Temperature dependence of recoilless fraction in tungsten

The Mössbauer recoilless fractions of¹⁸²W,¹⁸³W,¹⁸⁴W, and¹⁸⁶W have been calculated by using Born-von Karman model of lattice vibrations. There is a good agreement between the experimental and calculated results.     

Mössbauer recoilless fraction of solid krypton

Mössbauer recoilless fraction for solid krypton has been computed at various temperatures, using lattice dynamical method. An interatomic potential represented by Kihara core model of molecules has been used in these calculations, whose parameters have been computed using latest crystal data on sublimation energy and lattice spacing. The theoretical results have been compared with experimental data.     

Molecular motion in restricted geometries

Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations. An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.      

Time dependence of the source recoilless fraction for a cobalt-57 in rhodium source

The effect of source resonance self absorption (SRSA) has been measured for⁵⁷Co in a rhodium matrix by determining the ratio of the intensity of the 14.4 keV gamma transition to that of the associated 22 keV rhodium x-ray, directly counting both intensities from a stationary source. A ratio, ρ, is defined asf _s/f _so, wheref _s is the recoil-free fraction with the SRSA effect present, andf _so is in the absence of SRSA effects [1]. The value of ρ is to within an additive constant, proportional to the gamma over the X-ray intensities. In the absence of SRSA it is expected that both the 14.4 keV and the 22 keV decay rate would be identical to that of the⁵⁷Co parent, and ρ would be unity and time independent. But because of SRSA a fraction of the 14.4 keV gammas are reabsorbed by⁵⁷Fe, which is increasing as a result of⁵⁷Co decay, and hence, ρ decreases with time. By direct measurement our results confirm that the effect of SRSA can be appreciable even over short time intervals, and we find that in many cases SRSA may have a pronounced effect on Mössbauer experiments, affecting both the line width andf-value, and contrary to general practice in the field, needs to be evaluated in high intensity source experiments.     

Quantum Hall effect in restricted geometries

The confinement of electrons in narrow quasi-two-dimensional conducting channels, modelled with a parabolic well, leads to asymmetric Hall plateaus about complete Landau-level fillings and to saw-toothed oscillations of the dc resistivity _xx as a function of the magnetic field B. The peaks in _xx are displaced to lower B and drastically reduced from their wide-channel values. The peak values of _xx increase with increasing channel width. The corrections to σ_yx for finite channel widths and the response to oscillating electric fields are evaluated.     

Heat transport of helium II in restricted geometries

A modified version of Altmann's rigorous cellular method is presented. The new approach completely removes various practical drawbacks of Altmann's original method without affecting its unmatched flexibility. Instead of expanding the wave function in terms of partial waves which are generated at the band energies sought, we use partial waves and their energy derivatives at some suitably chosen energy. The one-particle energies are found by minimizing the expectation value of the true crystal Hamiltonian. Thereby non-spherical oscillations of the crystal potential within each cell are included. Moreover, the new approach is shown to be easily adaptable to a fully relativistic treatment. Open structures, such as covalently bound crystals, thin films, clusters of atoms and molecules, are just as tractable as close-packed structures. The progress achieved is demonstrated by first results on the electronic structure offcc lanthanum (including relativistic effects) and that of a (100) tungsten monolayer. The new approach is conceptually related to Andersen's LMTO-method with which it shares decisive virtues. However, the formal structure of our method is considerably simpler and its accuracy appears to be somewhat larger.     

Low-temperature helium in bulk and in restricted geometries

Experiments with helium in thin films and small pores show that the bulk properties are greatly modified. With ⁴He, the onset of superfluidity is depressed and the specific heat anomaly is rounded and also depressed to lower temperatures. With ³He new features are found such as surface magnetism and free path effects. Bulk superfluid ⁴He is shown to have a sharp transition to turbulence, and bulk ³He the characteristics of a Fermi fluid.     

The recoilless fraction of the oxide surface layer formed on small iron particles

Small iron particles with oxide surface layer were studied by Mössbauer Spectroscopy at various temperatures in the range of 5K to 300K. The Debye temperature of the oxide layer and the temperature dependence of f2/f1 were obtained, where f1 and f2 are the recoilless fractions for the inner α-iron core and the oxide layer respectively. Furthermore, the method of estimating the thickness of the oxide layer was improved.     

The exact Mössbauer recoilless fraction in fractals ATT≠0

In this paper a detailed calculation of the Mössbauer fraction for a general non-Euclidean dimensionality is given. The result shows that the contributions to the Mössbauer effect in fractals come from three possible types of transition.     

