Luminescence of ion-irradiated α-quartz

Optical functionality of materials used in devices is the basis of modern photonics. It depends on selected photoactive impurities or low-dimensional structures, which can be tailored by ion implantation. The present survey covers cathode-luminescence spectroscopy performed after Ge, Ba, Na, Rb and Cs ion implantation in α-quartz, in connection with dynamic, laser-induced and chemical epitaxy of the surface layers amorphized during the ion irradiation. The correlations, which emerged for various luminescence bands, ion species and thermal processing methods, allows one to classify the bands into ion-specific and intrinsic ones. The microstructural properties measured by ion beam analysis and transmission electron microscopy will be combined with the luminescence data, and the role of photoactive defects in quartz and nanoparticles of the implants will be discussed. The technologically most attractive case of double Rb/Ge implantation will be highlighted, which combines the achievement of full chemical epitaxy (due to the alkali ions) and a high light output (due to Ge atoms). Finally, some investigations suggested to better understand the interplay between nanostructure and light emission in ion-doped quartz and the enormous potential for photonic application will be outlined.     

Au diffusion in α-Ti

The solubility and diffusion of Au in -Ti have been studied in a 823–1023 K temperature range using the Rutherford backscattering technique. For this purpose we have implanted Au into -Ti samples. Our results show that the solubility of Au varies between 0.2 and 0.35 at.%. In addition, we found that the diffusion coefficients follow a normal Arrhenius behavior with Q=260 kJ/mol and D _o=1.9×10^–5 m²/s¹. These values are typical for a substitutional diffusion mechanism.     

Lead diffusion in α-Zr

Pb diffusion in α-Zr matrix between 823 and 1123 K was measured using heavy ion Rutherford backscattering spectrometry (HIRBS) technique. A deviation from the Arrhenius law was observed, with two different regions. At low temperatures the activation energy Q is close to the expected value for a substitutional diffuser but the pre-exponential factor D₀ is higher than expected. Close to the phase transition temperature the opposite occurs, with a low Q value. This behavior is similar to the one observed for Hf and self-diffusion in α-Zr. Received: 29 September 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999     

4.2 GeV α-^12C Elastic Scattering Based on α-Particle Model

Based on the α-particle model of ^12C nucleus, the differential cross sections for α-^12C elastic scattering at incident energy of 4.2 GeV have been calculated within the framework of Glauber multiple scattering theory. The results show that the main features of the measured angular distribution of the cross sections can be reasonably described. The parameterized α-α scattering amplitude, which is the basic input to construct the α-^12 C scattering Glauber amplitude in the a-particle model, is obtained by fitting the α-α scattering data.     

α-Decay half-lives, α-capture cross sections, and α-nucleous interaction

The expression for the α-nuclear potential was found. This potential describes the α-decay half-lives for the ground states of nuclei and the α-particle capture cross sections for ⁴⁰Ca, ⁴⁴Ca, ⁵⁹Co, ²⁰⁸Pb, and ²⁰⁹Bi nuclei well. The potential was used to calculate the probabilities of α-transitions from the ground states of parent nuclei to different excited states of daughter nuclei. Simple analytical relations for calculating α-decay half-lives for transitions between the ground states of nuclei are determined.     

Zeeman effect in α-quartz

The helical crystal structure in α-quartz acts as the natural micro-solenoids for an electromagnetic wave passing through them, producing a longitudinal magnetic field in the direction of the optical axis. The longitudinal magnetic field further induces the Larmor frequency for the rotation of the bound electrons. The calculated Larmor frequency was experimentally confirmed by monitoring a line splitting of the infrared OH-band in the transmission spectra of α-quartz. A shift in the resonance frequency of the OH-band is equal to the Larmor frequency induced by the natural Zeeman effect.     

Topological Insulators in α-Graphyne

In this paper, we investigate topological phases of α-graphyne with tight-binding method. By calculating the topological invariant Z2 and the edge states, we identify topological insulators. We present the phase diagrams of α-graphyne with different filling fractions as a function of spin-orbit interaction and the nearest-neighbor hopping energy. We find there exist topological insulators in α-graphyne. We analyze and discuss the characteristics of topological phases of α-graphyne.     

