High resolution measurements of energy spectra of protons scattered from silicon crystals in the case of planar channelling

The energy spectrum of backscattered protons in the case of incidence along several planar directions shows a fine structure near the high-energy edge. This structure, an oscillatory dependence of the probability of backscattering vs. depth in the crystal, offers a possibility to study the proton trajectory in the lattice and also to obtain the stopping power of protons near planes in silicon.

Application of a simple model for the proton trajectory yields a stopping power near the planes 4 to 5 times higher than for random incidence. These effects have been observed using primary energies in the range 40–140 keV and for incidence along (110). (111), (100) and (112) planes.     

摩擦基板间液晶薄层的分子场理论

提出一种新的表面作用势用于描写摩擦基板对液晶分子的作用.通过分子场理论研究摩擦基板间的液晶薄层,基板作用使液晶分子沿面平行排列.采用Lebwohl Lasher模型,将分子质心固定在简单立方晶格的格点上.对20个分子层构成的液晶薄层进行了数值计算.存在3种分子优先取向方向不同的状态,优先取向方向沿摩擦方向的状态具有最低自由能,并考虑了基板作用引起的双轴对称性.     

Chaotic behavior of channeling particles

Channeling describes the collimated motion of energetic charged particles along the lattice plane or axis in a crystal. The energetic particles are steered through the channels formed by strings of atomic constituents in the lattice. In the case of planar channeling, the motion of a charged particle between the atomic planes can be periodic or quasiperiodic, such as a simple oscillatory motion in the transverse direction. In practice, however, the periodic motion of the channeling particles can be accompanied by an irregular, chaotic behavior. In this paper, the Moliere potential, which is considered as a good analytical approximation for the interaction of channeling particles with the rows of atoms in the lattice, is used to simulate the channeling behavior of positively charged particles in a tungsten (100) crystal plane. By appropriate selection of channeling parameters, such as the projectile energy E(0) and incident angle psi(0), the transition of channeling particles from regular to chaotic motion is demonstrated. It is argued that the fine structures that appear in the angular scan channeling experiments are due to the particles' chaotic motion.     

Study of Friction between Liquid Crystals and Crystalline Surfaces by Molecular Dynamic Simulations

The lubrication characteristics of liquid crystal (LC) molecules sheared between two crystalline surfaces obtained from molecular dynamics (MD) simulations are reported in this article. We consider a coarse-grained rigid bead-necklace model of the LC molecules confined between two atomic surfaces subject to different shearing velocities. A systematic study shows that the slip length of LC lubrication changes significantly as a function of the LC-surface interaction energy, which can be well described though a theoretical curve. The slip length increases as shear rate increases at high LC-surface interaction energy. However, this trend can not be observed for low interaction energy. The orientation of the LC molecules near the surface is found to be guided by the atomics surfaces. The influence of temperature on the lubrication characteristics is also discussed in this article.     

Deflection of MeV protons by an unbent half-wavelength silicon crystal

The interaction of a 2?MeV proton beam with an ultrathin unbent Si crystal was studied through simulation and experiment. Crystal thickness along the beam was set at 92?nm, i.e., at half the oscillation wavelength of the protons in the crystal under planar channeling condition. As the nominal beam direction is inclined by less than the critical angle for planar channeling with respect to the crystal planes, under-barrier particles undergo half an oscillation and exit the crystal with the reversal of the transverse momenta; i.e., the protons are "mirrored" by the crystal planes. Over-barrier particles suffer deflection, too, to a direction opposite that of mirroring with a dynamics similar to that of volume reflection in a bent crystal. On the strength of such coherent interactions, charged particle beams can be efficiently steered through an ultrathin unbent crystal by the same physical processes as for thicker bent crystals.     

Band crossing and surface states in solids

A symmetry criterion for the existence of surface states in model calculations in solids is derived. The criterion applies to surfaces that coincide with symmetry planes of the ideal crystal. A detailed analysis is carried out for one-dimensional model calculations and it is shown how band crossing can be interpreted in the framework of the crystal symmetry.     

Reflection and transmission of phonons at a planar interface between two dissimilar solids

Two semi-infinite dissimilar crystals with, however, the same crystal structure and lattice parameter are in contact at a planar interface. Using a simple force constant model, restricted to near-neighbour interaction reflection and transmission coefficients for sound waves propagating along one solid are calculated. At low frequencies the reflection and transmission coefficients are determined solely by the force constants and the atomic masses in the two media. At high frequencies the transmission coefficient becomes small if the force constant at the interface between the two media is weak. Information of the local force constant in the interface region can be gained at least in principle, by measuring the reflection and transmission coefficients at high frequencies.     

