Phonon-assisted electronic excitation transfer near distortive phase transitions

An anomalous temperature dependence of the phonon-assisted electronic excitation transfer rate is to be expected near a distortive phase transition, when the energy mismatch between donor and acceptor energy levels is close to that of a soft optical mode, or an order-parameter-sensitive hard mode. In the case of electron coupling to soft acoustical modes, it is shown that the Debye model used in current theories breaks down near T_c.     

Quasiparticle picture of quarks near chiral phase transition

We study how the quasiparticle picture of the quark can be modified near but above the critical temperature (T _c) of the chiral phase transition; we incorporate into the quark self-energy the effects of the precursory soft modes of the phase transition, i.e. ‘para-σ(π) meson’. It is found that the quark spectrum has a three-peak structure near T _c: We show that the additional new spectra originate from the mixing between a quark (antiquark) and an antiquark-hole (quark-hole) caused by a “resonant scattering” of the quasi-fermions with the thermally-excited soft modes.     

Relaxation and collective motion in superconductors: a two-fluid description

A simple, unified discussion of branch imbalance and gap relaxation in superconductors is presented. Both phenomena are treated within the framework of the ordinary Boltzmann equation, supplemented by the BCS gap equation. We show that the physics of the process commonly referred to as quasiparticle branch imbalance relaxation may be understood simply if one introduces a two-fluid model for the charge in the superconductor, and regards the process as one in which charge associated with the normal component is converted into charge associated with the superfluid. We derive in detail the exact solutions of the Boltzmann equation, which are valid near T_c, and allow for the effects of anisotropy. We discuss the comparison between relaxation rates measured in the superfluid and those obtained from normal state measurements and calculations. We then derive a set of two-fluid hydrodynamic equations based on the two-fluid model for the charge, and find that the current of charge associated with the normal component is not in general equal to the usual normal current. On the basis of these equations we derive expressions for the characteristic quasiparticle diffusion length near phase slip centers, and for the frequency of the recently observed collective mode. We compare our result with those of both microscopic and phenomenological calculations.     

Anomalous resistivity in structural phase transition of IV–VI

The contribution of a carrier-soft optical phonon interaction to the electrical resistivity of IV–VI compounds is discussed. The calculated resistivity based on a soft mode theory shows a large increase near the transition temperature T_c. It is foound that the observed increase of resistivity of p-SnTe is reproduced by the carrier-soft TO phonon interaction.     

Central peak in the strontium titanate crystal near the tetragonal-to-cubic phase transition

Raman spectra of the SrTiO3 crystal have been measured in wide temperature (22?C316 K) and frequency (2?C1020 cm^?1) ranges. It has been shown that a central peak appears in low-frequency Raman spectra at temperatures above 70 K. In the spectral geometry with polarization rotation near the temperature T _c = 106 K of the cubic-to-tetragonal phase transition, the central peak exhibits properties of the order-disorder phase transition. Such a behavior of the central peak has been explained by the interaction of the low-frequency soft mode E _g with the relaxation mode near T _c .     

Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

Dynamic critical behaviour at the structural phase transition in SrTiO3

The soft mode frequency involved in the structural phase transition in SrTiO₃ is measured with high accuracy. Two regimes are observed: a Landau regime remote from T_c and a critical regime near T_c with exponents of 0.5 and 0.33, respectively. This gives evidence for a proportionality of this dynamic quantity with the static order parameter also in the critical regime. The results for the soft mode linewidth are close to the theoretical expectations.     

