Magnetic behaviour of isolated Fe and Ni ions measured directly after implantation into II-VI-semiconductors

Abstract

The local magnetic behaviours of isolated Fe and Ni impurities in the II-VI semiconductors ZnS and ZnTe have been studied directly after recoil implantation. From the measured magnetic properties, which are detected by the observation of the perturbed angular distribution, orbital contributions to the hyperfine field as well as contributions from spin magnetism are found to be present at both impurities. The interpretations on the basis of the intermediate ligand field model suggest that Fe and Ni ions are found to exist in 2+ and in 1+ states and that electronic excitations may play a significant role even microseconds after the implantation.     

Intermediate energy (e, 2e) processes on C60: A distorted wave Born approximation study

Triple differential cross-sections (TDCS) for ( e ,2 e ) processes on C₆₀ have been calculated in the plane wave Born and distorted wave Born approximations using a jellium shell model to describe the target valence states. The peculiarities of these TDCS are demonstrated by comparison with results for atomic hydrogen. Ionisation into a resonant state leads to dramatic modifications of the TDCS. This effect could also be observed in a surface ( e ,2 e ) experiment in specular geometry using a thin film of physisorbed C₆₀. Received 14 April 2000 and Received in final form 27 July 2000     

Intermediate range order in supercooled water

ABSTRACT

The temperature dependence of the heights of the first and second x-ray diffraction peaks in supercooled water measured down to 244?K are found to display very different behaviours. While the first peak intensity remains essentially constant, the second peak increases strongly with decreasing temperature. In real space this is concomitant with the reduction of the number of non-bonded interstitial molecules between the first and second shells. It is found that although the first O-O shell in supercooled water is unchanged upon supercooling, the variations in intermediate range order are mainly associated with the growth of a predominantly tetrahedral network that is distinctly different from ice-Ih. Moreover, in this temperature regime we find a direct correlation between the height of the second diffraction peak and the intensity changes in the 2nd, 3rd, 4th and 5th peaks in the oxygen-oxygen pair distribution function.     

Density dependence of mean kinetic energy in liquid and solid hydrogen at 19.3 K

We have measured, using the TOSCA spectrometer at ISIS, the inelastic incoherent neutron scattering spectrum of liquid and solid para-hydrogen along the T = 19.3 K isotherm. From the high-energy region of the spectrum, where the Impulse Approximation for the Centre of Mass motion applies, we have been able to extract the mean translational kinetic energy, which, as expected, turns out rather different from the classical value and density dependent. We find that the density behaviours in the liquid and the solid phase are slightly different. This confirms a similar feature already observed in liquid and solid helium at T = 6.1 K [M. Celli, M. Zoppi, J. Mayers, Phys. Rev. B 58, 242 (1998)]. The spectra from the solid phase have been also analysed in the low-energy region and allowed us to derive the Debye-Waller factor of solid para-hydrogen as a function of density. The comparison with the available experimental data in the literature is rather good and confirms the excellent performances of TOSCA in the spectroscopic analysis of the condensed phases of para-hydrogen.     

Angular Dependence of L X-ray Relative Intensities of Uranium and Thorium at 59.5 keV

INTRODUCTION

The knowledge of relative intensities of K, L and higher shell X-rays is needed for the investigation of various phenomena in atomic physics. In earlier studies, L X-ray relative intensities and energies have been reported for many elements by several investigators [1–5], but the angular dependence of L X-ray relative intensity has not been reported. According to Flugge et a1 [6], if the angular momentum J=1/2 and J>1/2 of the originating shell, then the angular distribution of X-rays is isotropic and anisotropic, respectively. From this idea, L_α, L_l and some of L_β, intensities are dependent on changing the scattering angles. In this study, we have measured the angular dependence of L_α, La_l, L_α/L_β, and L_α/L_γ, intensity ratios of U and Th at different angles varying from 45° to 135°.     

Neutron scattering on the strongly correlated electron CeNi Cu system: from non-magnetic behaviour to long-range magnetic order

The ground state of the strongly correlated electron CeNi_1-xCu_x compounds has been investigated by means of neutron scattering experiments. Thus, magnetic diffraction was performed for compounds showing long-range magnetic order (x > 0.2). An evolution from a collinear ferromagnetic structure for x =0.6 to a simple antiferromagnetic one for CeCu takes place through some more complex magnetic structures for intermediate compositions. The magnetic moments are continuously reduced when the Ni content increases reflecting the progressive enhancement of the Kondo screening. The large reduction found for x =0.6 compound is discussed and the existence of a spin glass like component of the magnetic moment cannot be discarded. From the quasielastic spectra, we have obtained the Kondo temperatures which are close to the magnetic ordering ones. The quasielastic line-width evolves from a linear temperature dependence to a T ^1/2 behaviour when approaching the non-magnetic limit. Then, this system provides an interesting example for the evolution of unstable 4 f shell relaxation regimes when modifying the hybridisation strength. Received 22 May 2000     

