The growth phenomenon observed in amorphous metallic alloys irradiated with energetic heavy ions is now well established but yet poorly understood. The experiment reported here shows, for the first time, that an external applied stress induces a drastic amplification of the phenomenon.
The authors have calculated the optical properties (dielectric function, reflectivity and electron energy loss function) of 15 metals, including all the 4d series, some 3d metals and some noble and simple metals. The calculations are based on the LMTO band structure with optical matrix elements. The very good quantitative agreement with optical measurements provides an a posteriori justification of the use of local density functional method in the calculations of the dynamical response in metals and also allows an analysis of the general trends in optical properties from the electronic structure point of view.
Single crystals of lead have been subjected to plastic strain at low temperatures and the resulting effects on the high-field transport properties have been studied. The increase in elastic electronic scattering is pronounced and correlates well with the flow stress, except at the lowest stress values. The high-field electronic thermal resistivity is shown to obey an appropriate Matthiessen's rule, in contrast to the situation that is usually found at zero field. The lattice thermal conductivity is not noticeably affected by the strain, a result consistent with calculations but contrary to the data obtained recently on pure K (Fletcher, Phys. Rev. vol.36, p.3042, 1987). With higher magnetic fields the present techniques might provide a useful alternative to those already in existence in studying phonon-dislocation effects.
On the basis of a uniaxially anisotropic and nonparabolic single-band model with a Fermi surface of corrugated circular cylinder type, analytical and numerical results for the wave-vector-dependent response function chi 0(q) of non-interacting electrons at zero temperature are given. It is found that chi 0(q) shows a flatness in a finite volume of the q space. For the single-corrugated cylindrical Fermi surface the plateau boundary of chi 0(q) is given by surface sections at which diametral and non-diametral Kohn anomalies appear. The flatness in chi 0(q) results in a linear relationship between the paramagnon mass enhancement and the Stoner enhancement factor, which is discussed with respect to the applicability to heavy-fermion metals.
An icosahedral phase has been observed for the first time in the equilibrium immiscible Fe-Cu system. The new metastable phase was formed by directed xenon ion mixing of Fe/Cu multilayers at room temperature and without any amorphous-to-quasi-crystalline phase transition. The atomic concentration of this phase was determined by in situ energy-dispersive spectroscopy (EDS) to be around Fe60Cu40.
The authors have investigated the electronic structure of the non-superconducting compound YBa2Cu3O6. They find the compound to be metallic with a somewhat reduced density of states (DOS) at the Fermi level, as compared with its sister superconducting compound YBa2Cu3O7. The structure in the DOS at the pyramidally coordinated copper site and the four oxygen atoms which form a square planar configuration around it are not substantially modified in YBa2Cu3O6 relative to YBa2Cu3O7. But the absence of linear chain oxygen atoms transforms the associated copper atoms into Cu+ and the oxygen atom with which this Cu interacts becomes almost ionic in nature, with vanishingly small DOS at the Fermi level at both sites. It thus appears that the Cu-O linear chains play an important role in superconductivity in YBa2Cu3O7.
On propose une equation permettant d'exprimer le frottement interne du aux joints de grains. On utilise cette equation pour ajuster les courbes experimentales de frottement interne associees au mouvement des joints de grains
For pt.I see ibid., vol.18, p.1995 (1988). The Hall coefficient RH has been measured for a series of (Ag0.5Cu0.5)1-xGex amorphous alloys in the temperature range 1.5-300 K. The value of RH for x=0.2 is temperature independent over the whole temperature range, whereas that for x=0.5, 0.6 and 0.7 exhibits a T1/2 temperature dependence below about 50 K. The temperature-independent Hall coefficient can be taken as evidence for the negligible contribution of the quantum corrections, lending support to the analysis based on the Boltzmann-type ordinary scattering mechanism for the low-resistivity data in the preceding paper. On the other hand, the square-root temperature dependence of the Hall coefficient provides clear evidence for the dominant role of the electron-electron interaction in high-resistivity alloys.
Starting with the real metallic potentials constructed from a full non-local model pseudopotential theory, the author carries out two computer simulation studies for yttrium and zirconium monatomic glasses. By comparing the Wendt-Abraham parameter, RLG, at the liquid-glass transition point obtained with previous results for chromium and various other, seemingly different monatomic glasses such as the Lennard-Jones, the soft core, etc., the author finds that the Wendt-Abraham criterion satisfies RLG=0.1+or-0.02 and may well be a universal constant. The physical indication of this RLG value is interpreted and various ubiquitous structural features related to liquid-glass transition are also presented.
Teynman's variational method and the arbitrariness of the Stratonovich-Hubbard transformation in the functional integral method are investigated.By making use of their results the dynamical spin susceptibility is given in the Gaussian approximation and it is applied to the calculation of the paramagnetic susceptibility of Ni.
The migration energies of tri- and tetravacancies in copper are calculated to be 0.56 eV and 0.38 eV, respectively, using a new energy minimisation scheme and a first-principles interatomic potential. Comparison with the previously calculated single and divacancy migration energies of 0.82 and 0.55 eV, respectively, indicates that the tri- and tetravacancies are very mobile in copper. The calculated binding and migration energies were used in a rate equation model which predicts the concentrations of vacancy defects during quenching and electron-irradiation experiments. The predicted activation energy of the model follows the same general behaviour as that measured in annealing experiments. The maximum concentrations of tri- and tetravacancies predicted by the model are very low (less than 0.001 p.p.m.), which suggests that these defects many not be observable using experimental techniques.