Quasiparticles in a strongly correlated liquid with the fermion condensate: applications to high-temperature superconductors

A model of a strongly correlated electron liquid based on fermion condensation (FC) is extended to high-temperature superconductors. Within our model, the appearance of FC presents a boundary separating the region of a strongly interacting electron liquid from the region of a strongly correlated electron liquid. We study the superconductivity of a strongly correlated liquid and show that, under certain conditions, the superconductivity vanishes at temperatures T > T _c ≈ T _node, with the superconducting gap being smoothly transformed into a pseudogap. As a result, the pseudogap occupies only a part of the Fermi surface. The gapped area shrinks with increasing the temperature and vanishes at T = _T*. The single-particle excitation width is also studied. The quasiparticle dispersion in systems with FC can be represented by two straight lines, characterized by the effective masses and, intersecting near the binding energy that is on the order of the superconducting gap. It is argued that this strong change of the quasiparticle dispersion upon binding can be enhanced in underdoped samples because of strengthening the FC influence. The FC phase transition in the presence of the superconductivity is examined, and it is shown that this phase transition can be considered as driven by the kinetic energy.     

Phenomenological description of the microwave surface impedance and complex conductivity of high-T c single crystals

Measurements of the microwave surface impedance Z _s (T) = R _s (T) + iX _s (T) and the complex conductivity σ_s(T) in the ab-plane of high-quality high-T _c YBCO, BSCCO, TBCCO, and TBCO single crystals are analyzed. Experimental data of Z _s (T) and σ_s(T) are compared with calculations based on a modified two-fluid model that includes a temperature-dependent quasiparticle scattering and a unique temperature variation of the density of superconducting carriers. We describe the agreement and disagreement of our analysis with the salient features of the experimental data. We review the existing microscopic models based on unconventional symmetry types of the order parameter and on novel quasiparticle relaxation mechanisms.     

Ferroelectric phase transition in SbSI type crystals

In this work Lorenz field coefficients F for SbSI type crystals are estimated. Also transverse and longitudinal frequencies (ω _T and ω _l) of optical vibrations are determined. From a study of these quantities it was concluded that a combination of F is an inadequate condition for ferroelectric phase transitions to take place. Taking the electronic structure, chemical bond model, electronic potential and condition ω _T—>0 into account, the phase transition in SbSI type crystals was investigated. The dependence of the electronic potential upon the composition of V-VI-VII crystals and mixed crystals has revealed the factors that change the phase transition temperature T _c and the dynamics of the soft mode.     

Some expected anomalies in the impurity vibronic spectra of ferroelectrics

The influence of the soft phonon branch on some characteristics of the impurity vibronic absorption spectra in ferroelectrics is qualitatively analyzed and illustrated using simple models. The following quantities are discussed: the intensity, the temperature shift and the half width of the no-phonon line, the one-phonon wing, the integral intensity of a parity forbidden band. It is shown that near the phase transition point (T > T_c) all these characteristics must show anomalous dependencies on temperature, if compare with usual matrix crystals. The observation of “plateaus” may be expected in these dependencies. Various aspects of obtaining information from measured impurity vibronic spectra of ferroelectrics are discussed.     

Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Retardation of dislocations by the Cherenkov mechanism in crystals with a soft mode

The retardation mechanism of dislocations in crystals with a soft mode on account of Cherenkov emission of soft optical phonons is considered in the present work. The conditions in which this retardation mechanism appears are examined for various orientations of the wave vector q_c characterizing structural phase transition. It is shown that the threshold velocity of the retardation mechanism in the case where q_c¦V (V is the velocity of motion of the dislocations) decreases on approaching the phase-transition temperature T_c in proportion to ¦t_c–T_c¦. The dependence of the dynamic retardation coefficient B on the velocity V and the closeness to the point of structural phase transition is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 59–64, December, 1988.     

