J. Keinonen

Dual origin of defect magnetism in graphene and its reversible switching by molecular doping

Control of magnetism by applied voltage is desirable for spintronics applications. Finding a suitable material remains an elusive goal, with only a few candidates found so far. Graphene is one of them and attracts interest because of its weak spin-orbit interaction, the ability to control electronic properties by the electric field effect and the possibility to introduce paramagnetic centres such as vacancies and adatoms. Here we show that the magnetism of adatoms in graphene is itinerant and can be controlled by doping, so that magnetic moments are switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions; one from itinerant magnetism and the other from dangling bonds. Our work suggests that graphenes spin transport can be controlled by the field effect, similar to its electronic and optical properties, and that spin diffusion can be significantly enhanced above a certain carrier density.

Hydrogen migration in diamond-like carbon films

Properties of physical vapor deposited diamondlike carbon (DLC) films and the migration of hydrogen in H+ and 4He+ ion implanted and hydrogen co-deposited DLC films have been studied. Measurements utilizing Rutherford backscattering spectrometry showed that the films studied have an average mass density of 2.6±0.1 g/cm3. The bonding ratio sp3/sp2 is typically 70% measured with the electron spectroscopy for chemical analysis technique. Impurities and their depth distributions were deduced from the particle induced x-ray emission and secondary ion mass spectrometry (SIMS) measurements. Distributions of implanted and co-deposited hydrogen were measured by the nuclear resonance reaction 1H(15N,αγ)12C and SIMS. It was found that annealing behavior of implanted H in DLC has a diffusion like character. The obtained diffusion coefficients resulted in the activation energy of 2.0±0.1 eV. It was observed that in H co-deposited DLC films the temperature of H release varied between 950 and 1070u2009°C depending on the H ...

A low-level detection system for hydrogen analysis with the reaction 1H(15N,αγ)12C

Abstract A spectrometer has been constructed for measurements of low hydrogen concentrations by detecting 4.43 MeV γ-rays from the reaction 1H(15N,αγ)12C at the 6.39 MeV resonance. It consists of an annular large-volume (2600 cm3) BGO detector, an annular plastic-detector anticoincidence shield around it, and Pb and Cd shields. The spectrometer is placed in the target hall of the laboratory located 20 m deep inside the granite bedrock. The background counting rate arising mainly from impurity of the BGO materials is 0.023 Hz in the energy range 3.9–4.9 MeV. The detection sensitivity of 10 at.ppm H in silicon substrates was determined by measurements of concentration distributions of 2 × 1013–1 × 1017 40 keV protons cm−2 in single-crystal silicon substrates. The range profiles of 40 keV protons simulated by molecular dynamics calculations resulted in the correction factor of 1.21 for the commonly used electronic stopping power of silicon given by Ziegler et al. [The Stopping Power and Ranges of Ions in Matter (Pergamon, New York, 1985) vol. 1].

Electronic stopping power for 7Li, 11B, 12C, 14N and 16O at energies 0.4 to 2.1 MeV/nucleon in Ta and Au, and for 12C at energies 0.4, 0.8 and 1.4 MeV/nucleon in 18 elemental solids

Abstract Stopping powers have been measured for 0.4 to 2.1 MeV/nucleon beams of 7 Li, 11 B, 12 C, 14 N and 16 O in Au and of 7 Li, 12 C and 14 N in Ta. The results are compared with the predictions of an empirical model. By application of the Doppler-shift attenuation method, the measured electronic stopping powers for 12 C in Ta and Au are used to deduce the mean lifetime 58 ± 5 fs for the 4.439 MeV level in 12 C. Deviations of the electronic stopping power from the predictions of the empirical model have been studied for 12 C at energies 0.4, 0.8 and 1.4 MeV/nucleon in 18 Z = 14–82 elemental solids by application of the Doppler-shift attenuation method and the lifetime value of the 4.439 MeV level.