Polycrystalline Co silicides and Co germanides of various stoichiometric compositions have been examined by ECSA experiments. From core level shifts and XPS valence band spectra it has been concluded that the bonding is similar in the silicides and germanides and can be expressed in terms of hybridisation, and that the chemical bonding is stronger in the silicides than in the germanides. Furthermore, the authors report on samples of Si and Ge which have been implanted with Co at an energy of 80 keV. The valence band spectra showed that in these samples the bonding of the Co atoms is similar to CoSi2 and CoGe2, respectively. Experimentally determined depth profiles of these samples are comparable with Monte Carlo calculations.
Neel's phenomenological model of magnetic surface anisotropy is generalised to hexagonal close-packed structures in order to determine the magnetic surface anisotropy of cobalt. The results obtained are in fair agreement with experimental values for the Au(111)/Co interface. It is also shown that Neel's model allows the surface roughness to be easily taken into account.
Weiler and Schaefer (1985) have recently published vacancy formation parameters for indium from a detailed analysis of positron lifetime spectra. Flower et al. (1985) have simultaneously published the pressure variation of the elastic constants. It is shown that these two sets of data when combined with earlier self-diffusion studies are interconnected through a thermodynamic relation published by Varotsos and Alexopoulos (1986).
The authors have observed the (X-band) electron spin resonance in the equiatomic intermetallic compounds XPtSn, XRhSb and XNiSn, where X=U, Th, Hf, Zr, and Ti. These compounds crystallise in the cubic MgAgAs-type structure and exhibit a semiconducting behaviour in the electrical resistivity at higher temperatures. The ESR lineshapes in bulk samples is of the Dysonian form with an A/B ratio larger than three, which is characteristic of conduction electron spin resonance (CESR). In powdered samples the Dysonian lineshape and the A/B ratio depend on the ratio of sample diameter to the skin depth. The g-values are close to the free-electron value (g=2.0023) except for the U compounds which show both a large g-shift and linewidth. The effective spin density was mostly found to be of the order of 1020 cm-3 at room temperature as measured by comparing the intensity of the ESR signal with that of a known amount of DPPH. The ESR signal of Gd doped into the Hf and U compounds was clearly observed at temperatures where no signal could be found in the pure host materials. Here a negative residual linewidth was found in UPtSn, while the other compounds with Gd obeyed the normal Korringa relation ( Delta H=aT+b) with small thermal broadening and negative g-shift in HfPtSn and positive shift in URhSb.
The triple-point transition temperature in niobium-hydrogen has been determined from DSC measurements during heating and cooling as a function of hydrogen content for both protium and deuterium. The Delta H values derived for the reaction 1/2H(D)2(g)+ alpha to beta from these DSC measurements and data for the reaction 1/2H(D)2(g)+ alpha to alpha ' are -48.7 kJ/mol H and -52.2 kJ/mol D. Hysteresis for the triple-point transition has been measured and found to be the same for niobium-protium and niobium-deuterium. The magnitude of the temperature hysteresis is related to the expected free energy dissipation for the phase transition which occurs through the triple point. The magnitude of the isotope effect for the triple point is considered theoretically.
In an attempt to understand the nature of the magnetisation at and around Fe sites in dilute CrFe antiferromagnetic alloys, neutron polarisation analysis has been used to isolate the magnetic diffuse scattering in a single crystal of Cr-2.8 at.% Fe and a polycrystalline sample of Cr-4 at.% Fe. The magnetic diffuse scattering gives a direct measure of the spatial magnetic moment distribution at and around impurity sites in a magnetic host. It has been found that the Fe possesses an ordered antiferromagnetic moment which is coupled into the SDW in both alloys. For the 2.8% Fe sample, the iron moment is (2.13+or-0.05) mu B, and the chromium moment is (0.72+or-0.02) mu B. For the 4% Fe sample, the moments are (1.8+or-0.1) mu B and (0.72+or-0.02) mu B for iron and chromium respectively. No significant disturbances on the Cr neighbours of Fe atoms have been found.
The atomic diffusion of Cu in solid Au:xCu alloys (x=5.7, 8.2, 9.1, 10.4, 13.2, 17.3 at.%) has been studied by means of solute (63Cu) nuclear spin relaxation (NSR) experiments in the rotating frame (T1p) between ambient temperature and 1100 K. Surprisingly, two distinct diffusion-induced maxima in the relaxation rate could be observed. The high-temperature mechanism around 950 K corresponds to the translational diffusion of Cu in the solid solution phase, as is suggested by comparison with tracer diffusion data. The second relaxation mechanism occurs at fairly low temperatures ( approximately=700 K) with a maximum intensity around 8 at.% Cu. The origin of this contribution to the diffusional NSR rate is not quite clear. The authors attribute it to a short-range ordering effect of Cu atoms in the Au matrix. The activation enthalpies of both types of atomic motion depend only slightly on the Cu concentration.
Experimental results on chromium are reinterpreted in terms of a modification of the thermally activated polarisation domain model, to include interactions between domains together with the effects due to internal strains. The authors suggest that various 'anomalous' peaks observed in ultrasonic attenuation data can be understood as arising because of domain-domain interactions together with internal stresses, and they point out that the associated critical point might be experimentally observable through the application of a compressive uniaxial stress.
Sub-eutectic solid solubilities in the silver-copper system have been determined using high-temperature X-ray diffractometry and applying Vegard's rule. The temperature at which solid solubility first becomes perceptible is found to be Tv approximately=380 degrees C by X-ray analysis. Thermoelectric measurements on the Ag-Cu couple showed no sharp changes in TEMF with temperature, but gave a minimum also at 380 degrees C.
