Scanning probe microscopy on superconductors: Achievements and challenges

A dislocation dynamical model of the reaction-diffusion type is used to describe the spatio-temporal dynamics of Lüders band propagation in polycrystals. The diffusive nature of dislocation glide is traced back to the random crystallographic orientation of the active slip systems. The role of pile-ups in dislocation multiplication is accounted for by a dynamical generalization of the Hall-Petch law. It is argued that Lüders bands in polycrystals are related to a bistable dynamics of mobile dislocations. Further results obtained cover the dependences on material parameters and deformation conditions of (1) the occurrence, (2) the strain, propagation velocity and width of Lüders bands, and (3) the upper and lower yield stresses. These results are in good agreement with experimental findings.     

Partition of excitation energy between reaction products in heavy ion collisions

In the single-particle approach a partition of the excitation energy between the reaction products in deep inelastic collisions of heavy ions are investigated. The role of the particle-hole excitations and the nucleon exchange is considered. The ratio of the projectile excitation energy to the total excitation energy for the reactions²³⁸U(1468 MeV)+¹²⁴Sn,²³⁸U(1398 MeV)+¹¹⁰Pd,⁵⁶Fe(505MeV)+¹⁶⁵Ho,⁷⁴Ge (629 MeV)+¹⁶⁵Ho and⁶⁸Ni(880 MeV)+¹⁹⁷Au is calculated. The results of calculations are in good agreement with the experimental data.We are grateful to Dr. N.V. Antonenko for valuable discussions. This work was supported partly by the Russian Minister for Education and Research under the Grant N2-61-13-28.     

Spatio-temporal aspects of low-temperature thermomechanical instabilities: A model based on dislocation dynamics

The occurrence of plastic instabilities which are accompanied by a significant heat release is a typical feature of the plastic behaviour of metals deformed at sufficiently low temperature. This phenomenon may be studied within the framework of a dislocation-dynamical model. The influence of the heat which is released by the deformation process on the dislocation velocity, and thus on the deformation dynamics, is taken into account. In particular, the influence of the spatial coupling which arises from heat conduction on the spatio-temporal behaviour of the deformation process is studied.     

Reconstruction of internal waves in oceanic waveguides

Reconstruction of the temporal variability of an intense internal wave field is studied by a numerical experiment. The inverse problem is solved using the data on the frequency shifts of the maxima of the inverted wave field. The influence of the amplitude of the internal wave on the efficiency of reconstruction is analyzed.     

Tunable optical limiting of gold nanorod thin films

Permalloy (Py) films were deposited on Si(111) or Corning 0211 glass substrates. There were two deposition temperatures: T _s=room temperature (RT) and T _s=270°C. The film thickness (t _f) ranges from 10 to 130 nm. The crystal structure properties of the films were studied by X-ray diffraction and transmission electron microscopy. Mechanical properties (including Young’s modulus E _f and hardness H _f) of each film were measured by the nanoindentation (NI) technique. E _f of the Py/Si(111) films was checked again by the laser induced surface acoustic wave (LA-SAW) technique. It was found that the NI technique is best suited for the measurements of E _f and H _f, but only when the sample belongs to the (soft film)/(soft substrate) system, such as the Py/glass film. For the (soft film)/(hard substrate) system, such as the Py/Si(111) film, the NI technique often provides higher values of E _f and H _f than expected. The anomalous phenomenon, associated with the NI technique may be related to the anisotropic crystal structures in the Py films on different kinds of substrates. From this study, we conclude that [E _f of Py/Si(111)]>[E _f of Py/glass] and [H _f of Py/Si(111)]>[H _f of Py/glass]. The good mechanical properties of the Py/Si(111) film make it a better candidate for recording head applications.     

On the electronic contribution to the elastic constants in strained quantum wire superlattices of non-parabolic semiconductors with graded structures: Theory and suggestion for experimental determination

Summary  We study the electronic contribution to the second- and third-order elastic constants in strained quantum wire superlattices of non-parabolic semiconductors with graded structures and compare the same with the constituent materials, by formulating the appropriate dispersion laws. It is found, taking InSb/GaSb quantum wire superlattice as an example, that the said contributions increase with decreasing thickness and with increasing electron concentration in oscillatory manners together with the fact that the influence of the finite interface width enhances their numerical values. An experimental method is suggested for determining the electronic contribution to the elastic constants in materials having arbitrary dispersion laws. In addition, the well-known results for constituent semiconductors in the absence of stress have also been obtained as special cases of our generalized formulations.     

