A light scattering investigation of the incommensurate transition in BaMnF4

The results of a Brillouin scattering investigation of BaMnF₄ near its incommensurate structural phase transition at T_i=247 K are reported. We have observed an anomalous dynamic central peak in the polarized (bb) spectrum, with $\text{q}\text{6}\text{c}$. This feature is visible only very near T_i, and possesses a width which is strongly wavevector dependent. The indicated behavior is Λ=D_tq², with D_t=0.14 ± 0.02 cm²sec^-1. It is identified as the result of a coupling between the LA phonon and the soft mode, where the latter contains a relaxing self energy due to a linear coupling to a diffusive process, possibly entropy fluctuations. The narrow central peak is not visible for $\text{q}\text{6}\text{a}$. In addition, we report an anomalous dispersion (～4%), in the LA mode propagating along $\text{c}$ (V_cc) near T_i.     

Central peak for a scalar ϕ 4-model: mode coupling approach revisited

We reinvestigate the mode coupling approach to the central peak which occurs in the vicinity of a structural phase transition at T _c. For a scalar ? ⁴-model it is shown that the use of renormalized vertices leads to quite different results compared to recent calculations with bare vertices. Particularly, we prove that the latter are obtained in leading order of the anharmonicity constant of the on-site potential from a perturbational treatment of the renormalized vertices. Again, this mode coupling approach may yield a dynamical transition at a temperature T _c'(≥ T _c) at which the dynamics becomes nonergodic, i.e. a central peak occurs. For a ? ⁴- model with infinite range interactions our theoretical predictions are consistent with numerical results. Furthermore, if the fluctuations in the vicinity of Tc are Gaussian, no dynamical transition occurs above Tc. Therefore the temperature T ₀'obtained from the Ginzburg criterion sets an upper bound for T _c'. If a dynamical transition occurs, it is shown that the nonergodicity parameter as function of wave vector q and temperature T follows from an universal master function.     

Raman study of charge-density-wave phase transition in quasi-one-dimensional conductor (TaSe4)2I

Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe₄)₂I above and below the charge-density-wave (CDW) transition temperature (T_c=263 K). The Raman intensities of many peaks become intenser and two of the phonon peaks shift to higher frequency with decreasing temperature. Moreover a new broad peak at about 90 cm^?1 and a new peak around 166 cm^?1 appear in the low-temperature phase. The polarization characteristic shows that the former is assigned to totally symmetric mode. The damping constant of the phonon at 90 cm^?1 increases markedly with increasing temperature. The frequency shifts to higher frequency as the temperature increases and the coupling coefficient is approximately proportional to (T_c?T)^{$\text{1}\text{2}$}. This peak becomes Raman active owing to the CDW phase transition. The temperature dependence of the damping constant and the frequency shift may have a relation to the dynamical properties of the CDW phase transition.     

Fano interference between the E2 (17 cm−1) optical mode and one-phonon continuum scattering from β-AgI

Resonant interference between the E₂ (17 cm^?1) optical mode and the continuum of one-phonon partial density of states has been observed in β-AgI, and has been shown to follow Fano formalism. The temperature dependence of the coupling parameter q shows a substantial variation as T approaches T_c. A qualitative explanation of such behaviour is made in terms of phonon assisted activation of Frenkel defects, which increases more rapidly as the superionic transition temperature is approached.     

d-Wave superconductivity via buckling-like phonon mode

We follow the classic strong-coupling theory of superconductivity through electron-phonon interaction with a buckling-like phonon mode. We find a nonzero d-wave order parameter in the sense of the Eliashberg theory. We derive a zero temperature gap Δ(0,π) at the gap edge versus the electron-phonon coupling strength g² relation. We find that large enough value for Δ(0,π) as compared to those of high-T_c superconductors cannot be realized in the electron-buckling-like-phonon coupling on the CuO₂ planes.     

Soft modes and high Tc in Nb3Sn

We investigate the direct effects of phonon softening on T_c in Nb₃Sn. It is inadequate to explain the high T_c, which arises instead from an unusually large electron- phonon coupling to the two Γ'₁₂ optical phonon modes.     

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 .     

Tc map and superconductivity of simple metals at high pressure

We calculate T_c map in region of weak electron–phonon coupling based on simple phonon spectrum. By using linear-response method and density functional theory, we calculate phonon spectra and Eliashberg functions of simple metals under pressure. Based on the evolutions of superconducting parameters of simple metals on the T_c map with increasing pressure, we find that there are two different responses to pressure for simple metals: (1) enhancing electron–phonon interaction λ such as for La and Li, (2) increasing phonon frequency such as for Pb, Pt. The λ threshold effect is found, which origins from the competition between electron–phonon interaction and electron–electron Coulomb interaction and is the reason why T_c of most superconductors of simple metals are higher than 0.1 K.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Ab initio study on the high superconducting transition temperature in calcium under high pressure

For calcium in the phases IV and V, we estimated the superconducting transition temperature T _c by the use of the Allen–Dynes formula. Setting the effective screened Coulomb repulsion constant μ* at 0.1 in the formula, we obtained T _c =23.42 K at 100 GPa for Ca-IV and T _c =15.87 K at 120 GPa for Ca-V. In order to clarify the origin of such high values of T _c , first, we investigated the band character of electrons and found that the high T _c is not necessarily related to the so called s–d transfer. Then we analyzed the electron–phonon coupling at each phonon mode in Ca-V where the highest T _c in elements has been experimentally observed. As a result, we discovered that an optical mode at the Γ point has the strongest electron–phonon coupling. Such phonon mode can exist only in the complex crystal structure of Ca-V, and the result shows that the high T _c seems to be closely linked with the complex crystal structures like Ca-IV and Ca-V.     

