The sputtering of gallium arsenide at elevated temperatures

The energy distribution of atoms and molecules sputtered from a polycrystalline GaAs sample with a 6 keV Ar ion beam have been measured. The temperature of the target ranged from 30°C to 350°C. Total sputtering yield of the investigated sample has also been measured. The results clearly show that there is a large contribution of molecular component in the sputtered flux and that the molecular component increases above 250°C in comparison to the atomic components thus yielding an increase in the total sputtering yield, as observed previously by Brozdowska et al. The enhanced molecular component at temperatures above 250°C can be explained by the appearance of a spike effect. The results obtained at low temperature can be explained in terms of the collision cascade mode. There is no contribution of beam-induced thermal vaporization to the sputtering of GaAs. The experiment has been performed at the FOM-Instituut voor Atoom-en Molecuulfysica, Kruislaan 407, Amsterdam/WGM., The Netherlands.     

3d-Magnets at high temperatures

Recent achievements in solving the problem of the microscopic nature of the ferromagnetic-to paramagnetic-phase transition of ferromagnetic metals by employing the method of spin- and angle-resolved photoemission spectroscopy with synchrotron radiation (from the storage ring BESSY in Berlin) are briefly summarized. Emphasize is also given to the theoretical interpretation of the data, from which a short-range order in Fe near the Curie pointT _c of at least 4 Å has been derived. This excludes the validity of the disordered local moment picture for Fe at high temperatures.     

MAS-NMR at very high temperatures

We report MAS-NMR experiments at temperatures of approx. 1200 K using a CO(2) laser as the heating device. An internal NMR thermometer based on the (7)Li T1 data of Li(0.24)La(0.54)TiO(3) is used for temperature calibration. Using this setup, temperatures as high as 1191 K could be reached under MAS conditions as confirmed by the melting of Li(2)B(4)O(7) at 1191 K which could be followed by (7)Li-MAS-NMR.     

Heavy quarkonia properties at high temperatures

The spectra and e ⁺ e ⁻ decay widths of the heavy quarkonia as a function of the temperature of medium generated in the ultrarelativistic heavy-ion collisions are discussed. The fluctuations of the vacuum gluon fields are estimated within the instanton liquid model approach. It is noticed that the parameters that are considered can be used to indicate the gluon warming.     

Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures

Physical vapor deposition techniques such as sputtering and laser ablation – which are very commonly used in thin film technology – appear to hold much promise for the synthesis of nanocrystalline thin films as well as loosely aggregated nanoparticles. We present a systematic study of the process parameters that facilitate the growth of nanocrystalline metals and oxides. The systems studied include TiO₂, ZnO, γ-Al₂O₃, Cu₂O, Ag and Cu. The mean particle size and crystallographic orientation are influenced mainly by the sputtering power, the substrate temperature and the nature, pressure and flow rate of the sputtering gas. In general, nanocrystalline thin films were formed at or close to 300 K, while loosely adhering nanoparticles were deposited at lower temperatures. Received: 31 October 2000 / Accepted: 9 January 2001 / Published online: 26 April 2001     

Thermal model for nanosecond laser sputtering at high fluences

The vaporization effect and the following plasma shielding generated by high-power nanosecond pulsed laser ablation are studied in detail based on the heat flux equation. As an example of Si target, we obtain the time evolution of the calculated surface temperature, ablation rate and ablation depth by solving the heat flow equations using a finite difference method. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the variation of ablation depth per pulse with laser fluence is performed. Our numerical results are more agreed with the experiment datum than other simulated results. The result shows that the plasma shielding is very important.     

Dissociation of dimethyl ether at high temperatures

The decomposition of dimethyl ether (CH₃OCH₃) has been investigated behind incident shock waves in a diaphragmless shock tube using laser schlieren densitometry, LS (T = 1500–2450 K, P = 57 ± 4, 125 ± 5 and 253 ± 12 Torr). The LS density gradient profiles were simulated and excellent agreement was found between the simulations and experimental profiles. Rate coefficients for CH₃OCH₃ → CH₃O + CH₃ were obtained. They showed strong fall-off, and at the lower end of the experimental temperature range are close to the low pressure limit. First order rate coefficient expressions were determined over 1500 < T < 2450 K. k_57Torr = (3.10 ± 1.0) × 10⁷⁹T^?19.03 exp(?54417/T) s^?1, k_125Torr = (1.12 ± 0.3) × 10⁸³T^?19.94 exp(?55554/T) s^?1and k_253Torr = (1.02 ± 0.3) × 10⁷³T^?17.09 exp(?51500/T) s^?1. The effect of a roaming channel for decomposition of dimethyl ether was assessed and the best agreement was obtained with 1% dissociation of DME via the roaming path.     

Phonon linewidths of NaF at high temperatures

The linewidths of transverse acoustic modes in NaF are calculated for temperatures up to the melting point. We show that the widths decrease in a certain long wavelength region with increasing temperatures. The results are compared with recent neutron experiments up to 700 K.     

Ill-defined block-spin transformations at arbitrarily high temperatures

Examples are presented of block-spin transformations which map the Gibbs measures of the Ising model in two or more dimensions at temperature intervals extending to arbitrarily high temperatures onto non-Gibbsian measures. In this way we provide the first example of this kind of pathology for very high temperatures, and as a corollary also the first example of such a pathology happening at a critical point.     

Density of indium antimonide at high temperatures

The density of solid and liquid indium antimonide was studied by irradiating the samples with a narrow beam of monochromatic gamma-radiation in the temperature range of 293–1950 K, including the range of melting — crystallization. The measurement errors for the density and thermal expansion coefficients were ±(0.25–0.40) % and ± 4 %, correspondingly. The approximating equations and tables of reference data were obtained for the temperature dependence of thermal properties. Measurement results were compared with the known published data. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 06-08-00040).     

A nonideal Bose gas at high temperatures

Thermodynamic Green's functions are used to examine the properties of a Bose gas whose particles repel one another, not necessarily weakly, for finite temperatures T > T₀, in which T₀ is the phase-transition point and T/n^2/3 1. The excitation spectrum is deduced, together with the temperature dependence of the renormalized chemical potential and mass of the excitations.     

Observation of composite nuclei at very high temperatures

Analysis of data collected with a 19.5 MeV/A⁴⁰Ar beam on a target of²⁷Al shows strong evidences for the formation of very excited composite nuclei. Incomplete linear momentum transfer remains small and the excitation energies per nucleon can be estimated to be close to 5 MeV.     

Electrical resistivity of Co-Ga alloys at high temperatures

The rotating magnetic field method was used to investigate the temperature dependences of the electrical resistivity of Co-Ga alloys in the temperature range 800–1750C. The experimental data obtained for the liquid phase are compared with the results of a calculation made in the framework of the Faber — Ziman method as generalized by Evans to alloys of transition metals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 61–67, December, 1978.