Study of multiple impurity seeding effect using SONIC integrated divertor code for JT-60SA plasma prediction

In order to study the potential impurity seeding operation regime of the future fusion devices, the first application of the integrated divertor code SONIC to the Ar + Ne mixed-impurity seeding operation of JT-60SA steady-state high-β plasma has been carried out. In the case, Ne is added to Ar-only seeding, the separatrix electron density has fell into the desired low separatrix electron density of the scenario. This is mainly because the D⁺ flow velocity towards the inner divertor has been increased by the Ne seeding. The resultant friction force transports Ar impurities towards the inner divertor region, while impurities are stagnated in the top of scrape-off layer (SOL) in the Ar-only seeding case. The higher impurity radiation power in the divertor regions and lower one in the SOL region above the X point have been obtained in mixed-impurity seeding cases, which show similar tendency as the Ar + Ne mixed-impurity seeding experiment in JT-60 U. At the core edge, Z_eff has been slightly increased and the radiation power has been decreased as the Ne seeding rate increases. The core plasma/impurity transport has been also evaluated by the TOPICS code using the impurity density at the core edge computed by the SONIC as a boundary parameter. The results show lower Z_eff and radiation power, and higher electron temperature in the core in the mixed-impurity seeding cases. Above possible contributors to the better energy confinement indicate that the mixed-impurity seeding operation might be more effective than Ar-only seeding operation.     

Effect of impurity scattering of a tunneling electron on variable-range hopping conduction

Variable-range hopping conduction in semiconductors is determined by the asymptotic behavior of impurity wave functions on distances much larger than mean interimpurity separation. Scattering of an impurity electron by the other impurities situated near its tunneling path is shown to result in a correction a to electron localization lengtha. This correction depends on the impurity scattering length and impurity concentrationN and may be of the order ofa(Na ³ ) ora(Na ³ )^1/2.     

Oscillatory behaviour in the energy and nonlinear optical rectification spectra of elliptic quantum rings under electric field: influence of impurity and eccentricity

We have theoretically investigated the electronic properties and nonlinear optical rectification spectra of GaAs/AlGaAs anisotropic quantum ring, modelled by an outer ellipsis and an inner circle, in connection to the presence of a donor off-centre impurity, structural distortions and in-plane electric field. The one-electron energy spectrum and wave functions are found using the adiabatic approximation and the finite element method within the effective-mass model. The energy spectrum of concentric ring reveals an anomalous oscillatory behaviour in the region of relatively small values of the electric field (< 12?kV/cm) followed by linear Stark effect at higher field values. We showed that this unusual behaviour is strongly affected by the ratio of the outer/inner ring radii, the displacement of the inner circle (eccentricity) along the x or y axis and the impurity presence. The related nonlinear optical rectification spectra present maxima whose positions mirror this oscillatory behaviour and consequently can be used as an excellent tool to distinguish the presence of an impurity or the direction of the eccentricity.     

Changes in optical absorption in iron garnet films due to impurity incorporation

The incorporation of impurities in films of Bi-substituted iron garnets grown by liquid phase epitaxy has been studied by successively adding small amounts of SiO₂ and CaCO₃ to the melt before film growth. It is found that very small additions of CaCO₃ to the melt profoundly influences the optical absorption coefficient and the electrical properties. The impurity absorption ofn-type YIG doped with Si⁴⁺ and Pb²⁺ ions is investigated and its wavelength dependence compared to that ofp-type YIG. The optical transitions involved in the impurity absorption of iron garnets are discussed in terms of transition reaction, photon energy and dependence on the impurity concentration. Depending on the photon energy, one or several transitions may give rise to impurity absorption.     

