Doping effect on the phase transition temperature in ferroelectric SrBi2−xNdxNb2O9 layer‐structured ceramics: a micro‐Raman scattering study

The temperature dependence of Raman spectra for SrBi_2−xNd_xNb₂O₉ ceramics (x from 0 to 0.2) has been studied in a wide temperature range from 80 to 873 K. It is found that the peak position of the A_1g[Nb] phonon mode at 207 cm^–1, which is directly associated with the distortion of NbO₆ octahedron, decreases with increasing Nd composition, while the A_1g[O] phonon mode at 835 cm^–1 increases. Moreover, both the peak position and intensity of the A_1g[Nb] phonon mode reveal strong anomalies around the ferroelectric to paraelectric phase transition temperature. It indicates that the phase transition temperature decreases from about 710 to 550 K with increasing Nd composition, which is due to the fact that the introduction of Nd ions in the Bi₂O₂ layers reduces the distortion extent of NbO₆ octahedron. Copyright © 2011 John Wiley & Sons, Ltd.     

InGaN薄膜的紫外共振喇曼散射

利用325nm紫外光激发,对不同组分的In_xGa_1-xN薄膜的喇曼散射谱进行了研究.在光子能量大于带隙的情况下,观察到显著增强的二阶A₁(LO)声子散射峰.二阶LO声子峰都从一阶LO声子的二倍处向高能方向移动,移动量随样品In组分的增加而增大,认为是带内Frhlich相互作用决定的多共振效应引起的.分析了一阶LO声子散射频率和峰型与In组分的关系.在喇曼谱中观察到样品存在相分离现象,并与X射线衍射的实验结果进行 关键词： xGa_1-xN合金')" href="#">In_xGa_1-xN合金 紫外共振喇曼散射 二阶声子 相分离     

Pressure induced lattice anomalies in pnictides

Hydrostatic pressure Raman and synchrotron XRD measurements at room temperature have been carried out on a series of NdFeAsO_1−xF_y (Nd1111) oxypnictides in order to investigate pressure-induced lattice modifications. The synchrotron XRD data indicate that there is an increased deviation of the lattice constants from smooth pressure dependence in the superconducting compound, in close agreement with the results from SmFeAsO_1−xF_x (Sm1111), although the effect is less pronounced in Nd1111. As in Sm1111 the hydrostatic pressure Raman measurements show that the A_1g mode of the rare earth atom deviates from the linear pressure dependence. Anomalous pressure dependence and a hysteresis is observed in the phonon width of the phonon modes. The calculated Grüneisen parameter for the Nd phonon is very similar to the corresponding value for SmFeAsO_1−xF_x compound and it does not vary with substitution. For the As mode it has a lower value indicating a stiffer phonon with the increased ion size. In connection with the XRD measurements the Raman data show a sudden increase of the pressure-induced lattice anomalies close to doping where the compounds become superconducting.     

Anomalous phonon intensities in the Raman spectrum of disordered CsMg1−xCoxCl3

The five Raman-active $\text{k}\text{= 0}$ phonons have been measured at low temperatures and for a range of x in the disordered lattice CsMg_1-xCo_xCl₃. While the E_2g^a, E_2g^b, E_2g^c and A_1g modes at 55.0, 132.0, 189.0 and 255.0 cm^?1 for x = 0 exhibit normal one-mode behaviour, the intensity of the E_1g phonon at 127.5 cm^?1 has a most unusual concentration dependence which requires a new theory.     

