Model calculations for the phase transition in SmS

Assuming a simple model for the compound SmS we have performed calculations which indicate a definite tendency toward the observed semiconductor-metal phase transition under pressure. Our results lend support to the hypothesis that the transition is associated with a change in 4f occupancy of the Sm ion.     

Destruction of SmS single crystals during the semiconductor-metal phase transition under hydrostatic compression

The behavior of internal microstresses, the size of the X-ray coherent scattering region, and the residual amount of the metal phase during the cyclic loading of SmS single crystals at hydrostatic pressure above the critical pressure of the semiconductor-metal phase transition has been investigated. It has been shown that the samples are destroyed as the microstresses reach the values corresponding to the ultimate stress of SmS single crystals. As the number of loading cycles increases, the coherent scattering region gradually decreases, which is accompanied by a decrease in the amount of the metal phase in the samples.     

SmS phase transition in thin films prepared by reactive evaporation

Preparation and analysis of thin films containing samarium sulfide (SmS) is presented along with a review of relevant SmS properties. Films were deposited onto unheated substrates by the reactive evaporation of samarium in a backpressure of hydrogen sulfide. This technique yields films that contain significant quantities of impurities; however high-quality SmS crystals are also formed. A phase transition in the SmS crystals was observed both by spectrophotometry and x-ray diffraction. The film optical properties can be modeled with an effective medium calculation. The predicted spectra successfully reproduce the observed qualitative features over a wide range of wavelengths.     

The structure of a metallic-phase film produced by mechanical polishing of polycrystalline SmS

X-ray diffraction is used to study the structure of a metallic-phase film that forms during controlled polishing of homogeneous polycrystalline semiconducting Sm_1+xS samples. Structural changes that appear in the semiconducting phase under these conditions were studied. The x dependence of the thickness of the metallic layer forming on the sample surface is analyzed to explain the effect of excess samarium ions on the transformation parameters. The cause of the stabilization of the metallic modification of SmS after polishing is terminated is explained using estimates based on the measured sizes of coherent domains in samples of different compositions. The appearance and stabilization of the metallic phase are related to a decrease in and subsequent conservation of the coherent-domain size, respectively.     

Emf induced by a change in the samarium ion valence as a result of a phase transition in SmS single crystals

The mechanism of the emf appearing in a semiconductor under heating in the absence of external temperature gradients, an effect revealed by the present authors, is considered. The experiments were performed on samarium sulfide (SmS) single crystals. It is shown that the emf is generated by an abrupt change in the samarium ion valence, which results from the ion screening by the electrons activated into the conduction band. We succeeded in obtaining emf pulses 1.3 s long with an amplitude of up to 2.5 V at T ～ 460 K, as well as CW emf generation within the 375-to 405-K temperature interval with a maximum value of ～50 mV.     

Deformation phase transition in vanadium under high pressure

Within the framework of the Landau theory of phase transitions and the density functional method, it is shown that the structural transformation from a body-centered cubic phase to a rhombohedral phase revealed in vanadium at 69 GPa is a first-order deformation phase transition close to a second-order phase transition.     

Magnetic field diffusion during a metal-insulator phase transition

An analysis is made of the diffusion of a static (slowly varying) magnetic field in a conductor in which a first-order phase transition to the insulating state takes place under the action of the Joule heating. An investigation is made of the case of subsonic propagation of the phase boundary. A (V_1−X Cr_X)₂O₃ solid solution is analyzed as a model substance. The application of this effect in pulsed high-current circuit breakers is discussed. Zh. Tekh. Fiz. 68, 43–48 (December 1998)     

Concentration model of semiconductor-metal phase transitions in SmS

Model calculations explaining the mechanism of the semiconductor-metal phase transition in SmS are carried out. The model, slightly modified, draws upon methods employed earlier to account for the concentration mechanism of piezoelectric resistance and thermovoltaic effect in SmS. The stable results are obtained from calculations for the phase transition pressure under hydrostatic compression (P _c ～ 700 MPa at T = 300 K). On this basis, it is concluded that the 4f levels of samarium ions and their excited states determine the value of P _c . The proposed model is universal in character and can be applied to calculations of other effects in SmS, which are associated with Mott transitions and are accompanied by collective carrier delocalization.     