Polymerization Mechanism of α-Linear Olefin

利用密度泛函理论在B3LYP方法6-31G水平上对α-直链烯烃在TiCl₃/AlEt₂Cl催化体系的链增长规律进行了相应的计算．对反应物、产物以及各种可能的中间体和过渡态进行了全参数优化,在同一理论水平上对势能面上的全部驻点进行了振动频率分析,并从过渡态分别向反应物和产物方向进行了内禀反应坐标计算．计算结果表明,在TiCl₃/AlEt₂Cl催化剂优化的构型上首先形成配位化合物,进而形成过渡态,过渡态与配位化合物的能量差为40.687 kJ/mol,最后是双键打开和Ti-C(4)断裂,从而完成整个聚合反应历程．计算结果也初步表明随着α-直链烯烃碳原子数的增加,链增长机理未发生本质变化．由聚合反应活化能与α-直链烯烃碳原子数的关系可以看出,碳原子数为6～10时反应活化能较低,是聚合反应合成减阻剂的最佳选择．     

Spin-assisted photoluminescence of polycrystalline α-MnSe

We have studied photoluminescence of α-MnSe in the antiferromagnetic and paramagnetic phases. Two broad, possibly multi-component emission bands centred at about 1.53 and 1.66 eV are observed at 20 K. The frequencies and intensities of these bands show step-like shift in the vicinity of T_N. On the basis of difference in peak energies at T=1.8 K and T_N the d-p-d superexchange interaction in excited state is estimated to be J_2g(exc)=0.918 meV.     

Diffusion of Hf in α-Zr

The diffusion of Hf in -Zr has been studied in the temperature range 773–1115 K for the first time using the Rutherford backscattering technique. For this purpose we have used 250 Å thick Hf films, which were deposited on two types of Zr samples. Our results show that in both cases, the diffusion coefficients follow curved Arrhenius plots. For the pure Zr material the diffusion coefficients are systematically lower than for Zr samples of lower purity. These results indicate that the Hf diffusion mechanism is at least partially of extrinsic character. Finally it should be mentioned that the present data lie significatively below the values of self-diffusion in -Zr.     

Diffusion of Ta in α-Ti

The diffusion of Ta in the hcp (α) phase of high-purity Ti (99.99%) was studied at different temperatures from 911 K up to 1123 K. The Rutherford Backscattering Spectrometry (RBS) and Heavy Ion RBS (HIRBS) techniques were used to obtain the penetration profiles. The evolution of the diffusion coefficient, D, as a function of temperature follows prediction of the Arrhenius law. The activation energy of the diffusion process is (318±7)kJ/mol, similar to that corresponding to self-diffusion in α-Ti. On the other hand, the measured values of D are systematically lower than those corresponding to self-diffusion by a factor of approximately 5. This reduction could be explained by taking into account the mass difference between Ta and Ti. An increase of the diffusion coefficient was measured when the diffusion proceeds on a less pure Ti (99.9%) matrix. This increment is higher at lower temperatures. Received: 12 November 2001 / Accepted: 12 March 2002 / Published online: 5 July 2002 RID="*" ID="*" RID="*" ID="*" RID="**" ID="**"Corresponding author. Fax: +54-11/6772-7362, E-mail: dyment@cnea.gov.ar RID="*" ID="*"Members of the Carrera del Investigador Científico del Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina     

Transport coefficients in superionic α-AgI

A phenomenological theory is given for the time dependence of the relaxation process of an α-AgI sample which has been previously polarized by the application of a voltage. The electromotive force caused by the internal electric field due to the polarized charge distribution is given by a series of exponential terms as a function of Dt, with D being the effective diffusion constant and t the time. The diffusion constants are obtained at several temperatures. In addition, the AC conductivity was measured for the same specimen. The results are analyzed with the help of a generalized Drude theory. Using both the results of the diffusion constant D and the AC conductivity, deviations from the Nernst-Einstein's relation are derived. Furthermore, computer simulations on these quantities were carried out by using well-established pairwise potentials. The agreement is satisfactory. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.     