Phonon-assisted spin diffusion in solids

The spin flip-flop transition rate is calculated for the case of spectral spin diffusion within a system of dipolarly coupled spins in a solid where the lattice vibrations are present. Long-wavelength acoustic phonons time-modulate the interspin distance r_ij and enhance the transition rate via the change of the 1/r³_ij term in the coupling dipolar Hamiltonian. The phonon-assisted spin diffusion rate is calculated by the golden rule in the Debye approximation of the phonon density of states. The coupling of the spins to the phonons introduces temperature dependence into the transition rate, in contrast to the spin diffusion in a rigid lattice, where the rate is temperature-independent. The direct (one-phonon absorption or emission) processes introduce a linear temperature dependence into the rate at temperatures not too close to T = 0. Two-phonon processes introduce a more complicated temperature dependence that again becomes simple analytical for temperatures higher than the Debye temperature, where the rate is proportional to T², and in the limit T → 0, where the rate varies as T⁷. Raman processes (one-phonon absorption and another phonon emission) dominate by far the phonon-assisted spin flip-flop transitions.     

Fröhlich electron-phonon interaction in CdSxSe1-xnanocrystals

We demonstrate that the polar electron-phonon coupling in CdS_xSe_1-x nanocrystals increases with decreasing crystallite size. This result is supported by the size dependence of the Huang-Rhys parameter measured by absorption and photoluminescence spectroscopy and by microprobe resonant Raman measurements which allow us to determine the size dependence of the electron-LO phonon coupling and the relative strength between Fröhlich and deformation potential Raman polarizabilities.     

Survey of the pre-factor of the power-law frequency dependence in the silver conducting chalcogenide glasses

The variation of parameter A in the power-law conductivity dependence on the frequency,() = A ^s, for chalcogenide glasses (Ag₂S)_x(GeS₂)_1-x, where 0相似文献   

Numerical study of Fermi-Pasta-Ulam recurrence for water waves over finite depth

Highly accurate direct numerical simulations have been performed for two-dimensional free-surface potential flows of an ideal incompressible fluid over a constant depth h, in the gravity field g. In each numerical experiment, at t = 0 the free surface profile was in the form y = A ₀cos(2πx/L), and the velocity field v = 0. The computations demonstrate the phenomenon of Fermi-Pasta-Ulam (FPU) recurrence takes place in such systems for moderate initial wave amplitudes A ₀ ≲ 0.12h and spatial periods at least L ≲ 120h. The time of recurrence T _FPU is well fitted by the formula T _FPU(g/h)^1/2 ≈ 0.16(L/h)²(h/A ₀)^1/2.     

Bi-induced vibrational modes in GaAsBi

We have studied GaAs_1−xBi_x (up to x3%) using Raman scattering with two different polarization configurations. Two Bi-induced phonon modes are observed at 186 cm⁻¹ and 214 cm⁻¹ with increasing Raman intensity as the Bi concentration increases. By comparing Raman selection rules for the observed Bi-induced phonon modes with those for the substitutional N vibrational mode (GaN mode) in GaAsN, the phonon mode at 214 cm⁻¹ is identified as originating from substitutional Bi at the As site in GaAsBi.     

Raman spectra of single crystals of α-P4S3

Polarization data for the Raman-active k = 0 phonons of single crystals of α-P₄S₃ have been measured at 10 K and at high resolution. Of the 24 external and 60 internal phonons expected for the D_2h factor group of α-P₄S₃ we have observed 16 external and 38 internal phonons. The effect of the layer lattice has been proposed to explain the absence of some expected phonon structure. The present work together with an earlier study of the pressure and temperature dependence of the Raman-active phonons have enabled the 4A₁, A₂ and 5E internal molecular modes to be assigned. The resulting normal coordinates give good fits for the Raman-active bands of the heavy isotope P₄ ³²S₂ ³⁴S and for the mixed chalcogen species P₄S_{3 − x}Se_x.     

