Investigation of the structure and luminescence properties of CdF2-CaF2 : Eu superlattices on Si(111)

A complex investigation of CdF₂-CaF₂ : Eu superlattices with different bilayer thicknesses (2.0–17.5 nm) grown by molecular beam epitaxy on Si(111) has been carried out. The structural perfection of the layers and interfaces of the superlattices have been estimated from the X-ray diffractometry and reflectometry data. The possibility of producing short-period pseudomorphic superlattices with a period of approximately 2 nm has been established. It has been shown that these superlattices are characterized by a larger root-mean-square roughness amplitude of the interfaces as compared to the long-period superlattices. The specific features of cathodoluminescence spectra have been analyzed as a function of the superlattice period. It has been revealed that, with a decrease in the superlattice period, the intrinsic luminescence intensity of fluorides increases in comparison with the intensity of the luminescence associated with the emission of Eu²⁺ impurity ions; in this case, several Eu³⁺ luminescence bands appear in the spectrum. The possibilities of electron probe microanalysis for determining the ratio of thicknesses of individual layers in short-period superlattices have been demonstrated.     

IR spectroscopy of the structural states of ethylene in aerogel nanopores

We have investigated for the first time the combination vibrational absorption bands ν₅ + ν₉ and ν₁ + ν₁₁ of ethylene adsorbed by nanopores of a silicon dioxide aerogel in the 5800–6300 cm^–1 region. Our measurements show significant differences in the absorption spectra of gas-phase ethylene and ethylene found in the nanoporous structure of the aerogel, including a change in the shape of the absorption band contours, their frequency shift, and an increase in the absorption intensity.     

Vibrational studies of lithium-intercalated SnSySe2−y (0≤y≤2) layered compounds

We report the vibrational properties of lithium-intercalated SnS_ySe_2−y (0≤y≤2) layered compounds. Infrared absorption spectra have been recorded as a function of temperature in the frequency range 50–600 cm⁻¹. The new bands are interpreted as vibrational modes of lithium atoms against the nearest-neighbouring-chalcogen atoms located in the van der Waals gap. Results are analyzed in terms of stretching vibrations of Li-S₄ and Li-S₆ entities. From a comparison of the spectra, it is concluded that lithium ions are more likely to occupy tetrahedral sites in the lithium-rich phases. Using a simple model of lattice dynamics, the coupling force constants are determined and compared with those of the pristine materials. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Microscopic study of low-lying yrast spectra in <Superscript>100–108</Superscript>Mo isotopes

Variation-after-projection (VAP) calculations in conjunction with Hartree-Bogoliubov (HB) ansatz have been carried out for A=100−108 molybdenum (Mo) isotopes. In this framework, the yrast spectra with J _max ^π ≥10⁺, B(E2) transition probabilities, quadrupole (β₂) and hexadecapole (β₄) deformation parameters, moment of inertia (I) and square of cranking frequency (ω²) for even-even Mo isotopes have been obtained. The results of the calculation give an indication that it is important to include the hexadecapole-hexadecapole component of the two-body interaction for obtaining various nuclear structure quantities in these Mo isotopes.     

Annealing temperature dependence of Raman scattering in Si/SiO2 superlattice prepared by magnetron sputtering

Si/SiO₂ superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 °C is larger than that of nanocrystals annealed at 850 °C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm⁻¹ decreases, and the intensity of that at 520 cm⁻¹ increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm⁻¹ shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO₂ superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images.     

Near-barrier fusion reactions induced by α on197Au,193Ir,191Ir,185Re,181Ta,121Sb and69Ga nuclei

Fusion-evaporation cross-sections for the α-induced reactions upon¹⁹⁷Au,¹⁹³Ir,¹⁹¹Ir,¹⁸⁵Re,¹⁸¹Ta,¹²¹Sb and⁶⁹Ga nuclei at bombarding energies near the Coulomb barrier have been measured by off-line observation of the γ-rays emitted in the radioactive decay of the residual nuclei using stacked foil technique. The total fusion cross-section for the systems have been compared with simple statistical model calculations using the code ALICE/91 as well as with the coupled channel calculations that include the β₂ and ν₄ slatic deformations and dynamic couplings of the vibrational/rotational states of the target and the projet tile using the code CCDEF.     