Electric field gradients created by near neighbour impurity atoms in a cubic silver host

The electric quadrupole interactions produced by near neighbour impurity atoms of Cu, Au, Zn, In, Sn and Bi on¹¹¹Cd probe nuclei in a cubic Ag lattice were studied by the TDPAC method. The effects of the type of impurity and its concentration have been investigated. The results show the presence of a high-frequency interactionv _Q ^h superimposed to a smeared out low frequencyv _Q ^l . The high frequency interaction, in the range 20 to 600 MHz, is attributed to impurity atoms located in nearest neighbour sites, while the low frequency interaction, in the range 2 to 12 MHz, is generated by impurities distributed at various different atomic distances from the probe nuclei. Bothv _Q ^h andv _Q ^l increase with impurity concentration leaving the ratiov _Q ^h /v _Q ^l almost constant. The results show that the high frequencyv _Q ^h is linearly dependent on the solute valence, and a logarithmic function of the impurity concentration, in the range 0.5 to 4.5 at. %. Large size effects have been observed in CuAg and BiAg alloys. Instead for ZnAg and SnAg, thev _Q ^h andv _Q ^l variation is attributed basically to charge effects.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de Estudos e Projetos (FINEP).     

The behaviour of an isolated impurity near a phase transition in a spherical-like model

The behaviour of an isolated impurity in a crystal undergoing a displacive phase transition is investigated in an exactly soluble spherical-like model. We find, depending on the parameter of the impurity, either a global phase transition at the bulkT _c or a freezing-out of local order at a temperatureT _c ^loc >T _c driven by a soft local mode. We furthermore discuss the dynamic autocorrelation function of the impurity by introducing a phenomenological damping.Work Supported by The Swiss National Science Foundation     

A vibronic-structure analysis of optical spectra in ZnO:Ni3+ crystals by localized-vibration simulation

A theoretical study is reported of the vibrations associated with a Ni³⁺ impurity charged with respect to the ZnO lattice. The calculations were made by a recursive method in terms of the shell model for vibrations with different symmetries. The vibronic structure observed in the spectra of d-d intracenter transitions in the Ni³⁺ impurity has been interpreted using model calculations. Fiz. Tverd. Tela (St. Petersburg) 41, 618–622 (April 1999)     

Negative resistivity anomaly of unstable 4f impurities in metals

The minimum of the resistivity of dilute alloys with unstable Ce and Pr impurities is usually attributed to an abnormal increase of the spin scattering cross section of the inner 4f electron at low temperatures (4f Kondo effect). By comparison with the resistivity increments of stable rare earth impurities, we show that nearT=0 the anomalous increment is in fact consistent with simple potential scattering from the outer 5d6s valence electrons of fractional valent impurities. The observed decrease of the increment at elevated temperatures is too large to be due to 4f spin scattering in the unitarity limit or to a valence change. This decrease therefore implies some kind of shunting of the impurity potential scattering by an unknown mechanism connected with the valence instability of the impurity.     

Theoretical investigations of the local structure and the g factors for the tetragonal Ru3+ center in PbTiO3

The local structure and the g factors for the tetragonal Ru³⁺ center in PbTiO₃ are theoretically studied from the perturbation formulas of the g factors for a 4d⁵ ion in tetragonally distorted octahedra. From the studies, the distance between the impurity Ru³⁺ and the center of the oxygen octahedron is found to be about 0.283 Å, which is smaller than that (≈0.3 Å) for the host Ti⁴⁺ site. It appears that the impurity may not occupy exactly the host Ti⁴⁺ site but suffer a slight inward shift toward the center of the oxygen octahedron. The calculated g factors based on the above local structure show good agreement with the experimental values.     

Polarization of quadrupolar nuclei

A method of obtaining high polarization and pure spin states of impurity nuclei with a moderately strong quadrupole interaction in solid diamagnetic hosts whose nuclei have spin 1/2, a large g factor (like ¹H and ¹⁹F), and a high degree of polarization is proposed. The method employs cross-relaxation transitions of the impurity nuclei with the host spins (with adiabatic variation of the external magnetic field) and simple radio-frequency pulses that invert the host nuclei or give rise to two-spin resonance of the host and impurity nuclei. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 539–543 (10 October 1998)     

Effect of different impurity atoms on 1/<Emphasis Type="Italic">f</Emphasis><Superscript>α</Superscript> tunneling current noise characteristics on InAs(110) surface

The results of UHV STM investigations of tunneling current noise spectra in the vicinity of individual impurity atoms on the InAs(110) surface are reported. It was found that the power law exponent of 1/f ^α noise depends on the presence of an impurity atom in the tunneling junction area. This is consistent with the proposed theoretical model considering tunneling current through a two-state impurity complex model system taking into account many-particle interaction. The text was submitted by the authors in English.     