Unusual phase transition mechanism in rare-earth ethylsulfates

A hypothesis is put forward that at pressure 30 kbar f-orbitals begin to participate in generation of a chemical bond between a rare-earth ion and oxygen. This phenomenon is presumed to be responsible for spectral changes observed in praseodymium ethylsulfate at its compression.     

Dynamics of interphase domain walls during a Morin-type phase transition

The dynamics of 90-degree interphase domain walls during a first-order Morin-type spin-reorientation phase transition is studied theoretically. It is shown that an oscillatory motion of the walls with an amplitude that depends linearly on the field amplitude, as well as a drift motion of the wall at a velocity proportional to the square of the field amplitude, are driven by an external oscillating magnetic field. Drift of the entire domain structure as a whole is predicted to be possible. Fiz. Tverd. Tela (St. Petersburg) 41, 274–282 (February 1999)     

A new mechanism of surface polariton formation at a ferroelastic phase transition

Conditions are determined at which the combined effect of the lattice and the spatial dispersion gives rise to the formation of a surface polariton with the participation of a soft mode in the vicinity of a dipole ferroelastic phase transition.     

Structural phase transition mechanism in the alloy CuPd

The high temperature x-ray method is used to study A1-B2 phase transitions in Cu-Pd alloys near a composition of 40 at.% Pd. An anomalous increase in mean square atomic displacements and a non-monotonic change in the lattice parameter with temperature are observed at the onset of the phase transitions. It is shown that upon heating of the quenched solid solution above 300°C the solid solution decomposes with formation of a two-phase mixture A1 + B2. Activation energies of the processes accompanying the structural phase transitions are calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 67–72, June, 1988.     

Nonequilibrium phase transition in quenched samples of Fe0.1TiTe2

The influence of the heat treatment of Fe_0.1TiTe₂ samples on the appearance of thermally induced “levitation” is investigated. It is postulated that this phenomenon is most probably caused by undamped fluctuations of the electric charge on the surface of particles of the material. Fiz. Tverd. Tela (St. Petersburg) 41, 680–683 (April 1999)     

Thermal equilibrium during the electroweak phase transition

The perturbative effective potential for the Standard Model develops a barrier, at temperatures around the electroweak scale, which separates the minimum at zero field and a deeper non-zero minimum. This could create out of equilibrium conditions by inducing the localization of the Higgs field in a metastable state around zero. In this picture vacuum decay would occur through bubble nucleation. I show that there is an upper bound on the Higgs mass for the above scenario to be realized. The barrier must be high enough to prevent thermal fluctuations of the Higgs expectation value from establishing thermal equilibrium between the two minima. The upper bound is estimated to be lower than the experimental lower limit. This also imposes constraints on extensions of the Standard Model constructed in order to generate a strongly first order phase transition.     

Time-resolved X-Ray diffraction from coherent phonons during a laser-induced phase transition

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory coupled to analytic solutions for the laser-induced strain profile. The results are consistent with a 12 ps thermal electron-acoustic phonon coupling time together with an instantaneous component from the deformation-potential interaction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coherent atomic motion.     

Cubic-tetragonal phase transition in SrTiO3 revisited: Landau theory and transition mechanism

Abstract

Existing experimental data for the antiferroelastic phase transition in strontium titanate are reviewed and analysed using a Landau free energy of the form ΔG = 1/2Aθs (cothθs/ Tc-colb.θ/T)Q² + 1/4BQ ⁴ + 1/6CQ ⁶, with A = 0·6472 J K^?1mol^?1, B = 29·12 Jmol^?1, C = 39·27 Jmol, T _c= 105·6 K, θ _S = 60·8 K. The temperature dependence of the critical exponent is found to be due to the delicate balance between the Q ⁴ and Q ⁶ terms in the free energy expansion, and the saturation of the order parameter at low temperatures.

The spontaneous strains observed in this phase transition are not consistent with simple rotation of the TiO₆ octahedra around [001], An alternative model is proposed, where these octahedra expand in order to preserve the volume of the twelve-fold co-ordinated Sr site and the spacing between SrO₃ pseudo-closepacked layers.     

Curie temperature in InMnP and the mechanism of phase transition

We present the phase transition of high Curie temperature InMnP system grown by liquid phase epitaxy. InMnP system has a distribution of uniform islands on the surface layer which has a width of 40 nm and height of 30 nm. Two kinds of phase transition in the magnetization were observed. The origin of these structures and their influence for the magnetization are discussed from the view of two-dimensional Ising model.