A symmetry adapted approach to vibrational excitations in atomic clusters

An algebraic method especially suited to describe the strongly anharmonic vibrational spectra in molecules may be an appropriate framework to study the vibrational spectra of Na _n ⁺ clusters, where nearly flat potential energy surfaces and the appearance of close lying isomers have been reported. As an illustration we describe the model and apply it to the Be₄, H ₃ ⁺ , Be₃ and Na ₃ ⁺ clusters. Presented by A. Frank at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997. This work was supported in part by the European Community under contract No. CI1*-CT94-0072, DGAPA-UNAM under project IN101997 and Spanish DGCYT under project PB92-0663.     

α-, β-和γ-Si3N4 高压下的电子结构和相变: 第一性原理研究

本文采用第一性原理框架下的赝势平面波方法结合振动类德拜模型研究了α,β和γ-Si₃N₄在高温下的点阵常数,弹性常数和弹性模量.研究发现三种同质异相体的体模量都很高.β-Si₃N₄在低温下表现出脆性,在高温下则表现出延展性.γ-Si₃N₄在低温和高温下都是脆性的共价化合物.β → γ 相变的相界斜率为正值,说明在较高温度时合成γ-Si₃N₄所需的压强也较高.α → γ 相变的相界可以表示成 P=16.29- 1.835-10^-2 T+9.33945-10^-5T²-2.16759-10^-7T³+2.91795-10^-10T⁴.本文还分析了Si₃N₄同质异相体在高压下的态密度和能带.在α-Si₃N₄中主要是Si-s, p和N-s,p的轨道杂化对晶体的稳定性起作用.α和β-Si₃N₄都具有Γ_V-Γ_C类型的间接带隙(分别是4.9~eV和4.4~eV)而γ-Si₃N₄具有直接带隙(3.9~eV). 研究还发现α-Si₃N₄和β-Si₃N₄的价带顶分别沿着Γ-M和Γ-A方向.本文的计算结果和已有的实验数据是一致的.     

A spectroscopic study of M@C82 metallofullerenes: Raman, far-infrared, and neutron scattering results

82 metallofullerenes have been studied at room temperature by Raman (for M=La, Y, Ce, Gd), far-infrared (FIR) (for M=La, Y, Ce), and inelastic neutron scattering (INS) (for M=La, Y) spectroscopy. Raman and FIR spectra suggest that these metallofullerenes have a common dominant, if not a single, structure of the C₈₂ cage and a similar bonding of the encapsulated metal ion, i.e. the bonding is primarily electrostatic and the metal atoms are in the same oxidation state (+3). The metal ion vibrations are located around 160 and 50 cm^-1. INS reveals no gap between internal vibrational and external vibrational and rotational modes in the range ∼50–200 cm^-1 as is typically observed for other fullerides and also predicted by our model calculations. Presumably this is due to strong intermolecular interactions between M@C₈₂ units in the bulk sample. The studied metallofullerenes are air sensitive, and degradation in air could be followed by changes in the Raman spectra. Received: 24 August 1997/Accepted: 26 September 1997     

Structure-sensitive changes in the terahertz absorption spectra of merocyanine dye derivatives

The low-frequency vibrational modes of the series of merocyanines (malononitrile derivative) have been investigated by pulsed terahertz spectroscopy. The terahertz absorption spectra are shown to contain both intermolecular and intramolecular vibrational modes in the range of 0.15–3.45 THz (5–115 cm⁻¹). An unambiguous correlation is established between the purposeful modification of the molecular structure of merocyanine dyes and the change in their terahertz absorption spectra.     

Inhomogeneous ferromagnetic resonance modes in [Fe/Cr]n superlattices with a high biquadratic exchange constant

In a set of [Fe/Cr]_n superlattices, magnetization curves and spectra of ferromagnetic resonance under an in-plane magnetic field have been studied at room temperature. Along with the acoustic branch, several additional branches have been observed in resonance spectra. Resonance spectra have been calculated analytically for a structure with an infinite number of layers and numerically for finite numbers of layers in real samples using a model of biquadratic exchange taking account of the fourth-order magnetic anisotropy. A possibility of describing both static and resonance properties of the system in terms of this model has been demonstrated. Zh. éksp. Teor. Fiz. 116, 1817–1833 (November 1999)     

Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag₄ clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag₄ ^-/Ag₄/Ag₄ ⁺ system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag₄. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm^-1 and an anharmonicity χ _e2νχ _eχ _e of 2.65 cm^-1 for the active vibrational mode. Received 30 November 2000     

Thermoelectric power in superlattices of nonparabolic semiconductors with graded interfaces under magnetic quantization

Summary We study the thermoelectric power of the electrons under magnetic quantization in III–V, II–VI, PbTe/PbSnTe and strained layer superlattices with graded interfaces and compare the same with the corresponding bulk specimens of the constituent materials by formulating the respective expressions incorporating the broadening. It is found, by taking GaAs/Ga_1−x Al _x As, CdS/CdTe, PbTe/PbSnTe and InAs/GaSb superlattices with graded interfaces as examples, that the thermoelectric power exhibits oscillatory dependence with the inverse quantizing magnetic field due to Shubnikov-de Hass effect and increases with decreasing electron concentration in an oscillatory manner in all the aforementioned cases. The thermopower in graded superlattices is greater than that of constituent bulk materials together with the fact that the oscillations in superlattices show up much more significantly as compared to the respective constituent materials. In addition, the well-known expressions for bulk specimens of wide-gap semiconductors have also been obtained as special cases from our generalized expressions under certain limiting conditions.     

Fine structure of the strength function for β<Superscript>+</Superscript>/<Emphasis Type="Italic">EC</Emphasis> decay of <Superscript>160<Emphasis Type="Italic">g</Emphasis></Superscript>Ho (25.6 min)

A strength function for the β⁺/EC decay of the deformed ^160g Ho (25.6 min) nucleus has been obtained from experimental data. The fine structure of the strength function S _β(E) is analyzed. It is found to have a pronounced resonant structure for Gamow-Teller transitions. In S _β(E) with μ_τ = +1 the Gamow-Teller resonance has been observed to split into two components. This splitting is associated with anisotropy of isovector density oscillation in deformed nuclei. The β⁺/EC strength function for first-forbidden transitions is obtained in the Coulomb (ξ) approximation. It is shown that S _β(E) for first-forbidden transitions does not have a pronounced resonant structure. The text was submitted by the authors in English.     

Growth of thin, crystalline oxide, nitride, and oxynitride films on NiAl and CoGa surfaces

At 300 K, an amorphous Al-oxide film is formed on NiAl(001) upon oxygen adsorption. Annealing of the oxygen-saturated NiAl(001) surface to 1200 K leads to the formation of thin well-ordered θ-Al₂O₃ films. At 300 K, and low-exposure oxygen atoms are chemisorbed on CoGa(001) on defects and on step edges of the terraces. For higher exposure up to saturation, the adsorption of oxygen leads to the formation of an amorphous Ga-oxide film. The EEL spectrum of the amorphous film exhibits two losses at ≈400 and 690 cm^-1. After annealing the amorphous Ga-oxide films to 550 K thin, well-ordered β-Ga₂O₃ films are formed on top of the CoGa(001) surface. The EEL spectrum of the β-Ga₂O₃ films show strong Fuchs-Kliewer (FK) modes at 305, 455, 645, and 785 cm^-1. The β-Ga₂O₃ films are well ordered and show (2×1) LEED pattern with two domains, oriented perpendicular to each other. The STM study confirms the two domains structure and allows the determination of the two-dimensional lattice parameters of β-Ga₂O₃. The vibrational properties and the structure of β-Ga₂O₃ on CoGa(001) and θ-Al₂O₃ on NiAl(001) are very similar. Ammonia adsorption at 80 K on NiAl(111) and NiAl(001) and subsequent thermal decomposition at elevated temperatures leads to the formation of AlN. Well-ordered and homogeneous AlN thin films can be prepared by several cycles of ammonia adsorption and annealing to 1250 K. The films render a distinct LEED pattern with hexagonal [AlN/NiAl(111)] or pseudo-twelve-fold [AlN/NiAl(001)] symmetry. The lattice constant of the grown AlN film is determined to be a_AlN= 3.11 Å. EEL spectra of AlN films show a FK phonon at 865 cm^-1. The electronic gap is determined to be E_g= 6.1±0.2 eV. GaN films are prepared by using the same procedure on the (001) and (111) surfaces of CoGa. The films are characterized by a FK phonon at 695 cm^-1 and an electronic band gap E_g= 3.5±0.2 eV. NO adsorption at 75 K on NiAl(001) and subsequent annealing to 1200 K leads to the formation of aluminium oxynitride (AlON). An oxygen to nitrogen atomic ratio of ≈2:1 was estimated from the analysis of AES spectra. The AlON films shows a distinct (2×1) LEED pattern and the EEL spectrum exhibits characteristic Fuchs-Kliewer modes. The energy gap is determined to be E_g= 6.6±0.2 eV. The structure of the AlON film is derived from that of θ-Al₂O₃ formed on NiAl(001). Received: 21 March 1997/Accepted: 12 August 1997     