Potential energy of interaction between a fast multiply charged ion and crystal atoms with regard for the Pauli principle

Expressions for the potential energy of interaction between a fast neutral atom or a multiply charged ion and a crystal atom are obtained, both with and without regard for the Pauli principle. Results from computer simulation of the heating of silver ions with a fixed value of the charge state in passing through the (110) planar channel of a silicon crystal are presented. It is shown that the rate of dechanneling can be estimated by the form of the potential energy of interaction of an incident ion with a continuous potential of the planar crystal channel.     

Volatile Phthalocyanines: Vapor Pressure and Thermodynamics

The current review summarizes the data of saturated vapor pressure and thermodynamic parameters of the sublimation process of different phthalocyanines as reported in the literature as well as in our work. The volatility of phthalocyanines is analyzed from the standpoint of their molecular and crystal structure. The differences in the saturated vapor pressure value of the investigated phthalocyanines may reach some orders of magnitude and are determined by the Van der Waals and electrostatic interaction of the peripheral atoms of adjacent molecules, as well as to a specific interaction, the type and number of which depend on the type of molecules packing in the crystal.     

Study of intrinsic anchoring in nematic liquid crystals based on modified Gruhn-Hess pair potential

A nematic liquid crystal slab composed of N molecular layers is investigated using a simple cubic lattice model, based upon the molecular pair potential which is spatially anisotropic and dependent on elastic constants of liquid crystals. A perfect nematic order is assumed in the theoretical treatment, which means the orientation of the molecular long axis coincides with the director of liquid crystal and the total free energy equals to the total interaction energy. We present a modified Gruhn-Hess model, which is relative to the splay-bend elastic constant K₁₃. Furthermore, we have studied the free nematic interfacial behavior (intrinsic anchoring) by this model in the assumption of the perfect nematic order. We find that the preferred orientation at the free interface and the intrinsic anchoring strength change with the value of modification, and that the director profile can be determined by the competition of the intrinsic anchoring with external forces present in the system. Also we simulate the intrinsic anchoring at different temperatures using Monte Carlo method and the simulation results show that the intrinsic anchoring favors planar alignment and the free interface is more disordered than the bulk.     

Pressure-induced dislocation amorphization of crystals

In terms of the isotropic elastic crystal model, it is shown that the formation of planar layers consisting of edge dislocation pileups is advantageous in energy if the shear modulus of the crystal is far lower than the bulk modulus. As pressure rises, the dislocation radius decreases, which can destroy the crystal structure.     

The ordering of crystallographic shear planes: Theory of regular arrays

In certain non-stoichiometric transition metal oxides, crystallographic shear planes are found instead of isolated oxygen vacancies. These planes tend to form arrays with regular spacings. The interactions responsible for this ordering have not been understood in the past. We propose that the major interaction comes from elastic strain, with smaller electrostatic terms: the configuration of the planes is one which minimises the elastic strain energy. Quantitative results are given for a realistic model, and agree with the main features observed experimentally. Thus the regular array of planes is stable, and the equilibrium separation of planes in the array is about half that predicted for an isolated pair of planes. The interaction between isolated planes proves an oscillatory function of their separation. One can understand from its detailed form why the observed mean spacing gets smaller as the planes come together to form larger groups.     

The Hausdorff chirality measure and a proposed Hausdorff structure measure

The Hausdorff chirality measure quantifies the chirality of a geometric representation of an object by measuring the degree of coincidence of the object with its mirror image. It can also allow comparison between a chiral dopant and host molecules which may illuminate mechanisms for chirality transfer. It has been applied to real molecules very infrequently in comparison to application of chiral indices as it is complex and time consuming to calculate. In this paper we introduce and verify a simulated annealing algorithm for the Hausdorff chirality measure that has proven rapid, robust and relatively simple to apply. We verify the method, finding good agreement between its results and those of Mislow and co-workers. We introduce a Hausdorff structure measure that does not permit overlap and allows a structure to be built one molecule at a time. We present results for a simple model and real biphenyl molecules and discuss promising building blocks of crystal and incommensurate structures formed in relation to experimental results.     

Adsorbate-adsorbate interactions and the ordering of organic monolayers on metal surfaces

To study the ordering of molecules adsorbed on single-crystal substrates, a molecular cross-section (MCS) is defined, which measures the surface area occupied by each molecule. With this MCS, a two-dimensional packing coefficient C_2D is then defined for ordered arrays of adsorbed molecules. Values and trends for MCS and C_2D are discussed for known surface structures, especially for benzene adsorbed on metal surfaces. The packing is found to be generally less dense at surfaces than one would expect from comparison with packing in three-dimensional organic crystals. The Van der Waals packing energy and the repulsive dipole-dipole energy are also computed to study this issue. The lack of close-packing is attributed to the need to respect structural coincidence with the substrate and/or co-adsorption of small molecules like CO. These concepts are then applied to the prediction of the long-range order that a monolayer of adsorbed molecules may adopt: thereby possible adsorption structures can be defined, restricting the number of possibilities in a further structural determination.     