Photochromic effect in Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals doped with molybdenum

The steady-state and photoinduced absorption spectra of Bi₁₂SiO₂₀ crystals doped with molybdenum are investigated. It is demonstrated that the impurity absorption is associated with the Mo⁶⁺ and Mo⁵⁺ ions occupying the positions close to the tetrahedral Si⁴⁺ position. The mechanism of the photochromic effect is proposed. This mechanism involves the change in the charge state of molybdenum impurity ions according to the scheme Mo_Si⁶⁺ + e ⁻ → Mo_Si⁵⁺.     

Investigations on the local structure and g factors for the interstitial Ti3+ in TiO2

The EPR g factors gi (i = x, y, z) for the interstitial Ti³⁺ in rutile are theoretically studied from the perturbation formulas of these parameters for a 3d¹ ion in rhombically compressed octahedra. The ligand octahedron in the impurity center is found to be less compressed than that on the host interstitial site due to the Jahn-Teller effect. The local compression parameter (≈0.026) and the rhombic distortion angle δϕ′ (≈0.7°) around the impurity Ti³⁺ are smaller than the host values (≈0.091 and 3.5°). The theoretical g factors based on the above local structural parameters are in good agreement with the experimental data. In addition, the g factors for a tetragonal interstitial Ti³⁺ center are also reasonably interpreted.      

Theory studies of the local lattice distortions and the EPR parameters for Cu2+, Mn2+ and Fe3+ centres in ZnWO4

The local lattice distortions and the electron paramagnetic resonance (EPR) parameters (g factors, hyperfine structure constants and zero-field splittings) for Cu²⁺, Mn²⁺ and Fe³⁺ in ZnWO₄ are theoretically studied based on the perturbation calculations for rhombically elongated octahedral 3d⁹ and 3d⁵ complexes. The impurity centres on Zn²⁺ sites undergo the local elongations of 0.01, 0.002 and 0.013 Å along the C₂ axis and the planar bond angle variations of 8.1°, 8.0° and 8.6° for Cu²⁺, Mn²⁺ and Fe³⁺, respectively, due to the Jahn–Teller effect and size and charge mismatch. In contrast to the host Zn²⁺ site with obvious axial elongation (～0.31 Å) and perpendicular (angular) rhombic distortion, all the impurity centres demonstrate more regular octahedral due to the above local lattice distortions. The copper centre exhibits significant Jahn–Teller reductions for the spin-orbit coupling and orbital angular momentum interactions, characterised by the Jahn–Teller reduction factor J (≈0.29 ? 1). The calculated EPR parameters agree well with the experimental results. The local structures of the impurity centres are analysed in view of the corresponding lattice distortions.     

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

Summary The simple cut criterion based on the accurate determination of the radii of the ions in alkali halides and previously introduced by the authors for forecasting off-centre configuration of Li⁺ and F⁻ has been extended to heavy ions (Ag⁺ and Cu⁺). It has been found that this criterion is valid for the Ag⁺ ion, whereas for Cu⁺ gives a less precise forecast because of the lack in knowledge of the effective partial charge on Cu⁺ ion. It has been evidenced that the critical value of the ratior ₊ ^* /r ₊ between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Effect of Sr<Superscript>2+</Superscript> ion substitution with trivalent ions (Sc<Superscript>3+</Superscript>, In<Superscript>3+</Superscript>, La<Superscript>3+</Superscript>, Bi<Superscript>3+</Superscript>) on its ferroelectric instability in SrTiO<Subscript>3</Subscript>

