- and pressure-induced entropy changes. An analysis is presented of the mechanism responsible for the increase in magnetocaloric and barocaloric effects associated with change from the second- to first-order magnetic phase transition.     

Scattering of surface plasmon polaritons by a nanoparticle with the inclusion of the magnetic dipole contribution

Using the method of the tensor Green’s function of the wave equation, the differential and total cross sections for the scattering of surface optical electromagnetic waves (surface plasmon polaritons) by a spherical nanoparticle into surface plasmon polaritons and light have been obtained with the inclusion of the magnetic dipole contribution. Using the example of noble metal systems, it has been shown that the magnetic dipole contribution may significantly affect the angular dependence of the differential cross sections and increase their anisotropy as the plasmon wavelength increases.     

Attenuation of sound by inhomogeneities in the transition parameter and critical fluctuations near phase transitions

The temperature singularities in the critical fluctuations and the sound attenuation resulting from a nonzero inhomogeneous distribution of the transition parameter have been investigated. This effect is related to a rearrangement of the crystal lattice near an impurity center. It is shown that the corresponding contributions to the kinetic coefficients which have been studied are comparable to the correlation corrections but have a different dependence on the quantity (T – T_c), where T is the absolute temperature and T_c is the Curie temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 60–65, October, 1978.     

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)     

Nonlinear surface magnetic polaritons in a nonuniform magnetic field

High-frequency surface magnetic polaritons of finite amplitude propagating along the interface between a ferrite and a nonlinear insulator in a weakly nonuniform, shaft-shaped external magnetic field are investigated theoretically. The analysis is based on employment of the variational method together with bilinear relations having the form of Lorentz’s lemma. It is shown that the wave dispersion and the transverse profile of a wave along the field nonuniformity depend significantly on the amplitude of the wave. Zh. Tekh. Fiz. 68, 92–95 (September 1998)     

Magnetoelastic effects in rare earth metals and alloys near magnetic phase transitions

Magnetoelastocaloric effects in rare earth metals and alloys have been thermodynamically investigated. It is found that these magnets, along with the ordinary elastocaloric effect, exhibit a giant magnetoelastocaloric effect near the magnetic ordering temperatures, which is proportional to the temperature derivative of spontaneous magnetostriction.     

Microkinetics of the first- and second-order phase transitions

A model of the phase transition in a lattice of interacting nodes, in which each node is a statistical system with internal structure, is introduced. Configuration entropy of microscopic states of the node is defined as a basic parameter of the model. In the frame of the model the first- and second-order phase transitions are considered in details. The distinction between them on the microscopic level is analyzed. Phase diagrams have been calculated in the mean-field approximation. Changes of the phase diagrams and modifications of phase transitions under external pressure and irradiation are investigated in the frame of the microkinetic approach. Results are referred to real systems.     

Acoustic dispersion near structural phase transitions

We show on various examples, that elastic constant measurements provide a valuable tool for studying the dynamics of solids near phase transitions. Many experimental methods used in the investigation of phase transitions (NMR, neutron scattering, etc.) are able to give information on the dynamics, but in different regions of q- and ω-space, thereby probing different dynamical processes.

A comparison of the dynamics obtained from the elastic measurements with other techniques (NMR) demonstrates this very clearly for the cases of KSCN and C_60.     

Wang-Landau multibondic cluster simulations for second-order phase transitions

For a second-order phase transition the critical energy range of interest is larger than the energy range covered by a canonical Monte Carlo simulation at the critical temperature. Such an extended energy range can be covered by performing a Wang-Landau recursion for the spectral density followed by a multicanonical simulation with fixed weights. But in the conventional approach one loses the advantage due to cluster algorithms. A cluster version of the Wang-Landau recursion together with a subsequent multibondic simulation improves for 2D and 3D Ising models the efficiency of the conventional Wang-Landau or multicanonical approach by power laws in the lattice size. In our simulations real gains in CPU time reach 2 orders of magnitude.