Structure and melting of dipole clusters

Two-dimensional microclusters made up of particles repelled by the dipole law and confined by an external quadratic potential are considered. The model describes a number of physical systems, in particular, electrons in semiconductor structures near a metallic electrode, indirect excitons in coupled semiconductor dots etc. Two competing types of particle ordering in clusters have been revealed: formation of a triangular lattice and of a shell structure. Equilibrium configurations of clusters with N=1–40 particles are calculated. Temperature dependences of the structure, potential energy, and mean-square radial and angular displacements are studied. These characteristics are used to investigate cluster melting. Melting occurs in one or two stages, depending on N. Melting of a two-shell microcluster takes place in two stages: at low temperatures—from the frozen phase to a state with rotationally reoriented “crystalline” shells with respect to one another, followed by a transition involving breakdown of radial order. Melting in a cluster made up of a larger number of shells occurs in one stage. This is due to the fact that the potential barrier to intershell rotation is substantially lower than that to particle jumping from one shell to another for small N, and of the same order of magnitude for large N. A method is proposed for predicting the character of melting in shell clusters by comparing the potential barriers for shell rotation and intershell particle jumping. Fiz. Tverd. Tela (St. Petersburg) 40, 1379–1386 (July 1998)     

Effects of void shape and orientation on the elastoplastic properties of spheroidally voided single-crystal and nanotwinned copper

ABSTRACT

Void nucleation, growth and coalescence have been identified as the leading cause of ductile damage in metallic materials. To understand the underlying deformation and damage mechanisms, extensive theoretical, experimental and simulation efforts have been attempted on spherically voided metals. In this work, molecular dynamics simulations are performed to analyze the uniaxial straining deformation behaviours of both single-crystal and nanotwinned copper materials embedded with a preexisting spheroidal void. The coupling effects among twin boundary, spheroidal void aspect ratio and orientation on unidirectional elastoplastic behaviours are systematically examined. The dislocation-induced plastic deformation mechanism is also examined and compared with the one due to a perfectly spherical cavity. Simulation results show that elastic modulus increases with both spheroidal void aspect ratio and orientation. So do the yield stress, the first peak stress and the plasticity index. Another peak stress exists for most cases, except for a prolate void embedded in nanotwinned specimens. The slope between peak stresses decreases with both the spheroidal aspect ratio and orientation. The incorporation of a twin boundary results in lower elastic modulus, higher yield strength and smaller plasticity index. For an oblate void, the twin boundary gives rise to more severe strain softening behaviour. The dislocation extraction algorithm illustrates that the continuous nucleation, propagation and reaction of dislocations emanated from both the void front and twin boundary are responsible for the ductile damage of spheroidally voided crystals. The lower dislocation densities found in nanotwinned specimens indicate the desired suppression effects of twin boundary on dislocation activities.     

Study of the 90Zr(p,α)87Y reaction at 22 MeV

The (p,α) reaction on ⁹⁰Zr has been studied in a high resolution experiment at an incident proton energy of 22 MeV. The cross section and asymmetry angular distributions for transitions to 36 levels of ⁸⁷Y with an excitation energy up to 3 MeV have been measured. DWBA analyses of experimental angular distributions, using either Woods-Saxon or Double Folded potentials for the exit channel, have been done, allowing either the confirmation of previous spin and parity values or the assignment of new spin and parity to a large number of states. The structure of low lying states of ⁸⁷Y has been studied in the framework of the shell model, using the OXBASH code. With the interaction PMM90 reasonable agreement is obtained for part of the negative parity spectrum. Received: 17 November 1997     

Simplified components of the spin-other-orbit interaction near the middle of the atomic f shell

To honour the memory of Brian Garner Wybourne, an analysis is presented of three components of the spin-other-orbit interaction for f electrons using the kind of Lie groups he would have been familiar with. The components have been named z ₆, z ₈ and z ₁₀. They all belong to the irreducible representation (IR) (30) of Racah’s group G₂. Near the middle of the f shell it is often found that fewer independent blocks of numbers are needed to express their matrix elements than the Wigner–Eckart theorem, generalized to the IRs U of G₂, would indicate. Each block corresponds to a given U and U?′, and possesses rows and columns labelled by the angular momenta L and L′. The number of independent blocks would be expected to be given by Racah’s multiplicity function c(UU?′ (30)); but near the middle of the shell the number c(UU?′ (20)) (or less) often suffices. For this to occur, z ₈ and z ₁₀ have to be replaced by linear combinations corresponding to IRs of the types (20)×(10) and (21)×(10) of the direct product group G_2A×G_2B, where A and B refer to electrons with their spins up (A) and spins down (B). A detailed example is provided by the IR (31) of G₂, which occurs in the configurations f ⁵ through f ⁹. In addition, two antiHermitian operators (z _a6 and z _a7) that also belong to the IR (30) of G₂ are discussed.     