Superconductivity and Peierls instability in coupled linear chain systems

We study the superconducting transition temperature (T_c) and the Peierls instability temperature (T_p) using Eliashberg type equations for both T_c and T_p self consistently with finite interchain coupling. We show that T_c > T_p below a critical electron-phonon coupling constant which depends on the bare phonon frequency. This determines an upper bound on T_c so that for higher transition temperatures T_p > T_c and superconductivity is unlikely. Higher values of T_c are possible if the interchain coupling is increased above a critical value where the Peierls instability is suppressed.     

Levelcrossing Experimente im Tm I-Spektrum

A semi phenomenological dynamical theory for the rotations of the BO₆ octahedra in ABO₃ perovskites is proposed, which accounts for the soft mode and the central peak forT≧T _c observed by neutron scattering in SrTiO₃. The neutron scattering cross section is discussed. The EPR line width is predicted to diverge as ?^?ν(1–2η) and as ?^?ν(2–2η) for three- and two dimensional correlations with ?=(T-T _c )/T _c . The theory is generalized to other structural transitions.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Hyper-Raman scattering spectra in the low frequency range in cubic BaTiO3 and the mechanism of the phase transition

The hyper-Raman spectra in the cubic phase of BaTiO₃ are studied with special emphasis on the lowest-freauency phonon mode. The spectral structure is found to change as the temperature T is raised from T_c (130 °C) to 400 °C. It is shown that the overall experimental results can be well accounted for in terms of the critical dynamical theory based on pseudospin-phonon coupled system developed by Y. Yamada et al. As the consequence, it is found that the coupling between TO phonons and pseudospins presumably constituted by flipping motion of Ti ions causes the cubic-tetragonal phase transition, and plays an important role in understanding the dynamical aspects including the anomalous increase in dielectric constant as T approaches T_c from above.     

非晶态超导体的2△0/(kBTc)和声子谱参量

提出了能够很好地描述非过渡金属无序和非晶态超导体的2Δ₀/(k_BT_c)与声子谱参量之间关系的一个公式:2Δ₀(k_BT_c=4.95[1-(T₀<ω>^1/2)/A(1/(λω₀)+1/(20λ<ω>)+1/(20<ω>))]。计算了大量已知声子谱的非晶和无序超导体的能隙2Δ₀对T_c的比,结果表明在百分之几的范围内与实验值符合。指出了非过渡金属和合金的非晶态超导体,既可以是一个2Δ₀/(k_BT_c)值远大于BCS理论值(3.53)的强耦合超导体,也可以是一个2Δ₀/(k_BT_c)值比BCS理论值还要小得多的弱耦合超导体。 关键词：     

Nonlinear dynamics of a two-dimensional lattice

The dynamics of the nonlinear excitations in a two-dimensional (2D) φ⁴-diatomic lattice, with nonlinear on-site electron-phonon coupling at the polarizable ion site has been presented, without considering the self consistent phonon approximation. One of the major results obtained from our calculations is in the understanding of continuous structural phase transition, where we have obtained the minimum in soft mode frequency at a soft mode temperatureT _s (>T _c), not at critical temperatureT _c. This occurs due to the anisotropy of such 2D systems.     

Dynamical properties of the Peierls-Fröhlich state on the many-phonon-coupling model

The amplitude and phase phonons and the frequency dependent conductivity below the mean-field Peierls-Fröhlich transition temperature T_c, and the Kohn anomaly and fluctuation induced charge-density-wave conductivity above T_c, are discussed on the basis of the many-phonon-coupling model recently introduced by Rice, Duke and Lipari. For dominant intramolecular phonon coupling an isotope effect in T_c is related to the isotopic shift in the small polaron binding energy.     

Effects of spin-phonon interaction on transport, magnetic properties and the oxygen isotope effect in R1−xAxMnO3

We consider the spin-phonon (s-p) coupling as an important correction to the double exchange and the dynamic Jahn-Teller effect in colossal magnetoresistance systems R_1−xA_xMnO₃. The effect of s-p on transport, magnetic properties and the oxygen isotope effect has been studied by utilizing the perturbation theory and mean field theory. It is indicated that (i) a formula for the shift of the ferromagnetic (FM) transition temperature T_c is given, from which it could be seen that the s-p coupling could yield an excitation gap for long-wavelength acoustic spin waves and an applied field could improve T_c, (ii) the relation T_c∼m^−1/2 (m is the mass of the anion) is obtained, which suggests that the oxygen isotope effect is dominated by both the s-p and e-p interaction, (iii) it could cause the hardening of phonon frequency in the FM state, (iv) s-p could lead to the increase of the resistivity which is proportional to T³ at low temperature and proportional to T at high temperature in the ferromagnetic metallic region of R_1−xA_xMnO₃.     

有效声子谱对超导体临界温度的影响

本文由超导体强耦合能隙方程出发,对64种有效声子谱求得了近3500个临界温度的数值解。它们说明,在谱面积A不变时,T_c具有条件极值(T_c/A)_max。T_c主要取决于α²F(ω)上峰的位置及其面积,与峰的宽度关系不大。控制<ω>及<ω²>两个参量时,用双δ谱来代替L谱所产生的误差为3.2％。本文分析并澄清了文献中关于“λ=2极限”的争论。 关键词：     

Observation of a central peak in lead germanite by light scattering

Brillouin-Raman scattering measurements on single domain lead germanate over the temperature interval 300–455 K reveal an unresolved (less than 80 MHz) central peak. At about T_c?20 K this peak begins to increase in intensity, reaching a maximum at T_c, and then falls away rapidly with increasing temperature above the transition. The central peak anomalous intensity, which is visible to the naked eye, is much larger than the intensity contributions due to phonons, including the ferroelectric soft mode, and appears to be a static effect.