Electronic modifications and surface reactivity of an ion bombarded graphite surface

The first steps of structural and electronic modifications of a graphite surface bombarded with argon, hydrogen and deuterium ions were investigated using high resolution electron energy loss spectroscopy (HREELS). The energy and the damping of the low energy plasmon mode of graphite (E//C mode) were studied with respect to the bombardment settings. We show that argon bombardment affects the energy of the plasmon mode, while no similar change is observed after hydrogen (deuterium) bombardments. This can be related to the variation of inter-planar distance between two graphene layers. Moreover, the damping of the plasmon mode can be correlated with the interstitial defect concentration. Concerning the reactivity of the bombarded surfaces, we demonstrate that deuterium bombardment produce a non-deuterated surface. This last is very reactive to a further atomic deuterium exposure, as it is shown by the formation of C-D bondings. The deuterated sites can be removed after thermal annealings between 473 and 783 K. The occurrence of a chemical erosion mechanism accompanying this deuteration is discussed.     

Exchange fields in Gd–Fe intermetallics studied by inelastic neutron scattering

Inelastic neutron scattering (INS) was used to study the magnetic excitations in the compounds Gd_1−xY_xFe₂ and Gd_2−xY_xFe₁₄B. Three inelastic magnetic lines were observed in the INS spectra of these compounds, the two smaller lines appearing as shoulders to the main line. From the concentration-dependent shift of the dominant magnetic line we have obtained experimental information of the strength of the Gd–Gd interaction in these materials and showed that the Gd–Gd contribution to the total molecular field experienced by the Gd moments is roughly an order of magnitude smaller than the contribution of the Gd–Fe interaction. Our results are compared with several earlier published results, including results of electronic band structure calculations.     

The use of high-frequency filters in the investigation of the coloured-random-noise properties of cosmic-ray data

Summary This paper investigates the influence of high-frequency filtering the background white noise on the determination of the coloured-random-noise properties of cosmic-ray experimental signals. We consider fractal methods (Grassberger-Procaccia method, scaling exponent method and fractal length method) and statistical methods (comparison with the Gaussian curve, and the multivariate scaling-analysis technique). The investigation is carried out using two experimental time series with differentcolour (spectral indices α=1.6 and α=1.2) and their computer simulations of pure noise-freecoloured-random-noise signals. The results of the investigation is that the use of high-frequency filtering allows the algorithms employed in the various analyses to see the physical characteristics of the particular data set; the procedure should be reserved, however, to researchers with a good experience in the methods of fractal analysis so that they can identify and discard the artifacts introduced by the truncation applied to the power spectrum.     

Statistics of inter-particle distances and angles in plasmas

Accurate treatment of the plasma density effects requires a detailed knowledge of the spatial distribution of individual ions around a test ion. In the present work, rigorous expressions are derived for the main 2- and 3-particle spatial distribution functions involving the nearest neighbor (NN) and the next-nearest neighbor (NNN) ions. These expressions, valid for both ideal and nonideal plasmas, present the distributions as functionals of the potentials U _NN and U _NNN at the nearest and next-nearest ion locations. All of the distribution functions except one are derived and discussed in the present work for the first time ever. For utilization of our results in practical calculations, we suggest semi-empirical expressions for U _NN and U _NNN in the ion-ion coupling parameter range 0 ?Γ < 1. In order to test the accuracy of our expressions for U _NN and U _NNN we conduct Molecular Dynamics (MD) simulations. The simulations utilize the pure Coulomb particle-particle interaction potentials, regularized at close range to avoid classical Coulomb collapse, and are free from the assumptions made to find U _NN and U _NNN. Thus, the results of the MD simulations provide an independent test of our theoretical results. Excellent agreement has been found between the results of the theory and of the MD simulations. Finally, we outline the implications of the present findings on the problem of tunneling and charge exchange in dense plasmas. Received 27 October 2000 and Received in final form 30 January 2001     

Determination of Easy Axis in a Chemical Ferromagnet, 4-(p-Chlorobenzylideneamino)–TEMPO (TEMPO=2,2,6,6-Tetramethylpiperidin-1-Oxyl)

