Microhardness studies in alkali halide mixed crystals

Microhardness measurements done in KCl, KBr and in different compositions of KCl-KBr mixed crystals show that it varies nonlinearly with composition. In order to investigate the nature of defects, several techniques such as etching, ionic conductivity and dielectric loss have been employed which showed that the mixed cristals of KCl-KBr are more defective, containing a high concentration of dislocations, low-angle grain boundaries and vacancies as compared to the end products KCl and KBr. These imperfections appear to be responsible for the nonlinear variation of microhardness in mixed crystals. The microhardness studies also revealed that the difference in size of the ions constituting the mixed system are responsible for the internal strains which in turn give rise to imperfections affecting the microhardness of mixed crystals.     

Entangling atoms in photonic crystals

We propose a method for entangling a system of two-level atoms in photonic crystals. The atoms are assumed to move in void regions of a photonic crystal. The interaction between the atoms is mediated either via a defect mode or via a resonant dipole-dipole interaction. We show that these interactions can produce pure entangled atomic states. We analyze the problem with parameters typical for currently existing photonic crystals and Rydberg atoms and we show that the atoms can emerge from photonic crystals in entangled states. Depending on the linear dimensions of the crystal we estimate that a pair of atoms entangled in a photonic crystal can be separated by tens of centimeters. Receive 11 June 1999 and Received in final form 4 October 1999     

The effect of deformation and forest dislocations density on the activation volume in zinc single crystals

The obstacle density calculated from the experimental values of the activation volume is compared with the density of forest dislocations. The density of obstacles is about two or three order higher than that of the forest dislocations. It is assumed that nonconservative jog motion is the rate-controlling mechanisms in stage A, while in stage B the activation volume is more directly determined by interaction with the forest dislocations.     

Sound velocity anisotropy in cubic crystals

Simple analytical expressions may be derived for sound velocities in cubic crystals by using lattice harmonics or functions which are invariant under the crystal symmetry operations. These expressions are in good agreement with the exact results for typical crystals such as metallic iron and potassium fluoride.     

Local-field anisotropy in cholesteric liquid crystals

The first experimental values of the Lorentz tensor components L _j for the cholesteric and smectic A phases of derivatives of cholesterol have been obtained using the dispersion of the refractive indices in the visible range. The temperature dependence of the components L _j has been determined; it is invariant with respect to the cholesteric-smectic A phase transition. The effect of the isotropization of the Lorentz and local field tensors with decreasing anisotropy of the molecular polarizability has been revealed.     

Muon capture in atoms,crystals and molecules

The slowing down and capture of negative muons in solids is discussed on the basis of classical equations of motion where the energy dissipation is described in terms of frictional forces. Transport equations determining the energy distribution of the muons in the target are formulated and solved for various models. Using a statistical model of the atom it is shown that the muons are captured typically at energies of a few tens of electron volts, that the angular momentum distribution of the muons at capture is almost statistical, and that this distribution is not expected to be qualitatively changed by the subsequent cascade to the tightly bound orbits. In mixtures of atoms with atomic numbers Z₁ and Z₂ the capture ratio is to a good approximation proportional to the atomic concentration and, in the statistical model, proportional to $\text{(}\text{Z}{}_1\text{Z}{}_2\text{)}{}^{\mathrm{<mtext>7</mtext><mtext>6</mtext>}}$. Calculations are also performed with more accurate atomic models, and it is shown that capture ratios as well as angular momentum distributions are influenced by the ionicity of the atomic bonds and the atomic shell structure. No systematic study of these effects has been made but the few results obtained seem to be in reasonable agreement with experiments.     

Enhancement of phase space density by increasing trap anisotropy in a magneto-optical trap with a large number of atoms

The phase space density of dense, cylindrical clouds of atoms in a 2D magneto-optic trap is investigated. For a large number of trapped atoms (>10(8)), the density of a spherical cloud is limited by photon reabsorption. However, as the atom cloud is deformed to reduce the radial optical density, the temperature of the atoms decreases due to the suppression of multiple scattering leading to an increase in the phase space density. A density of 2 x 10(-4) has been achieved in a magneto-optic trap containing 2 x 10(8) atoms.     

The lattice dynamics of systems with several identical atoms per unit surface cell: a unit cell spectral density

In studies of the lattice dynamics of surfaces in which there are many identical atoms in the unit cell, for example a reconstructed surface, there is a problem in presenting the results. The problem arises because each atom in the unit cell has a different spectral density and it is not, in general, clear which of the many densities should be presented. We propose here that in such studies a “unit cell spectral density”, one which replaces the collection of densities for individual atoms with a single density, should be used. The relationship of this unit cell spectral density to experimental phonon measurements is also discussed.     