铌酸钡钠和钽酸锂铁电相变的实地观察

本文报道了用透射电子显微镜实地观察铌酸钡钠和钽酸锂铁电相变的实验结果。在室温电畴的尺寸为0.2—1μm。当温度增加到接近居里点时,形成准规则排列的微畴结构,其尺寸为几百埃,实验结果用稍微修正了的模型进行了解释。在钽酸锂中当接近铁电相变温度时,发现了强烈的辐照损伤,这可能跟相变的软模有关。 关键词：     

Lattice distortion-induced phase transformations in La1 − y Pr y MnO3 + δ manganites

The structural and magnetic phase transformations that occur in the system of self-doped La_{1 ? y} Pr _y MnO_{3 + δ} (δ ≈ 0.1, 0 ≤ y ≤ 1) manganites in the temperature range 4.2–300 K are studied by X-ray diffraction and measuring the temperature and field dependences of dc magnetization. The low-temperature magnetic phase transformations induced by the substitution of Pr for La correlate well with the structural phase transformations at T = 300 K, which indicates a strong coupling of the electronic and magnetic subsystems of La_{1 ? y} Pr _y MnO_{3 + δ} manganites with the crystal lattice. The anomalies of the magnetic and structural properties detected in this work in the form of peaks and inflection points in the concentration dependences of the magnetization and lattice parameters of the pseudocubic phase of La_{1 ? y} Pr _y MnO_{3 + δ} (0.1 ≤ y ≤ 0.7) in the temperature range 4.2–300 K are explained in terms of the existing concepts of the effect of Fermi surface nesting on the renormalization of the density of states and the hole dispersion near E _F in the presence of a strong coupling of holes with low-frequency optical phonons, which results in their transformation into quasiparticles. The narrow peak in the magnetization curve M(y) of La_{1 ? y} Pr _y MnO_{3 + δ} that is detected near y = 0.3 at T = 4.2 K is assumed to correspond to the peak of coherence of quasiparticles with a low energy of coupling with the crystal lattice near E _F, which was found earlier in the photoelectron emission spectra of manganites. The disappearance of the narrow magnetization peak with increasing Pr concentration is explained by the transition of charge carriers from the mode of “light” holes weakly coupled to one of the soft phonons to the mode of “heavy” holes strongly coupled to several phonons. The transition between phases with strongly different effective quasiparticle masses proceeds jumpwise; that is, it has features of the metal-insulator Mott transition and is accompanied by the transition from 3D to 2D quasiparticle motion in planes ab.     

Anomalous properties of crystals with ferroelectric phase transition induced by off-center ions

It is given the theoretical study of some properties of strongly polarizable dielectric crystals in which off-center impurity ions induce ferroelectric phase transition. The spontaneous polarization, transition temperature, soft mode frequency, dielectric susceptibility, ultrasonic attenuation, nuclear spin-lattice relaxation are analyzed. The theory explains observed in K_1?xLi_xTaO₃ saturation of remanent polarization with off-center Li⁺ concentration increasing, close to $\text{x}$ dependence of phase transition temperature, the anisotropy of ultrasonic attenuation, the absence of anomalies of Li nuclear spin-lattice relaxation rate near T_c.     

The nature of anharmonicity and anomalous piezoelectric properties of ferroelectric SbSBr crystal

The contribution of soft mode at Sb atom's sites, to the temperature dependences of Sb atom's equilibrium position's difference Δz(T) has been studied theoretically, when SbSBr crystal is deformed along a(x), b(y) and c(z)-axis in paraelectric phase and is deformed along c(z)-axis in ferroelectric phase. The largest change of Δz₃₃(T) occurs in the ferroelectric phase near the phase transition temperature in the range from 16 K to 21 K. The temperature dependence of Sb atom's equilibrium position's displacements Δz₃₃ is very similar to the temperature dependence of experimental piezoelectric modulus, when SbSBr crystal is deformed in the direction of c(z)-axis in ferroelectric phase.     

Persistence of tetragonal raman lines in cubic KNbO3

The temperature dependence of the vibrationnal modes in KNbO₃ around the cubic-tetragonal phase transition is studied by Raman scattering measurements. An unexpected intense spectrum is observed above the transition. In addition to broad bands probably due to second order process, it consists of forbidden lines which are characteristic of the tetragonal phase. In particular the presence of the hard component of the soft ferroelectric cubic mode until 30° above T_c can be attributed to the existence of a precursor order near the transition.