Influence of phonon confinement,surface stress,and zirconium doping on the Raman vibrational properties of anatase TiO2 nanoparticles

We present a comprehensive analysis of the Raman spectra of pure and zirconium‐doped anatase TiO₂ nanoparticles. To account for the wavenumber shifts of the E_g(ν₆) mode as a function of particle size (L) and dopant concentration (x), a modification of the standard phonon confinement model (PCM) is introduced, which takes into account the contribution of surface stress by means of the Laplace–Young equation. Together with X‐ray diffraction (XRD) and transmission electron microscopy data, our analysis shows that the surface stress contribution to the observed blue shift of the Raman wavenumber is of the same magnitude as the spatial phonon confinement effect. Annealing experiments show that Zr‐doped nanoparticles exhibit retarded grain growth and delayed anatase‐to‐rutile phase transition by up to 200 K compared to pure anatase TiO₂. XRD shows that Zr doping leads to a unit cell expansion of the anatase structure. Applying the modified PCM to the x‐dependent variations of the E_g(ν₆) Raman mode, the mode‐Grüneisen parameter is found to increase abruptly at x > 0.07 with a concomitant mode softening. This coincides with the x range over which the Zr cations are reported to be displaced from their position in the tetrahedral lattice, and where Zr precipitation occurs upon annealing. The results have implications for the interpretation of Raman spectra of ionic metal oxide nanoparticles and how these are modified upon cation doping. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of spin–phonon coupling in LiFe1 − xMnxPO4olivines

LiFe_{1 − x}Mn_xPO₄ olivines are promising material for improved performance of Li‐ion batteries. Spin–phonon coupling of LiFe_{1 − x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature‐dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm⁻¹ is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature‐dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin–phonon coupling quickly decreases. Our analyses show that the quick decrease of spin–phonon coupling is due to decrease of the strength of spin–phonon coupling, but not change of spin‐ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin–phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li‐ion batteries. Copyright © 2015 John Wiley & Sons, Ltd.     

Superconductivity induced phonon anomalies in the Raman spectra of Zn and Ni doped YBa2Cu3O7

Here we present Raman spectra of YBa₂(Cu_1–x Zn _x )₃O₇ and YBa₂(Cu_1–x Ni _x )₃O₇ as a function of temperature and Zn or Ni content. The temperature dependence of two modes at 340 and 440 cm^–1 is analyzed. Similarly to the infrared measurements it is found that Zn substantially suppresses the superconductivity induced phonon softening whereas, Ni does not affect much that effect. Moreover, the superconductivity induced phonon stiffening of the 440 cm^–1 mode completely disappeared with the Zn doping. We find this behaviour might support the model where Zn acts effectively as a magnetic pair breaker.     

The number of independent traces and supertraces on symplectic reflection algebras

It is shown that A:= H₁, _η (G), the sympectic reflection algebra over ?, has T_G independent traces, where T_G is the number of conjugacy classes of elements without eigenvalue 1 belonging to the finite group G ? Sp(2N) ? End(?^2N) generated by the system of symplectic reflections.

Simultaneously, we show that the algebra A, considered as a superalgebra with a natural parity, has S_G independent supertraces, where S_G is the number of conjugacy classes of elements without eigenvalue -1 belonging to G.

We consider also A as a Lie algebra A^L and as a Lie superalgebra A^S.

It is shown that if A is a simple associative algebra, then the supercommutant [A^S, A^S] is a simple Lie superalgebra having at least S_G independent supersymmetric invariant non-degenerate bilinear forms, and the quotient [A^L, A^L]/([A^L, A^L] ∩ ?) is a simple Lie algebra having at least T_G independent symmetric invariant non-degenerate bilinear forms.     

Resistivity,thermopower and single-particle tunneling studies on some zinc-doped yttrium barium copper oxide superconductors

Abstract

Resistance and thermopower measurements have been made on a series of compounds, YBa₂Cu₃- _x Zn _x O_7-y, with x = 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below T_M , the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et al. From the plot of logarithm of resistivity vs. (T_M ? T) ^1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of Å, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above T_c . This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with T_c , and 2Δ/kT_c has an approximate value of 5.5.     

Si–Si optical phonon behavior in localized Si clusters of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanocrystals

Raman spectra acquired from Si _x Ge_1−x -nanocrystal-embedded SiO₂ films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si _x Ge_1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.     

