Temperature dependence of the linewidth of the first-order Raman spectrum for GeO2 powder

The first-order Raman spectrum of A_1g mode in GeO₂ powder was measured at a temperature range of 130–973 K and the linewidths at these temperatures were obtained. The temperature dependence of the linewidths was analysed by the phonon dispersion curves based on the rigid ion model, and the results show that it was caused perhaps by the cubic anharmonic term in crystal potential energy.     

Phonon deformation potentials for the corundum structure of sapphire

A theoretical Raman polarization analysis is proposed for the corundum structure of sapphire (α‐Al₂O₃) and validation experiments conducted with the purpose of retrieving the full set of phonon deformation potentials (PDPs). From the theoretical side, the change in force constants under stress/strain has been expressed in matrix form, and close‐form solutions were obtained for the eigenvalues that take into account the local dependence of oblique phonons on crystallographic orientation (i.e. uncoupling the effects of local crystal orientation and stress tensor from the shifts of Raman bands). From the experimental side, controlled (uniaxial) stress fields were applied to sapphire parallelepiped bars (along known crystallographic axes) while Raman spectra were systematically recorded along the bar thickness. An untextured alumina polycrystal with fine grain size was also investigated according to the same procedure. As a result of this set of experiments, PDPs for both A_1g and E_g vibrational bands could be retrieved. Validation of PDP constants was obtained by measuring the steeply graded stress fields developed ahead of a surface crack propagated along an arbitrary crystallographic direction in the R‐plane of the sapphire crystal. Copyright © 2011 John Wiley & Sons, Ltd.     

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb₂AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb₂AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E_2g (ω₁) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E_2g (ω₂) and A_1g (ω₄) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E_2g (ω₂) and A_1g (ω₄) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.     

Properties of ground state and anomalous quantum fluctuations in one-dimensional polaron-soliton systems—the effects of electron-two-phonon interaction and non-adiabatic quantum correlations

Based on the Holstein model Hamiltonian of one-dimensional molecular crystals, by making use of the expansion approach of the correlated squeezed-coherent states of phonon instead of the two-phonon coherent state expansion scheme, the properties of the ground state and the anomalous quantum fluctuations are investigated in a strongly coupled electron-phonon system with special consideration of the electron-two-phonon interaction. The effective renormalization (αi) of the displacement of the squeezed phonons with the effect of the squeezed-coherent states of phonon and both the electron-displaced phonon and the polaron-squeezed phonon correlations have been combined to obtain the anomalous quantum fluctuations for the corrections of the coherent state. Due to these non-adiabatic correlations, the effective displacement parameter αi is larger than the ordinary parameter α (0) i . In comparison with the electron-one-phonon interaction (g) corrected as αig, we have found the electron-two-phonon interaction (g1) corrected as αi2 g1 is enhanced significantly. For this reason, the ground state energy (E(2) 0 ) contributed by the electron-two-phonon interaction is more negative than the single-phonon case (E(1) 0 ) and the soliton solution is more stable. At the same time, the effects of the electron-two-phonon interaction greatly increase the polaron energy and the quantum fluctuations. Furthermore, in a deeper level, we have considered the effect of the polaron-squeezed phonon correlation (f-correlation). Since this correlation parameter f > 1, this effect will strengthen the electron-one and two-phonon interactions by fαig and f2αi2 g1, respectively. The final results show that the ground state energy and the polaron energy will appear more negative further and the quantum fluctuations will gain further improvement.     

Micro‐Raman investigation of nanosized zinc ferrite: effect of crystallite size and fluence of irradiation

Zinc ferrite nanoparticles of different sizes ranging from 12 to 62 nm were synthesized by using the nitrate route. These nanoparticles were irradiated by a 100 MeV oxygen beam at two fluences: 1 × 10¹³ and 5 × 10¹³ ions/cm². It was observed that modes corresponding to cubic spinel phase were retained after the irradiation in all the systems. The variation in the parameters of various modes follows phonon confinement, while this effect seems to violate in irradiated specimen. It was found that the irradiation‐induced changes in the modes F_2g(2) and F_2g(3) depend on whether the crystallite size of the pristine sample is less than, equal to or greater than the phonon confinement length, while this length is not dominant for the irradiation‐induced changes in the mode A_1g. The changes in various parameters of the modes are attributed to the combined effect of the restructuring of the chemical species and ion‐induced defects. Copyright © 2011 John Wiley & Sons, Ltd.     