EPR of radiation defects in LiBaF 3 crystals

EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S =1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F -centre model, taking reduced hfs values of the F -centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of the g -tensor with its main axis along the [1 v 0 v 0] axis of the crystal.     

Theoretical study of the A 3 0 + ← X 1 0 + and B 3 1 ← X 1 0 + transitions in the Cd-rare gas van der Waals molecules

Excitation spectra arising from A ³ 0 ⁺ ← X ¹ 0 ⁺ and B ³ 1 ← X ¹ 0 ⁺ electronic transitions in the Cd-rare gas (RG) van der Waals molecules are calculated using newly obtained theoretical potential curves for these species. In the molecular structure calculations, Cd²⁰⁺ and RG⁸⁺ cores are simulated by energy-consistent pseudopotentials which also account for scalar-relativistic effects and spin-orbit (SO) interaction within the valence shell. Potential energies in the Λ S coupling scheme have been obtained by means of ab initio complete-active-space multiconfiguration self consistent-field (CASSCF)/complete-active-space multireference second-order perturbation theory (CASPT2) calculations with a total 28 correlated electrons, while the SO matrix has been computed in a reduced CI space restricted to the CASSCF level. The final Ω potential curves are obtained by diagonalization of the modified SO matrix (its diagonal elements before diagonalization substituted for the corresponding CASPT2 eigen-energies). The spectroscopic parameters for the ground and several excited states of the Cd-RG complexes deduced from the calculated potential curves are in quite reasonable agreement with available experimental data. In addition, the radial Schr?dinger equation for nuclear motion was solved numerically with the calculated potentials to evaluate the corresponding vibrational levels and radial wavefunctions. The latter have been used in the calculation of the appropriate Franck-Condon factors to yield information on relative intensities of the vibrational bands of the Cd-RG complexes. The theoretical vibrational progressions are discussed in the context of experimental spectra. Received 10 August 2000 and Received in final form 7 November 2000     

Hydrogen bonding and clusters in supercritical methanol–water mixture by neutron diffraction with H/D substitution combined with empirical potential structure refinement modelling

ABSTRACT

Neutron diffraction measurements of H/D isotopic substitution have been performed for seven H/D substituted methanol-water mixtures of 0.3?mol fraction of methanol (x_M) under the supercritical (618?K, 100?MPa) and ambient (298?K, 0.1?MPa) conditions. The seven structure factors obtained were subjected to an empirical potential structure refinement (EPSR) modelling to derive all site-site pair correlation functions, coordination number distributions, spatial density functions, and cluster distributions. Water has a four coordinated structure in the first coordination shell under both ambient and supercritical conditions; however, the spatial density distribution of water molecules in the second coordination shell is delocalised under the supercritical condition. The mean coordination number of all atomic pairs with hydrophilic interactions decreases in the supercritical state. On the other hand, the mean coordination number of interactions between the hydrophobic part of methanol and water molecule is less sensitive to temperature. In the supercritical condition, water clusters with a wide size distribution are generated in a methanol-water mixture as well as in pure water. Since the critical temperature of a methanol-water mixture is lower than that of pure water, it can be concluded that the addition of methanol can generate fragment water clusters at a lower temperature.     

Theoretical study of the SrLi+ molecular ion: structural,electronic and dipolar properties

ABSTRACT

The structural and electronic properties of (SrLi)⁺ molecular ion have been determined by the use of ab initio approaches. Potential energy curves (PECs) with their spectroscopic constants (R_e, D_e, ω_e, T_e, B_e and ω_eχ_e) have been calculated. Also, the vibrational properties and the electric dipole moments, either permanent (PDM) or transition (TDM) ones, have been investigated and analysed. Large Gaussian basis sets, the full valence configuration interaction (FCI) and the formalism of non-empirical pseudo-potential including the two approaches, effective core potential (ECP) and core polarisation potential (CPP), are analysed. Therefore, (SrLi)⁺ is considered as a two effective electrons system. Numerous excited states of symmetries ^1,3Σ⁺, ^1,3Π and ^1,3Δ dissociating below the ionic limit Sr²⁺Li^? have been investigated. This study shows interesting behaviours around the avoided crossing related to charge transfer involving important processes in physics and astrophysics such as dynamics, pre-dissociation and inelastic transitions.     