Identification of vacancy charge states in diffusion of arsenic in germanium

Diffusion of As into Ge from a GaAs overlayer deposited on p-type Ge substrates has been studied by means of secondary ion mass spectrometry. A concentration-dependent diffusion of As atoms was observed in addition to the concentration-independent diffusion of Ga and As atoms. The concentration dependence is explained by a Fermi-level-dependent diffusion model. Arsenic atoms are shown to diffuse through Ge vacancies with the charge states 2− and 0. No presence of the singly negatively charged vacancies was observed, indicating that Ge vacancy could be a negative U center.

First-principles simulation of collision cascades in Si to test pair-potentials for Si-Si interaction at 10 eV–5 keV

Abstract Interatomic potentials for Si-Si interaction are tested at energies of 10 eV–5 keV for Si ions in ion-beam amorphized Si by simulating range distribution data with the molecular dynamics method. The range profile of 1 × 10 16 10 keV 30 Si + cm −2 implanted into originally crystalline silicon was measured using a nuclear reaction technique. An interatomic repulsive potential from first-principles calculations is proposed for the Si-Si interaction. The dependence of the number of vacancies produced in low-energy collision cascades on the potential is demonstrated by simulations of the cascade dynamics.

Defect formation in implantation of crystalline Si by MeV Si ions

The distributions of vacancy‐type defects and displaced Si atoms in Si(100) produced by the room‐temperature implantation of 1014–1016 12‐MeV 28Si+ ions/cm2 are measured with low‐energy positron‐ and ion‐beam techniques. The observed damage regions are reproduced by computer simulations. The distribution of displaced Si atoms coincides with the deposited energy distribution in elastic collisions. At the fluence of 1×1016 Si+/cm2, no crystalline structure was found in the peak region of the deposited energy at the depth of z=6 μm. Saturation of the divacancy concentration was observed at the ion fluences 3×1015 Si+/cm2 close to the surface (z 1 μm). In the region z 1 μm. This is also found in the simulated spatial structure of collision cascades.

Search for low-energy resonances in 21Ne(p, γ)22Na and 22Ne(p, γ)23Na

Abstract The reactions 21 Ne(p, γ) 22 Na and 22 Ne(p, γ) 23 Na have been investigated at E p (lab) = 70–355keV. Neon gas enriched to 91% in 21 Ne and to 99% in 22 Ne was recirculated in a differentially pumped gas target system of the extended-static and quasi-point supersonic jet type. For 21 Ne(p, γ) 22 Na, new resonances were found at E p = 126, 272, 291 and 352 keV. The 291 keV resonance corresponds to a new unbound state in 22 Na. Excitation energies, γ-ray decay schemes, resonance widths and strengths as well as J π assignments are reported for all the resonances. Information on low-lying states in 22 Na is also obtained. Of the 9 expected resonances in 22 Ne(p, γ) 23 Na none has been observed. Upper limits on their ωγy strengths are presented. The astrophysical as well as the nulcear structure aspects of the results are discussed.

Stopping power for low-velocity heavy ions: (0.01-0.9) MeV/nucleon Si ions in 18 (Z = 13–79) metals

Abstract The stopping power for 29 Si ions in Al, Ti, V, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Re, Pt and Au has been studied in the energy region (0.01-0.9) MeV/nucleon by application of a technique of nuclear physics, the inverted analysis of Doppler-shift attenuation data. Generally, the measured values are considerably higher at low energies (less than 0.4 MeV/nucleon) and show different energy dependence than the predictions of the commonly used empirical electronic stopping powers by Ziegler, Biersack and Littmark [1] [The Stopping and Ranges of Ions in Matter (Pergamon, New York, 1985) Vol. 1]. The uncertainty of the electronic stopping powers determined is typically ±6%.

Electronic stopping power of Si and Ge for MeV-energy Si and P ions

The electronic stopping powers of Si and Ge for 0–30 MeV 29Si and 29P ions are reported. The stopping power was studied by application of a technique of nuclear physics, the inverted analysis of Doppler‐shift attenuation data. The measured values at 30 MeV are about 15% lower and at 2 MeV considerably higher than the predictions of the commonly used empirical electronic stopping powers by J. F. Ziegler, J. P. Biersack, and U. Littmark [The Stopping Power and Ranges of Ions in Matter (Pergamon, New York, 1985), Vol. 1]. The experimental nuclear stopping power was taken into account in the deduction of the electronic stopping power.