石墨带的晶格动力学研究

基于晶格动力学理论,采用力常数模型,计算了石墨带的声子色散关系、振动模式密度和比热.计算结果表明,石墨带的声子谱特征介于一维碳纳米管和二维石墨片之间.扶手椅型和锯齿型石墨带的中、高频声子支分别与锯齿型和扶手椅型碳纳米管的类似.由于声子限域效应,低频声子支随着石墨带带宽的改变出现明显的频移现象.振动模式密度在高频区几乎不敏感于带宽,而低频区的峰位随着带宽的增加而逐渐向低频移动.此外,无论是在低温还是高温,比热都随着带宽的增加而逐渐降低,呈现量子尺寸效应.在300K时,比热可以拟合成C_V=C_Vg+A/n,其中C_Vg为石墨片的热容,而A/n项反映了石墨带中边缘效应对比热的影响. 关键词： 石墨带 声子色散关系 比热     

Prediction of superconductivity of Ta2AlC: in situ Raman spectrometry and density functional investigations

In this paper, in situ Raman spectra of Ta₂AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line‐width of E_2g (ω₃) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line‐widths of E_2g (ω₁) and A_1g (ω₄) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta₂AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Electron spin resonance and the spatial extension of the relaxed excited state ofF- andF A -centres

By means of an ESR detection method based on the optical pumping technique theg-factors and ESR-linewidths of the relaxed excited state (RES) of a number ofF- andF _A -centres have been measured. From the linewidths information on the spatial distribution of the RES is obtained: The data are interpreted in terms of a very diffuse wavefunction which is essentially the same for all crystals when scaled with the lattice constant. A numerical value for the parameter describing the spatial extension is given.     

第一性原理计算MgB2薄膜拉伸对超导电性的影响

应用全势线性响应线性糕模轨道方法计算MgB₂的电子能带结构、声子谱及电声子耦合常数,并讨论MgB₂的超导电性.通过比较MgB₂薄膜双轴拉伸前后超导电性的变化可以看出，随着a轴晶格常数增大和c轴晶格常数减小,声子谱中硼的E_2g声子频率显著下降，使得电声子耦合强度λ和声子对数平均频率ω_ln增强,提高了MgB₂薄 关键词： 超导电性 能带结构 声子频率 电声子耦合     

Zeeman splitting factor of the Er3+ ion in a crystal field

Numerical computations are presented on the energy levels of the Er³⁺ ion in crystalline fields of cubic, trigonal, tetragonal and orthorhombic symmetry. Zeeman splitting factors were obtained from the level splitting in an additional magnetic field. For the quartet Γ₈ states in cubic symmetry the Zeeman effect is described by an effective Hamiltonian ℋ= gμ_BBJ+ uμ_BBJ³ with the parametersg andu calculated for mixed fourth- and sixth-order potentials. For the eight doublets in the lower symmetry of an axial trigonal or tetragonal crystal field the principalg tensor components g_∥ and g_⊥ were calculated. The results of such calculations for a ground-state doublet can exactly account for the experimental data obtained on around 70 erbium centers in various crystalline hosts. However, sometimes different sets of parameters give comparably good results. An empirical rule of constant trace g_∥ + 2g_⊥ is supported by the calculations. In contrast to analytical treatments the effect of the crystalline field can be followed over a continuous range of the crystal field parameters. This allows one to establish relations on the relative signs of tensor components. It is found that the measured trace of tensors |g_∥| + 2|g_⊥| is not always equal to their real trace g_∥ + 2g_⊥. In an exploratory calculation a nonaxial center was simulated in an orthorhombic field, with calculation of the three principal values g_x, g_y and g_z. A good agreement is obtained for the recently reportedg values of an erbium center in silicon.     

Iron content and temperature dependences of diffuse scattering in Ti–(50–x)Ni–xFe (6 ≤ x ≤ 10) alloys

Diffuse scatterings appearing in electron diffraction patterns of Ti–(50???x)Ni–xFe (x?=?6, 7, 8, 10, in at.%) alloys were investigated. In the alloys, martensitic (R-phase) transformation is suppressed down to 4.2 K, but the electrical resistivity exhibits a local minimum at T _min (210 K, 195 K, 180 K and 140 K for x?=?6, 7, 8, 10, respectively). The following results were obtained for all the alloys. Diffuse scattering appears below T _min and its intensity maximum is located at an incommensurate position of g?+??ζζ0?*, where g is a reciprocal lattice vector of the B2-phase. The value?ζ?at T _min is significantly smaller than 1/3 and increases with decreasing temperature; it decreases with increasing Fe content. The value of?ζ?at T _min agrees with the length of nesting vector obtained by a band calculation, suggesting that the diffuse scattering is caused by the nesting effect of the Fermi surface in the B2-type structure.     

ESR spectroscopic studies of alkylammonium,pyridinium and arsonium solid salts of C 60 −

The synthesis of a series of new solid salts of the monoanion of C₆₀ using a variety of countercations, e.g., tetraalkylammonium, pyridinium and tetraalkylarsonium cations, and their initial characterization by ESR spectroscopy are reported. Both the size of the countercation and the solvent used in the synthesis of the C₆₀ monoanion salts have been found to affect the ESR spectral characteristics of the resulting salts. For example, for countercations less than 10 Å in diameter, the ESR spectral linewidths decrease as the countercation diameter decreases. In a similar manner, those salts believed to be solvent free display narrower ESR spectral linewidths and somewhat largerg-values than those in which solvent is believed to be incorporated in the crystal structure of the salt in question. Finally, ESR spectroscopy was used to show that exposure of the salts reported herein to ambient atmosphere leads to a decrease in the original ESR signal and an increase of a sharp (i.e., narrow line) signal which is superimposed on the original broad line.     