The vibration frequencies of unstable ferroelectric and antiferrodistortion modes and the dependences of the energy on the ion displacement amplitude have been calculated within the generalized Gordon-Kim model for distortions along eigenvectors of these modes in the mixed compounds Sr_{1 − x} A _x Ti_{1 − x} /₄□ _x/4O₃ and Sr_{1 − y} A _2y /₃□ _y/3TiO₃ (A = Sc³⁺, In³⁺, La³⁺, Bi³⁺; □ is the vacancy). To compensate an excess positive charge, vacancies are introduced into the Ti⁴⁺ or Sr²⁺ site. Calculations have been performed in the “daverage” crystal approximation for impurity concentrations of 0.25 and 0.50. To this end, a set of 40 atomic superlattices with various orderings of heterovalent ions Sr²⁺ and impurity A ³⁺ has been considered. It has been found that each impurity type, independently of charge balance, induces ferroelectric instabilities in doped compounds. In the case of doping with In³⁺ and La³⁺ for concentration x = 0.25, the possibility of rotating the polarization vector has been shown.     

Single passage NMRON and the spin-temperature question

The analysis of spin-lattice relaxation (SLR) of very dilute impurity nuclei in ferromagnetic hosts, as encountered in NMR of oriented nuclei (NMRON), usually neglects the spin-spin interaction, assuming a no-spin-temperature model. However, the regime of impurity systems for which this assumption is valid has not been established. the relaxation following single passage in⁶⁰CoFe<110> and⁵⁵CoFe<100> has been measured, and is compared with calculations made with and without a spin-temperature constraint.     

Theoretical studies of the spin Hamiltonian parameters and local structures for WO3 doped Zn3(PO4)2ZnO nanopowders

The spin Hamiltonian parameters (SHPs) and the local structures for impurity W⁵⁺ in the Zn₃(PO₄)₂ZnO nanopowders doped with WO₃ under different concentrations are theoretically investigated using the perturbation calculations of these parameters. The exponential functions of the related quantities (cubic field parameter Dq, covalency factor N, relative tetragonal compression ratio τ and core polarisation constant κ) of concentration x with totally four adjustable coefficients a, b, c and d are adopted to fit the concentration dependences of the experimental d-d transition bands and SHPs. The impurity W⁵⁺ centres demonstrate moderate tetragonal compression ratios τ (～3.1%) due to the Jahn–Teller effect. With the increase of WO₃ concentration, Dq and N show moderately decreasing rules, while τ and κ exhibit slightly and moderately increasing tendencies with x, respectively. The mechanisms of the above concentration dependences of these quantities are analysed from the modifications of the local crystal-field strength and electron cloud density around the impurity W⁵⁺ with the variation of x. Present theoretical studies would be useful to the exploration of the structural properties and optical applications for WO₃ doped Zn₃(PO₄)₂ZnO nanopowders.     

Investigations of EPR parameters and impurity structure for Co<Superscript>2+</Superscript> in Ca(OH)<Subscript>2</Subscript> crystal

The electron paramagnetic resonance (EPR) parameters (g _‖ andg _⊥ factors and hyperfine structure constantsA _‖,A _⊥) for Co²⁺ in Ca(OH)₂ are studied from the second-order perturbation formulas on the basis of the cluster approach. In these formulas, the contributions to EPR parameters from the state interactions and covalency effects are considered and the parameters related to both effects are obtained from the optical spectra and impurity structure of the studied system. From the study, it is found that the β angle between the metal-ligand bond and the C₃ axis changes from 61° in a pure crystal to 53.68(26)° in the impurity center of a Co²⁺-doped Ca(OH)₂ crystal because of the impurity-induced local lattice relaxation. The reduction of the angle β in the impurity center is also supported by the result obtained by analyzing the EPR zero-field splitting for Mn²⁺ in the same Ca(OH)₂ crystal. The EPR parameters of Ca(OH)₂:Co²⁺ are also reasonably explained by considering the suitable local lattice relaxation.     

Anderson impurity model: Double occupancy contribution to the magnetic susceptibility

After solving the single Anderson impurity model (SIAM) within the non-crossing approximation with a finite Coulomb repulsion, U, and vertex corrections (NCAf²v), we focus on the magnetic susceptibility. Using the same diagrammatic expansion the susceptibility can be dressed with two factors, namely, the double state occupancy and the vertex corrections. In this work we analyse the effect of double occupancy on the dynamic and static susceptibility as a function of U and on the degeneracy of the total impurity angular moment, S.