Thickness distribution of thin amorphous chalcogenide films prepared by pulsed laser deposition

Amorphous chalcogenide thin films were prepared from As₂Se₃, As₃Se₂ and InSe bulk glasses by pulsed laser deposition using a KrF excimer laser. Thickness profiles of the films were determined using variable angle spectroscopic ellipsometry. The influence of the laser beam scanning process during the deposition on the thickness distribution of the prepared thin films was evaluated and the corresponding equations suggested. The results were compared with experimental data.     

Correlations of projectile-like fragments in heavy-ion reactions at Fermi energy

Correlations between pairs of projectile-like fragments, emitted by the system ¹⁶O$ + $¹⁹⁷Au at the laboratory bombarding energy of 515MeV, have been studied under two stipulated conditions: 1) at least one member of the pair is emitted at an angle less than the grazing angle for the system, 2) both the members of the pair are emitted at angles larger than the grazing angle. A surprisingly large difference, by more than an order of magnitude, is found between the correlations for the two cases. This observation could be explained on the basis of a simple semi-classical break-up model. Further analysis of the variation of the charge correlation function with the difference in the nuclear charges of the correlated pair showed trends which are consistent with an “inelastic break-up process”, in which the projectile breaks up at the radius of contact, in such a way that, one fragment (preferably the lighter) is emitted to one side within the grazing angle, while the other orbits around the target nucleus for a while and emerges on the other side, at a negative scattering angle, much like in a deep inelastic scattering.     

The effect of angular-momentum dissipation on fluctuations of excitation functions in heavy-ion collisions

We discuss here the effect of dissipation of relative angular momentum on fluctuations of excitation functions in dissipative heavy-ion collisions. Dissipation and fluctuation of relative angular momentum modify and smooth the time-angle localization of the rotating dinuclear system. The secondary maxima in the energy correlation function of the cross-section shift to smaller values of the energy difference, the shift depending on the relaxation time and the diffusion coefficient for angular-momentum dissipation. The results are illustrated for the collision²⁸Si(E _lab=130 MeV)+⁴⁸Ti.Partly supported by the Alexander von Humboldt Foundation     

Mechanism of enhanced yield of light particles in compound nucleus formation: Diffusion description

In the formation of a compound nucleus the evolution of a dinuclear system is considered. The enhanced yield of light particles for some reactions is explained by the dynamic reasons. The role of quantum and thermal fluctuations is discussed. The results of the previous paper are confirmed.Authors wish to thank Prof. V.V. Volkov for his active attention and comments.     

Neutrino absorption by <Emphasis Type="Italic">W</Emphasis> production in the presence of a magnetic field

In this work we calculate the damping rate of the electron type neutrinos into W bosons and electrons in the presence of an external uniform magnetic field. The damping rate is calculated from the imaginary part of the W exchange neutrino self-energy diagram but in the weak field limit, and we compare our result with the existing one.     

Finite element simulation for estimating the mechanical properties of multi-walled carbon nanotubes

A finite element simulation technique for estimating the mechanical properties of multi-walled carbon nanotubes is developed. In the present modeling concept, individual carbon nanotube is simulated as a frame-like structure and the primary bonds between two nearest-neighboring atoms are treated as beam elements, the beam element properties are determined via the concept of energy equivalence between molecular dynamics and structural mechanics. As to the simulation of the interlayer van der Waals force which has intrinsic nonlinearity and complicated applying region, a simplifying method is proposed that the interlayer pressure caused by van der Waals force instead of the force itself is to be considered, and we make use of the linear part of the interlayer pressure near the equilibrium condition to avoid the nonlinearity in problem, then linear spring elements whose stiffness is determined by equivalent force concept can be utilized to simulate the interlayer van der Waals force such that significant modeling and computing effort is saved in performing the finite element analysis. Numerical examples for estimating the mechanical properties of nanotubes, such as axial and radial Young’s modulus, shear modulus, natural frequency, buckling load, etc., are presented to illustrate the accuracy of this simulation technique. By comparing to the results found in the literature and the possible analytical solutions, it shows that the obtained mechanical properties of nanotubes by the present method agree well with their comparable results. In addition, the relations between these mechanical properties and the nanotube size are also discussed.     