Theoretical analysis of spin-Hamiltonian parameters for the rhombic Cu2+ centres in CuGaSe2 crystals

The spin-Hamiltonian parameters (g factors g_i and hyperfine structure constants A_i, where i = x, y, z) of the rhombic Cu²⁺ centres in the CuGaSe₂ crystal are determined from the high-order perturbation formulae based on the cluster approach (sometimes also called two-spin-orbit parameter model). In the studies, some parameters in the analysis of g factors for the same centre within the tetragonal symmetry approximation in the previous paper are used, and the parameter due to the perturbation of rhombic crystal field caused by a charge compensator at, e.g., [110] direction are considered. As the result of a fitting process, the determined spin-Hamiltonian parameters are in reasonable agreement with the experimental values. The results are discussed.     

Theoretical explanation of electron paramagnetic resonance and optical parameters for Cu<Superscript>2+</Superscript> ion in LiNbO<Subscript>3</Subscript> crystal

The EPR parameters, anisotropic g-factors g _x , g _y and g _z for cu²⁺ ion and hyperfine structure constants A _x , A _y and A _z for Cu²⁺ in LiNbO₃ crystal are calculated by the method of diagonalizing the full Hamiltonian matrix. The crystal-field parameters contact with the crystal structure by the aid of the superposition model. The optical transition parameters are calculated using Zhao crystal-field model. The calculated results are in good agreement with the observed values. The results are discussed.      

Field Effect in the Linear and Nonlinear Conductivity of the Layered Quasi-One-Dimensional Semiconductor TiS3

Field-effect transistor structures based on whiskers of layered quasi-one-dimensional semiconductor TiS₃ have been fabricated. The dependences of the conductivity σ on the gate voltage V_g, as well as the current-voltage characteristics of whiskers (“source-drain”) at different V_g values, have been measured in the temperature range of 4.2-300 K. As the temperature decreases, the sensitivity of the conductivity to the gate voltage, α ≡ 1/σdσ/dV_g, increases in the range from 300 to 80 K and decreases sharply below 80 K, where the nonlinear conductivity begins to depend on V_g. The results can be explained by the formation of an electronic crystal at low temperatures.

     

A study of the magnetic,molecular and electronic structure of some alkali biphenyl ion pairs in the solid state

A magnetic resonance and susceptibility study of the molecular and magnetic structure of some alkali biphenyl (Bp) polyglyme single crystals and powders has been performed. For RbBp · 2Ttg (Ttg=tetraglyme), space group C2/c, and NaBp · 2Tg (Tg=triglyme), space group P2₁, the average g tensors have been measured. From the average g tensor and the known crystal structure of RbBp · 2Ttg the molecular g tensor of the Bp anion could be derived. The molecular g tensor was calculated with Stone's theory. Excellent agreement was found with the experimental values. With the aid of the molecular g tensor of the Bp anion and the average g tensor of NaBp · 2Tg, for which the crystal structure is unknown, the orientation of the symmetry axes of Bp with respect to the crystal axes could be calculated. Susceptibility measurements reveal ferromagnetic coupling in both KBp · 2Ttg and NaBp · 2Tg, whereas antiferromagnetic coupling exists in RbBp · 2Ttg. The results of alkali N.M.R. experiments are related to the molecular structure. A lower limit is found for the electrical conductivity of NaBp · 2Tg.     

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.     

Single‐crystal Raman spectroscopy of natural leiteite (ZnAs2O4) and comparison with the synthesised mineral

The oriented single‐crystal Raman spectrum of leiteite has been obtained and the spectra related to the structure of the mineral. The intensities of the observed bands vary according to orientation, allowing them to be assigned to either A_g or B_g modes. A_g bands are generally the most intense in the CAAC spectrum, followed by ACCA, CBBC, and ABBA whereas B_g bands are generally the most intense in the CBAC followed by ABCA. The CAAC and ACCA spectra are identical, as are those obtained in the CBBC and ABBA orientations. Both cross‐polarised spectra are identical. Band assignments were made with respect to bridging and non‐bridging As O bonds. Copyright © 2010 John Wiley & Sons, Ltd.     

Calorimetric fragility of maltitol

The heat capacity of maltitol has been measured with an adiabatic calorimeter for the crystal from 100 K to 425 K (T _m = 420 K), for the glass from 249 K to T _g (around 311 K) and for the liquid from T _g to 400 K. The heat of melting is 55.068 kJ/mol. The calorimetric glass transition occurs at about T _g = 311 K with a sudden jump of the heat capacity ΔC _p (T _g ) of about 243.6 J/(K mol). The excess entropy between the undercooled liquid and the crystal was calculated from the heat capacity data and was used to estimate the Kauzmann temperature T _K which was found 50 K below T _g . ΔC _p (T _g ) and T _K for maltitol were compared to other compounds like sugars, polyol and hydrogen bonded liquids. It has been found that the glass former maltitol is a "fragile" liquid on the thermodynamic point of view.