Multicrystal microundulator

The mode of propagation of relativistic, positively charged particles through a system of mutually oriented and periodically arranged ultrathin crystals whose thicknesses are equal to the half-period of the particle trajectory during planar channeling in a thick crystal is considered. In the case of an incidence angle that is less than the critical channeling angle, a certain fraction of particles is specularly reflected from the atomic planes of the crystal. Therefore, passing through a stack of crystals, a particle moves along quasiundulator trajectories. The characteristics of the radiation of a particle passing through such a “multicrystal microundulator” are found. The radiation spectrum is discrete, and the first-harmonic frequency and the number of harmonics in the spectrum are dependent on the distance between the crystals, the particle energy, and the potential of atomic planes of the crystal. Radiation is concentrated in a narrow cone in the direction of the average velocity of particles and is mainly polarized in a plane that is orthogonal to the atomic planes of the crystal. The microundulator can be composed of separate crystals with micron thicknesses and can be fabricated using modern methods of microlithography and micromechanics with deep, for example, plasmochemical etching of the crystal surface.     

Two Gaps in Semiconducting EuSbTe_3 Studied by Angle-Resolved Photoemission Spectroscopy

Using angle-resolved photoemission spectroscopy, we study the low-energy electronic structure of a layered ternary telluride EuSbTe_3 semiconductor. It is found that the photoemission constant energy contours can be well described by the simple two-parameter(t_(perp) and t_(para)) tight-binding model based on the Te orbitals in square-net planes of EuSbTe_3, suggesting its Te 5p orbitals dominated low-lying electronic structure, which is reminiscent of other rare-earth tritellurides. However, a possible charge-density-wave gap of 80 meV is found to persist in 300 K,which renders the unexpected semiconducting properties in EuSbTe_3. Moreover, we reveal an extra band gap occurring around 200 meV below the Fermi level at low temperatures, which can be attributed to the interaction between the main and folded bands due to lattice scatterings. Our findings provide the first comprehensive understanding of the electronic structure of layered ternary tellurides, which lays the basis for future research on these compounds.     

Bistable surface anchoring and hysteresis of pitch jumps in a planar cholesteric liquid crystal

The relationship between bistable surface anchoring and the pitch jump process is examined for a planar cholesteric liquid crystal. Introducing a high-order, azimuthal surface anchoring potential into a simple model to describe a cholesteric, we derive an expression for the director twist as the natural pitch of the liquid crystal is allowed to vary. Writing the energy in terms of the surface twist, we are able to determine the twists which minimize the total energy of the system. We demonstrate how a pitch jump is related to an energy exchange from one branch of metastable states to another. We then discuss how the co-existence of energy minima and their associated solution branches may help explain the thermal hysteresis observed experimentally in cholesterics in the neighbourhood of a pitch jump. The presence of a higher-order surface energy term can expand the range of anchoring strengths in which pitch jumps are possible. We also investigate the influence of bidirectional surface anchoring on the behaviour of the total energy. Intermediate quarter-turn pitch jumps can occur, depending on the relative strength of the high-order anchoring term, and these can have a significant effect on the system hysteresis.     

Threshold effects in homeotropically oriented nematic liquid crystals in an external electric field

The electric field-induced orientational transition in a homeotropically oriented nematic liquid crystal cell is investigated. The interaction with the field as a result of anisotropy of the permittivity and flexoelectric polarization is taken into account. For an arbitrary energy of interaction of the nematic with the substrate simple relations are derived for determining the threshold characteristics of the phase transition. It is shown that, in contrast with planar orientation, in fields above a critical value a periodic structure can occur only by virtue of the flexoelectric effect. The resulting dependences for the threshold parameters in the given geometry are exceptionally useful for determining experimentally the surface energy and the difference in the flexoelectric coefficients. Zh. éksp. Teor. Fiz. 116, 543–550 (August 1999)     

Planar Channelling Criticalities of MeV Protons in Si Crystal: Simulations, Evaluation and Applications

We reports a phase-space structure of MeV proton beam planar channelled along {110} planes in Si crystal using simulation results with the help of a computer code FLUX. The aim is to understand channelling conditions suitable for disorder measurement in crystals. Phase-space distribution of a planar channelled proton beam evolutes in a systematic fashion when it travels into the crystal. Planar channelled beam oscillates between phase-like and space-like conditions in which a part of the beam becomes under phase and space criticalities. These criticality conditions in planar channelling are analysed, explained and discussed with the perspective of defect measurement in crystals.