Diffusion-weighted spatial information from1H relaxation in restricted geometries

Summary NMR relaxation of water¹H confined in restricted geometries, whatever is the nature of the system (porous media saturated by water as well as biological tissues), exhibits common characteristics. Artificial microporous media saturated by water have been chosen as model systems to study the longitudinal and transverse relaxation of¹H magnetization of water molecules diffusing in restricted geometries. These systems are very stable, easy to prepare, with well-characterized pore size distribution and connections, and with highly homogeneous surface properties. The response was compared with that from more complex natural porous media. Scanning Electron Microscopy techniques demonstrated spatial characteristics and surface properties of the samples. The information content of longitudinal relaxation curves associated with spatial structure and due to restricted diffusion is shown in these samples. The effect on transverse relaxation of self-diffusion in the presence of spatially varying magnetic fields due to susceptibility differences is shown. A simple linear relationship has been found in all samples between the transverse relaxation rate and the interpulse delay in CPMG experiments, in spite of the variety of pore shapes and sizes. In general, one can say that relaxation curves beardiffusion-weighted information on the pore space framework. The role of the investigated relaxation mechanisms is important also in the response of biological tissues, including in the presence of MR Imaging contrast agents inducing microscopic magnetic-field gradients. Work partially supported by CNR and MURST Grants.     

Temperature and pressure variation of the recoilless fraction for119Sn:Pd Mössbauer system

The variation of the recoilless fraction ‘f’ as a function of temperature and pressure of¹¹⁹Sn embedded as a dilute impurity in palladium (Pd) host has been calculated after incorporating the changes in the force constant and mass at the impurity site using Debye model and phonon frequency spectrum (pfs). The comparison of the results have been made with the available experimental values.     

Goldanskii-Karyagin effect and external magnetic field method as tools to measure anisotropy of the recoilless fraction in amorphous materials

Diffraction of X-rays or neutrons can not be used to obtain details about the atomic vibrational anisotropy in the case of amorphous materials due to the lack of well-defined Bragg reflections. Mössbauer spectroscopy can yield some information in such cases, either via the Goldanskii-Karyagin effect or by applying a magnetic field, preferably along the beam axis. The latter method can be applied to the (preferably diamagnetic) samples exhibiting an electric quadrupole interaction (preferably non-axial) and the magnetic field should be chosen in such a way as to produce significant off-diagonal elements in the hyperfine hamiltonian. The external magnetic field method is capable of yielding much more information than the Goldanskii-Karyagin effect in most cases, provided sufficiently strong magnetic fields are available. Some examples of the¹²⁹I Mössbauer spectra have been calculated to show the usefulness and sensitivity of the external magnetic field method.     

Analytical representations of the Mössbauer recoilless fraction,Debye-Waller factors,lattice energies,and heat capacities

Simple analytical expansions are given for the recoilless fraction in Mössbauer spectroscopy, the Debye-Waller factor in X-ray scattering, and the lattice energy and heat capacity of solids. While this problem has been discussed in an earlier paper [1], computer technology has now advanced to the point that direct evaluations of the simple expansions of these quantities are useful for quick curve fitting to experimental data at any desired temperature, and these expansions are easier to evaluate than using graphs to estimate recoilless fractions and Debye temperatures. We compare this approach with a polynomial expansion in terms of Bernoulli numbers, which has only a limited domain of convergence. We explicitly evaluate the convergence of these Debye integral expansions as a function of the number of terms used and the time required.This work was prepared with the support of the U.S. Department of Energy, Grant No. DE-FG02-85 ER 45199.     

Temperature dependence of recoilless fractions in tungsten

The recoilless fractions of ¹⁸²W, ¹⁸⁴ W, and ¹⁸⁶W have been simultaneously measured from 30°K to 100°K in 10°K steps. They are found to agree with previous calculations of Raj and Puri.     

Anisotropic recoilless fraction and temperature dependent quadrupole interaction of substitutional 129I in Si due to Jahn-Teller distortion

A dope concentration dependent quadrupole relaxation and a large anisotropy of the recoilles fraction are observed for substitutional iodine impurities in silicon, using ¹²⁹I Mössbauer spectroscopy. The results are interpreted in terms of a transition from a static to a dynamic Jahn-Teller distortion.     

Modeling of nonlinear and non-stationary multi-vortex behavior of CDWs at nanoscales in restricted geometries of internal junctions

We report on studies of stationary states and their transient dynamic for an incommensurate charge density wave (ICDW) in a restricted geometry of two spatial dimensions. The model takes into account multiple fields in mutual nonlinear interactions: the amplitude and the phase of the complex order parameter, and distributions of the electric and chemical potentials, of the density and the current of normal carriers. We observed spontaneous formation of vortices (the ICDW dislocations), and followed events of their creation and the subsequent evolution. The vortices appear when the voltage across, or the current through, the sample exceed a threshold. The number of vortices remnant in the reconstructed stationary state increases stepwise – in agreement with experiments, while a much greater number of vortices appears during the intermediate transient states. The vortex core concentrates the electric dipole leading to sharp drops of the electric and chemical potentials across the core. That can lead to enhanced inter-layer tunneling making the core to be a self-tuned microscopic tunneling junction. The results are applied to experiments on nano-fabricated mesa-junctions. They also appeal to modern efforts of the field-effect transformations in correlated electronic systems.     

On the energetics of Reissner Nordstrøm geometries

We point out the existence of a generalized ergosphere in the Reissner Nordstrøm geometry and we give an explicit formula to determine its range. These results are compared and contrasted with the ones obtained in the case of the Kerr solution. An explicit process of energy extraction from a Reissner Nordstrøm black hole is given.