The trapping sites for muon and muonium in ferromagnetic p-Cl–Ph–CH=N–TEMPO [(4-(p-chlorobenzylideneamino)–TEMPO (TEMPO = 2,6,6-tetramethyl-piperidin-1-yloxyl)] and the hyperfine interaction tensors for these sites are obtained using first-principles Unrestricted Hartree–Fock theory. The calculated hyperfine interactions are used to compare the calculated zero field muon spin rotation (μSR) frequencies for different choices for the easy axis and the observed frequency. It has been concluded that the two trapping centers that can best explain the observed μSR frequency are a trapped singlet muonium near the radical oxygen and a trapped muon site near the chlorine. The direction of the easy axis also is determined to be the b-axis of the monoclinic lattice. This direction for the easy axis is confirmed by determining the direction of the distributed magnetization in the molecular solid which leads to a minimum dipole–dipole interaction energy. The consequences of this agreement for the easy axis direction by two independent procedures are discussed. This revised version was published online in September 2006 with corrections to the Cover Date.     

A new μs isomer in 136Sb produced in the projectile fission of 238U

Rapidity correlations in 800 GeV proton interactions with emulsion nuclei are investigated for different targets and multiplicity regions. To study the energy dependence, the results have been compared with proton interactions at 200 GeV and 400 GeV. A common feature of all the interactions is the existence of strong, short-range correlations. However, no dependence of cluster parameters on primary energy or target mass is found. A marginal increase of correlation strength with multiplicity is observed. Received: 30 March 2001 / Accepted: 18 September 2001     

Neutron diffraction study of the magnetic ordering in the series R 2 BaNiO 5 (R = Rare Earth)

A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8?, b ≈ 5.8?, c ≈ 11.3?) is structurally characterised by the presence of flattened NiO₆ octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R₂BaNiO₅ ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R³⁺ cations are close to the free-ion value. The magnetic moment of Ni²⁺ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T _N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R³⁺, seems to occur below and very close to T _N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected. Received 19 July 2001     

Electron phonon interactions and superconductivity in α ′ -TTF[Pd(dmit)2]2

A simple model is developed to understand superconductivity in α ^′ -TTF[Pd(dmit)₂]₂. We include electron-intra molecular and intermolecular phonon interactions as the mechanism of superconductivity. Intramolecular vibrations included are the eight symmetric A_g modes of the Pd(dmit)₂ molecule. Intermolecular vibrations included are the longitudinal acoustic and transverse acoustic (LA and TA) modes of the Pd(dmit)₂ column. All the electron-phonon coupling constants are calculated from first principles. We find that largest el-intramolecular vibration coupling is to the A_g mode with the highest frequency (1449 cm^-1). The el-intermolecular coupling to the LA mode is found to be larger than the total el-intramolecular couplings. We also find el-(TA)phonon coupling to be at least an order of magnitude smaller than el-(LA)phonon coupling. Estimate of superconducting transition temperature is comparable to experimental result. We also provide a detailed discussion, employing the results of recent numerical calculations on two-chain Hubbard model and the specific material parameters, on the relative importance of el-ph and Coulomb-origin mechanisms of superconductivity in α ^′ -TTF[Pd(dmit)₂]₂ and TTF[Ni(dmit) ₂ ] ₂ . Received 29 March 2001 and Received in final form 7 August 2001     

Accurate Measurement of Small Spin–Spin Couplings in Partially Aligned Molecules Using a Novel J-Mismatch Compensated Spin-State-Selection Filter

The accurate measurement of small spin–spin coupling constants in macromolecules dissolved in a liquid crystalline phase is important in the context of molecular structure investigation by modern liquid state NMR. A new spin-state-selection filter, DIPSAP, is presented with significantly reduced sensitivity to J-mismatch of the filter delays compared to previously proposed pulse sequences. DIPSAP presents an attractive new approach for the accurate measurement of small spin–spin coupling constants in molecules dissolved in anisotropic solution. Application to the measurement of ¹⁵N–¹³C′ and ¹H^N–¹³C′ coupling constants in the peptide planes of ¹³C, ¹⁵N labeled proteins demonstrates the high accuracy obtained by a DIPSAP-based experiment.