Defect structures of ion-implanted α-tin

Single crystalline and polycrystalline -tin has been implanted at room temperature with 80-keV ions of radioactive^119m Sn,¹¹⁹Sb, and^119m Te. The radioactive nuclei decay to the Mössbauer level of¹¹⁹Sn. Mössbauer spectra of the emitted 24-keV radiation have been measured for different source temperatures by resonance counting techniques. Five individual lines in the spectra are characterized mainly by their isomer shifts and Debye temperatures. From these parameters the radiogenic¹¹⁹Sn atoms are concluded to be located in regular substitutional and interstitial lattice sites and in defect complexes. Simple models for the defects are proposed: A Sn-vacancy pair consists of Sn atoms on (nearly) substitutional sites with a dangling bond into an adjacent vacancy. In a complex oxygen-containing defect the Sn atoms have approximately a 5s ² configuration withp-bonds to two nearest neighbour atoms. Sn atoms, having an atomic 5s ² 5p ² configuration and large vibrational amplitudes, are concluded to be in non-bonding regular interstitial sites. For special implantation conditions minor fractions of SnO₂ molecules are formed in the bulk. The interstitial¹¹⁹Sn and the¹¹⁹Sn-vacancy pairs are proposed to represent elementary point defects in -tin. Conclusions are also drawn concerning the lattice location and the defects created in the implantation process of the implanted parent isotopes.     

Low-dimension self-interstitial diffusion in α-Zr

The mobility of self-interstitials in α-zirconium (α-Zr) is studied with molecular dynamic (MD) and molecular static (MS) simulations, using Ackland’s many-body inter-atomic potential. The basal crowdion configuration is found to be the ground state. Four types of diffusion jumps can be identified via MS, in-plane in-line, in-plane off-line, out-of-plane in-line and out-of-plane off-line. The in-plane migration is dominated by one-dimensional crowdion motion along the [110] directions, interrupted from time to time by off-line or out-of-plane jumps. Based on the MS results, the activation energies and pre-exponentials for the diffusion processes are determined by fits to the Arrhenius plots of D_c and D_a. The diffusional anisotropy factor D_c/D_a is also obtained, and compares well with experimental results. The mean frequency of each type of jumps is then found using Monte Carlo simulation, and is reported as a function of temperature. The mean lifetime and mean free path of the one-dimensional mobility are then obtained. The 1-D mean free path is found to be unimportant for sink separations involved under the usual irradiation damage conditions. Received: 4 March 2002 / Accepted: 4 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. E-mail: Chung.Woo@polyu.edu.hk     

Magnetotransport and superconductivity of α-uranium

We have measured the electrical resistivity, magnetoresistance and Hall effect on several new single-crystal samples and one polycrystalline sample of α-U. The residual resistivity ratios of these samples vary from 13 to 315. Matthiessen's law appears to hold above the onset of the charge-density wave phase transitions that begin near 43?K, but not below this temperature. Sharp features at all three charge-density wave transitions are observed and the effects of high magnetic fields on them are presented and discussed. The magnetoresistance is anisotropic, reaches 1000% at 2?K and 18?T and does not exhibit Kohler scaling. The Hall coefficient is positive, independent of magnetic field and slightly temperature dependent above about 40?K in agreement with earlier studies. Below 40?K the Hall coefficient changes sign as the temperature falls, varies with field and becomes much more strongly negative at the lowest temperatures than has been reported. Some of our results suggest that a spin-density wave may coexist with the charge-density wave states. Superconductivity is observed in two of our samples; we argue that it is intrinsic to α-U and suggest that it is consistent with a two-band model. Several parameters characterizing the transport and superconductivity of α-U are estimated.     

Nb diffusion in ferromagnetic α-Fe

The effect of the magnetic ordering on the diffusion of Nb in α-Fe was investigated. Measurements were made in the 673–1073 K temperature range using secondary ion mass spectrometry as analysis tool. By expanding the temperature range with respect to the previous works, the Arrhenius plot shows the variation of the slope at the Curie temperature and a soft-upward curvature in the ferromagnetic region. The curvature observed is well fitted by the model developed by Ruch et al. The parameter that takes into account the influence of the magnetic order in the vacancy formation and mobility during the Nb diffusion process is quite similar to that observed in other non-magnetic diffusers. A possible explanation for such an effect is given, supported by ab initio calculations. PACS  66.30.-h; 66.30.J-; 67.80.dk; 85.40.Ry     

Annealing behaviour of C-implanted α-Hf

The annealing behaviour of C in the group IVa metal Hf has been studied in the previously uninvestigated temperature range 650–800°C using ion-beam techniques. Diffusion couples were created by ion implantation. The time evolution of the carbon profiles were monitored by the use of the Nuclear Resonance Broadening (NRB) technique. The linear Arrhenius plot extracted from the measurements indicates that the diffusivity of implanted C in polycrystalline h.c.p. -Hf can be described by the activation energyQ=3.9±0.2 eV and the pre-exponential factor D₀=(50± ₄₇ ¹²⁰ )×10⁵ cm²/s. The implanted distributions were used to determine the solubility of C in -Hf for the first time.