Raman studies and optical properties of some (PbO)x(Bi2O3)0.2(B2O3)0.8−x glasses

Five (PbO)_x(Bi₂O₃)_0.2(B₂O₃)_0.8−x glasses, where x = 0, 0.2, 0.3, 0.4 and 0.6, were prepared. The dilatometric glass transition temperature (T_g) was found in the region 470 (x = 0)≥ T_g ( °C) ≥ 347 (x = 0.6), and the density (ρ) varied within 4.57 (x = 0) ≤ ρ (g/cm³) ≤ 8.31 (x = 0.6). Raman spectra indicated the conversion of BO₃ to BO₄ entities for low x values but for x > 0.3, namely, for x → 0.6, back‐conversion occurred, most probably. From the measurements of the optical transmission on very thin bulk samples, the room temperature optical gap values (E_g) were determined to be in the range 4.03 (x = 0)≥ E_g (eV) ≥ 3.08 (x = 0.6). The temperature (T) dependence of the optical gap (E_g(T)) in the region 300 ≤ T(K) ≤ 600 was examined and approximated by a linear relationship of the form of E_g(T) = E_g(0)− γT, where γ × 10⁻⁴(eV/K) varied from 5.1 to 6.8. The non‐linear refractive index (n₂) was estimated from the optical gap values and it was found to correspond to the n₂ values calculated from the experimental third‐order non‐linear optical susceptibility taken from the literature. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural properties and Raman spectroscopy of orthorhombic (Eu1−xPrx)0.6Sr0.4MnO3 (0≤x≤1.0)

The effect of Pr doping on structural properties and room temperature Raman spectroscopy measurements is investigated in manganites (Eu_1−xPr_x)_0.6Sr_0.4MnO₃ (0≤x≤1.0) with fixed carrier concentration. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in an orthorhombic distorted structure with a space group Pnma. It is evident that, with increasing Pr substitution, three types of orthorhombic structures can be distinguished. The phonon frequencies of the three main peaks, in room temperature Raman-scattering measurements, have been discussed together with their structural characteristics, such as bond-length, bond-angles, and the change of orthorhombic structure type. With the increase of Pr content, the mode at 491  cm⁻¹ also shows a corresponding change. A step effect becomes evident, which seems to indicate the close dependence between the frequency shift of this mode and the change in crystal symmetry. This further supports the notion that the mode at 491  cm⁻¹ is closely correlated with the Jahn–Teller distortion. Moreover, we have found that the lowest frequency peak (assigned as an A_1g phonon mode) depends linearly on the tolerance factor t.     

Vibrational Raman spectra of CdSx Se1‐x magic‐size nanocrystals

Resonant Raman scattering spectra of ultrasmall (<2 nm) magic‐size nanocrystals (NCs) are reported. The spectra of CdS and CdS_x Se_1‐x NCs, resonantly excited with 325 nm and 442 nm laser lines, correspondingly, reveal broad features in the range of bulk optical phonons. The relatively large width, ～50 cm^‐1, and downward shift, ～20 cm^‐1, of the Raman bands with respect to the longitudinal optical phonon in bulk crystals and large NCs are discussed based on the breaking of the translational symmetry and bond distortion in these ultrasmall NCs. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The corrections to scaling within Mazenko’s theory in the limit of low and high dimensions

We consider corrections to scaling within an approximate theory developed by Mazenko for nonconserved order parameter in the limit of low (d → 1) and high (d → ∞) dimensions. The corrections to scaling considered here follows from the departures of the initial condition from the scaling morphology. Including corrections to scaling, the equal time correlation function has the form: C(r, t) = f ₀(r/L)+L ^−ω f ₁(r/L)+…, where L is a characteristic length scale (i.e. domain size). The correction-to-scaling exponent ω and the correction-to-scaling functions f ₁(x) are calculated for both low and high dimensions. In both dimensions the value of ω is found to be ω = 4 similar to 1D Glauber model and OJK theory (the theory developed by Ohta, Jasnow and Kawasaki).