Andrey V. Solov’yov

Molecular dynamics study of the stability of a carbon nanotube atop a catalytic nanoparticle

AbstractThe stability of a single-walled carbon nanotube placed on top of a catalytic nickel nanoparticle is investigated by means of molecular dynamics simulations. As a case study, we consider the (12,0) nanotube consisting of 720 carbon atoms and the icosahedral Ni309 cluster. An explicit set of constant-temperature simulations is performed in order to cover a broad temperature range from 400 to 1200 K, at which a successful growth of carbon nanotubes has been achieved experimentally by means of chemical vapor deposition. The stability of the system depending on parameters of the involved interatomic interactions is analyzed. It is demonstrated that different scenarios of the nanotube dynamics atop the nanoparticle are possible depending on the parameters of the Ni-C potential. When the interaction is weak the nanotube is stable and resembles its highly symmetric structure, while an increase of the interaction energy leads to the abrupt collapse of the nanotube in the initial stage of simulation. In order to validate the parameters of the Ni-C interaction utilized in the simulations, DFT calculations of the potential energy surface for carbon-nickel compounds are performed. The calculated dissociation energy of the Ni-C bond is in good agreement with the values, which correspond to the case of a stable and not deformed nanotube simulated within the MD approach.

PBrS from Atomic Clusters and Fullerenes

This chapter is devoted to the description of the PBrS process in in collisions involving atomic clusters.

PBrS in Non-Relativistic Collisions of Structural Particles with Atoms and Ions

The qualitative picture of the PBrS process in a collision of two structural atomic particles is as follows. The colliding particles become mutually polarized and, thus, acquire induced dipole moments.

Stimulated Emission from CU

The possibility to generate a stimulated emission of the FEL type by means of a crystalline undulator is discussed. The considered range of photon energies, \(10^2\)–\(10^3\) keV, is inaccessible in conventional FELs. Three different schemes of achieving the lasing effect are presented: (a) a gamma-amplifier based on the use of a single CU; (b) a gamma-klystron which utilizes the sequence of two (or more) CUs; (c) a Crystalline Undulator Laser. We demonstrate, that despite a number of parasitic effects, present technologies are nearly sufficient to achieve the conditions needed to achieve the emission stimulation by means of CU. It is shown, that the brilliance of CUL radiation can be as high as \(10^{28}\)–\(10^{32}\) \(\mathrm {photons/s/mrad}^{2}/\mathrm {mm}^{2}/0.1\,\%\,\mathrm {BW}\) in the sub-Angstrom range of the emitted photon wavelengths.

Polarizational Bremsstrahlung in Collisions with Hydrogen Atom

The formulae presented in Chaps. 1 and 2 indicate that to a great extent the behaviour of the total BrS cross

CUs for Electrons and Heavy Particles

Specific features of an electron-based CU and a heavy-particle-based CU are discussed. For the electron channeling through PBCr, to overcome the restriction due to the smallness of the dechanneling length, the CU must operate in the regime of higher beam energies than a positron-based one does. However, the operation of electron-based CU is also feasible for the low-energy electron beams (below 1 GeV). An algorithm of a new classical Monte Carlo code which simulates the electron channeling process is presented. For a CU based on the heavy-projectile channeling the main restrictive factor is the photon attenuation. It is demonstrated that the most feasible devices are the proton-based CU (for the projectile energies \(\varepsilon \gtrsim 1\) TeV) and the muon-based CU (for \(\varepsilon \gtrsim 10^2\) GeV). In both cases the use of light crystals (diamond, silicon) is most promising.

Positron-Based CU: Illustrative Material

Results of numerical calculations of the CUR spectra emitted by ultra-relativistic positrons channeling in PBCr are presented. The interplay of the two motions of the channeling particles,—the channeling oscillations and the undulator motions, in forming the radiative spectrum is discussed. Description the computer codes which simulate the channeling trajectories and calculate the spectral and spectral-angular distributions is given. Peak brilliance of CUR is calculated using the parameters of positron beams from several modern \(e^{+}-e^{-}\) colliders. Analytical and numerical analysis of the influence of the periodic bending imperfectness on spectral distribution of CUR is carried out.

Experimental Studies of CUR

Results of existing experimental studies of the radiation from crystalline undulator as well as of the channeling phenomenon in periodically bent crystals are reviwed. As of now, the experiments with positron beams have not presented a convincing evidence of the CUR signal. This is mainly due to the insufficient quality of the beam and of the periodic structure of crystal. The first clear experimental evidence of CUR was obtained with low-energy electron beams (below 1 GeV) at MAinz MIcrotron (MAMI) facility (Backe et al., Nuovo Cimento C 34:157–165, 2011). The CUs used in the experiment were based on graded composition strained layers Si\(_{1-x}\)Ge\(_{x}\) and were manufactured in Aarhus University (Denmark).

Schemes for Periodic Bending of Crystals

Methods, proposed to construct a PBCr suitable for generation of CU radiation are described. The crystal can be bend dynamically by transmitting a monochromatic transverse AW along a crystallographic direction. As of now, this approach awaits its practical realization. Alternative scheme of statical bending include several methods which are currently used to manufacture CUs. One of these schemes uses the technologies of growing \(\mathrm{Si}_{1-x}\mathrm{Ge}_x\) mixtures with varying germanium content \(x\). Another method is based on making periodic deformations on the crystal surface which lead to those in the bulk. This can be achieved either by making regularly spaced grooves on the crystal surface (with the help of a diamond blade or by means of laser-ablation) or by depositing tensile strips onto the surface of a crystal. In connection with periodic surface deformations method, we present a formalism which allows on to analyze the variation of the bending amplitude across the crystal thickness.

Bremsstrahlung in Collisions of Structureless Charged Particles with Atoms and Ions

In this section we discuss the peculiar features of PBrS which distinguish this mechanism from OBrS and which strongly influence the total emission spectrum.