Quantum-well structured high-Tc superconductors prepared by soft-chemical method

We have synthesized quantum-well structured high-T_c superconductors through soft-chemical processes, where Bi-based cuprate lattices are regularly interstratified with atom, molecule, and ionic salt. In these materials, superconducting (S) and nonsuperconducting (I) layers are alternately stacked to form superlattices with atomically clear-cut interfaces between them. The intercalation technique, which means a reaction of incorporating chemical species (guest) into the pre-synthesized layered compound (host), could provides an easy route to superconducting superlattices with molecular-level layer-by-layer structural feature. The joint analysis of temperature-dependent magnetic susceptibilities and X-ray absorption spectroscopy indicated that superconducting transition temperature is predominantly governed by the electronic structure of superconductive CuO₂ plane.     

Millimeter and submillimeter-wave spectroscopy of silicon difluoride

The rotational spectra of ²⁸SiF₂, ²⁹SiF₂, and ³⁰SiF₂ in their ground vibrational states, as well as those of ²⁸SiF₂ in the v₁ = 1, v₂ = 1, v₃ = 1, and v₂ = 2 excited states have been studied in selected frequency regions between 80 and 700 GHz. Transitions involving a large range of quantum numbers have been observed, so that precise rotational and quartic centrifugal distortion constants could be determined for each of the spectra investigated. In addition, the complete set of sextic distortion constants was also obtained for the most abundant isotopomer in its ground vibrational state. The quadratic and cubic force constants of silicon difluoride have been refined by a least-squares procedure using a larger and more precise set of data.     

An approximation method for evaluating centrifugal distortion constants in XY2 bent symmetric nonhydride molecules

An approximation method is preser ted for an easier evaluation of centrifugal distortion constants τ_αβγδ in XY₂ bent symmetric ronhydride molecules, employing the known extremal properties of the intramolecular force constants. The formulas for τ_αβγδ thus obtained involve only atomic masses, geometry and normal vibrational frequencies of the molecule. The centrifugal distortion constants calculated under this approximation compare very well with experimental values obtained from the microwave spectrum.     

Optical investigations of Mn2+ ion in Cs2CdCl4 and CsCaCl3 single crystals

Summary Polarized absorption, emission and excitation spectra of Cs₂CdCl₄∶Mn²⁺ and emission as well as excitation spectra of CsCaCl₃∶Mn²⁺ single crystals at temperatures down to 77 K are reported. The general features of the spectra are fruitfully analysed within the framework of the strong field scheme of Tanabe and Sugano including Trees' correction. Some finer structure of the crystal field independent levels are observed which are rationalized in terms of vibrational mechanism. A detailed analysis of the spectra indicates octahedral site symmetry around the central metal ion. The authors of this paper have agreed to not receive the proofs for correction.     

Influence of size effects on <Emphasis Type="Italic">para</Emphasis>-dichlorobenzene lattice dynamics

We have obtained experimental low-frequency Raman spectra for light scattering by nanoparticles of para-dichlorobenzene of size ∼300–70 nm. As their sizes decrease, the line frequencies decrease. The size of the nanoparticles was determined using an electron microscope. We found that in the lattice vibrational spectrum of nanoparticles of size 300–70 nm, a summed spectrum appears from α-para-dichlorobenzene and β-para-ichlorobenzene. This is consistent with molecular dynamics calculations of the structure of the nanoparticle and calculations of the histograms for the lattice vibrational spectra by the Dean method.