Propagation of cylindrical shock waves in a rotating atmosphere under the action of monochromatic radiation

Summary  A model of cylindrical shock waves is discussed in a non-uniform rotating atmosphere under the action of monochromatic radiation. We have assumed that the radiation flux moves through a rotating gas with constant intensity and the energy is absorbed only behind the shock wave which moves in opposite direction to the radiation flux.     

Post-deposition processes for nanostructures formed by electron beam induced deposition with Pt(PF<Subscript>3</Subscript>)<Subscript>4</Subscript> precursor

Electron beam induced deposition was performed using a Pt(PF₃)₄ precursor gas. Self-standing nanowires were produced on the edge of a molybdenum film, followed by two post-deposition processes; electron beam irradiation at room temperature and heating at about 400 K in vacuum. The as-deposited nanowires were composed of an amorphous phase, of which the dominant composition was platinum but containing a small amount of phosphorus impurity. After irradiating with a 300 keV electron beam, the amorphous nanowires were transformed to crystalline ones. By heating, the as-deposited nanowires became single-crystal platinum with a large grain size and the phosphorus content disappeared.     

Time-resolved,stress-induced,and anisotropic phase transformation of a piezoelectric polymer

We investigate the time-dependent and anisotropic phase transformation of poly (vinylidene difluoride) (PVDF) under bending. Using combined techniques of an atomic force microscope and a Fourier transform infrared spectroscope, observation of surface morphology and phase transformation in time was made. Results showed that bending stress induces the transformation of amorphous, α,β, and γ crystalline phases. Specifically, the amorphous phase was transformed into the β phase when the bending force was applied. In addition, the transformation observed was time and direction dependent. The anisotropic behavior observed brings insights into the origin of the piezoelectricity of PVDF.     

Low-temperature STM/STS investigation of nanostructures created by an STM tip on Au(111) surfaces

The structural stability of rapidly solidified (about 10⁴ K/s) Sn–3.7Ag–0.9Zn eutectic solder was explored by high-temperature annealing. For the as-cast solders, the applied fast cooling rate had a significant influence on the microstructure of the solders. The faster the applied cooling rates, the smaller the β-Sn dendrites. After annealing at 473 K for 20 and 50 h, β-Sn dendrites congregated together into bulk ones for minimizing the interfacial energy, and Ag₃Sn intermetallic compounds (IMCs) as well as ternary Ag–Zn–Sn IMCs segregated on the grain boundary of the β-Sn dendrites. It seems that the coarsening of the β-Sn dendrites in the rapidly solidified specimen brought a significant softening during annealing of the explored Sn–Ag–Zn alloy. Finally, the β-Sn dendrites vanished gradually with increase of the annealing period, which leads to a kind of softening.     

Fringe waves radiated by a half-plane for the boundary conditions of Neumann

The fringe waves of the physical theory of diffraction are obtained in terms of Fresnel integrals for a half-plane satisfying the Neumann boundary condition. The approximate expressions of the radiated waves are also evaluated for sufficiently large wavenumbers. The fields are plotted and compared numerically.     

Solidification velocity in liquid silicon during excimer laser crystallization

Excimer laser crystallization (ELC) is commonly employed to fabricate low-temperature polycrystalline silicon. A time-resolved in-situ optical system with nanosecond response time is developed to monitor and record the phase transformation process during ELC. The average solidification velocity of liquid silicon (liquid Si) is investigated from the optical spectra recorded by a fast oscilloscope. It is found that the average solidification velocities of liquid Si in the partial-melting and complete-melting regimes are fundamentally different. In the partial-melting regime, the average solidification velocity decreases with increasing excimer laser energy density; while in the complete-melting regime, it increases abruptly due to the presence of deeply supercooled liquid Si.