Technical Reports: Atoms to Aerosols—The Chemical Dynamics Beamline

Chemical dynamics is the study of the elementary processes and interactions in chemistry. Fundamental properties such as dipole moments, ionization energies, electron affinities, proton affinities, and electronic structure all contribute to the photochemistry, radiationless processes and reactivity underlying all physical processes. The making and breaking of chemical bonds, and the energy partitioning in chemical systems after transformation, are also in the domain of chemical dynamics.

The valence shell, the outermost shell of electrons in a system, contributes most to the physical properties of material. It is these electrons that are shared in covalent bonding, transferred in ionic systems, coupled to form bands in bulk material, and interact most strongly with the environment.     

Core–Shell Nanoparticles Driven by Surface Energy Differences in the Co–Ag,W–Fe,and Mo–Co Systems

Core–shell nanoparticles are known to form in binary systems using a one‐step gas‐condensation deposition process where a large, positive enthalpy of mixing provides the driving force for phase separation and a difference in surface energy between component atoms creates a preferential surface phase leading to a core–shell structure. Here, core–shell nanoparticles have been observed in systems with enthalpy as low as ?5 kJ mol^?1 and a surface energy difference of 0.5 J m^?2 (Mo–Co). This suggests that surface energy dominates at the nanoscale and can lead to phase separation in nanoparticles. The compositions and size dependence of the core–shell structures are also compared and no core–shell structures are observed below a critical size of 8 nm.     

Quasiclassical description of electronic supershells in simple metal clusters

A quasiclassical method for calculating shell effects, which has been used previously in atomic and plasma physics, is used to describe electronic supershells in metal clusters. An analytical expression is obtained, in the spherical jellium model, for the oscillating part of the binding energy of electrons of a cluster as a sum of contributions from supershells with quantum numbers 2n _r +l, 3n _r +l, 4n _r +l,... This expression is written in terms of the classical characteristics of the motion of an electron with the Fermi energy in a self-consistent potential. The conditions under which a new supershell appears and the relative contribution of this shell are investigated as a function of the cluster size and form of the potential. Specific calculations are performed for a “square well” of finite depth. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 333–337 (10 September 1999)     

Investigation of the effect of large-scale shell asymmetry on Iskra-5 target operation

Experiments designed to investigate the effect of a controlled large-scale asymmetry of a shell containing deuterium-tritium fuel on the shell compression and neutron production under indirect (x-ray) action are performed on the Iskra-5 laser fusion device. The uniformity of the x radiation near the target is not worse than 3%, and the shell asymmetry is varied from 30 to 100%. The observed decrease of the experimental neutron yield as compared with experiments using symmetric targets is in satisfactory agreement with two-dimensional calculations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 161–165 (10 February 1998)     

Composition,temperature and geometric dependent hysteresis behaviours in Ising-type segmented nanowire with magnetic and diluted magnetic,and its soft/hard magnetic characteristics

Abstract

In this paper, a theoretical approach to evaluate the hysteresis behaviours of Ising-type segmented nanowire (ISN) comprising magnetic and diluted magnetic segments is defined. The dependency to the system parameters are calculated for optimising their performance in applications like magnetic sensors or recording media. The effects of the composition (p) and temperature (T) as well as crystal field on the hysteresis behaviours are investigated in detail. We studied the effect of the segment dimensions obtained from the exchange interactions. The coercivity (H_C) and remanence (M_r) of the ISN are derived from hysteresis loops. The phase diagrams are presented in the different planes as function of H_C and M_r to investigate the magnetic characteristics of the ISN. Under certain conditions, namely p = 0 and J_D = 0, we also examined the effect of temperature on the nanowire with magnetic and non-magnetic segments. The distinct hysteresis properties and soft/hard the magnetic characteristics depending upon these factors are observed. We found that the magnetic hardness decreases case as the temperatures increase as well as p and crystal field (Δ) decrease. Moreover, when the p increase and Δ decrease the triple hysteresis loop behaviour occurs in the system. Comparisons between the observed theoretical results and some experimental works of nanowire with hysteresis behaviours are made and a very good agreement is obtained.     

No-catalyst growth of vertically-aligned AlN nanocone field electron emitter arrays with high emission performance at low temperature

The AlN nanostructures with a wide band-gap of 6.28 eV are considered as ideal cold cathode materials because of their low electron-affinity. Many methods have been devoted to fabricating AlN nanostructures, but high growth temperature over 800°C and the use of the catalysts in most methods limit their practical application and result in their poor field-emission behaviours in uniformity. This paper reports that without any catalysts, a simple chemical vapour deposition method is used to synthesize aligned AlN nanocone arrays at 550°C on silicon substrate or indium tin oxide glass. Field emission measurements show that these nanocones prepared at low temperature have an average turn-on field of 6 V/μm and a threshold field of 11.7 V/μm as well as stable emission behaviours at high field, which suggests that they have promising applications in field emission area.