Thermalization of positrons and positronium atoms in ionic crystals

It is shown that total positron scattering on acoustic and optical lattice vibrations in atomic semiconductors and ionic crystals causes total positron thermalization before annihilation at both high and low (T≤10 K) temperatures.     

Microhardness and defectiveness of solid-solution crystals of alkali halide compounds of the system KCl-KBr

A study has been made of the dependence of microhardness (H), dislocation. density (N), vacancy-Schottky defect-density (_v), and length of the rays of etch-figure stars (L) on the composition of solidsolution crystals of the system KCl-KBr.The dependence of N on the composition of the crystals is represented by a curve with two maxima related to solid solutions with small additions of KCl and KBr. The curves of the composition dependence of H, L, _v, and melting point are asymmetrical; crystals with a smaller content of KCl in KBr correspond to a larger vacancy density _v, larger deviations from additivity of microhardness, and lower values of L and the melting point. With decrease of the bond forces in the crystal lattice of the solid solutions and with increase in the vacancy content, there is a reduction in the distance traversed by the leading dislocation under the action of a concentrated load on the crystal, a reduction in the length of the rays of the etch-figure stars, and an increase in microhardness.     

On the anisotropy of the FMR linewidth in iron-silicon single crystals

The ferromagnetic resonance linewidthH in in plane configuration was measured in single crystal discs of Fe-2·2% Si at frequency 36·14 GHz. Several methods of electrolytic and chemical polishing were used to minimize the effects of sample surface irregularities. By rotating the external field in the plane of the discs ((110) plane) the linewidth of well polished samples was found to be isotropic within the experimental errors (about 5%). Spurious anisotropy of linewidth was found in some imperfectly polished samples.     

Vibronic structure and the anisotropy of excitons in cubic LiH single crystals

The shape of one-phonon sidebands of exciton in LiH is studied both in reflection and edge luminescence spectra. The main structure of the shape is found to reflect the phonon density of states. This fact is interpreted as a result of a high anisotropy of the exciton band.     

Polarization anisotropy of optical transmission in opals and Langmuir-Blodgett crystals

The transmission spectra of thin-film colloidal photonic crystals with three-dimensional and one-dimensional-two-dimensional photonic energy band structures, i.e., opals and Langmuir-Blodgett crystals with a refractive index contrast of ∼1.5: 1.0, have been measured in linearly polarized light. It has been demonstrated that the polarization anisotropy in the light transmitted through the crystal is uniquely related to the diffraction resonance and that the degree of polarization can exceed 90%. A higher degree of polarization is provided by lattices that are characterized by a smaller attenuation of light polarized in the plane of incidence. It has been revealed that the diffraction resonances from the crystal planes for which the dispersions are in anticrossing with the dispersion of the growth planes acquire the same anisotropy. The general character of the results obtained has been confirmed by the fact that the polarization anisotropy identically manifests itself in colloidal crystals that have different symmetries and lattice orderings.     

Acceleration of atoms in crystals under plastic deformation

It is shown that a double-well potential can appear under mechanical loads for atoms in rows parallel to the axis of a screw dislocation and located near the nucleus of a dislocation and under dislocation slip conditions atoms can be accelerated up to energies much higher than the binding energy of atoms in a crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 499–504 (March 1997)     

Controlled entanglement of two atoms in photonic crystals

The exact entanglement dynamics of two dipole-dipole interacting two-level atoms coupled to a common photonic band-gap (PBG) environment has been investigated. We show that the detuning conditions and the dipole-dipole interaction (DDI) are two essential ingredients and their interplay plays a crucial role in controlling the entanglement of the two-qubit system. For the negative detuning, corresponding to the case where the atomic transition frequency is inside the band gap, the entanglement of the two-qubit system can survive in the long-time limit. For the positive detuning, although the fast disentanglement presents, the DDI effects can be used to fight against the deterioration of the entanglement. The theoretical results could be applied to the implementation of quantum information processing in nanostructured materials.     

Artificial ultrasound anisotropy of crystals in the range of X-ray frequencies

Phenomenon of artificial ultrasound anisotropy is revealed in crystals in the range of X-ray frequencies and an attempt is made to explain this effect theoretically for the case of the Bragg-Laue diffraction. It is established that an isotropic crystal becomes, under action of ultrasound, optically anisotropic like a uniaxial crystal with the optical axis along the direction of the external action; this axis is the symmetry axis and causes birefringence.