Bulk nanomaterials made of carbon nanotubes (CNTs) possess unique physical and mechanical properties that are used in a number of applications, for example, for creating mass or force sensors, ...protection against shock and vibration. One of the specific implementations of such materials is a CNT bundle in which aligned CNTs are held together due to van der Waals interactions. Linear and nonlinear dynamic excitations in such materials are still poorly understood. Here we study plane waves and spatially localized nonlinear dynamic modes in a CNT bundle under plane strain conditions, when the CNT cross sections completely determine the deformed state of the bundle. In particular, we analyze vibration spectrum of single CNT, dispersion curves of phonons propagating along the close packed direction of CNT bundle, compressive solitons and discrete breathers in the form of single CNT oscillating at large amplitude in the CNT bundle. It is shown that compressive solitons can propagate long distances in the bundles of CNTs having radius no more than about 0.8 nm. Discrete breathers can exist in bundles of CNTs of any radius. The results obtained revealed new channels of energy localization and transport in CNT bundles, which is important for their use in various technologies.
•Linear and nonlinear dynamics of carbon nanotube (CNT) bundles was investigated.•Bundles with CNTs of radius R<0.8 nm support propagation of compressive solitons.•Energy dissipation rate of a soliton increases as the diameter of the CNT increases.•Discrete breathers can be created by radial or angular vibrational modes of CNT.•Discrete breathers can exist in bundles of CNTs of any radius.
A general approach is applied to study a new type of intrinsic spatially localized vibrational modes in a defect free nonlinear crystal lattice, i.e., discrete breathers (DBs). For that, dynamics of ...eight delocalized nonlinear vibrational modes (DNVMs) of two-dimensional triangular lattice is investigated in three-dimensional single crystal of hcp Ti. Molecular dynamics simulations are performed using two interatomic potentials (Ti_EAM and Ti_MEAM). The eight DNVMs modeled with Ti_EAM potential are found to be unstable and dissipate their vibrational energy very rapidly. The usage of Ti_MEAM interatomic potential allows to excite stable two-dimensional (planar) DBs. These localized vibrational modes can be called DBs, since the frequency of atomic oscillations is above the upper edge of the phonon spectrum of Ti, and the atomic oscillations are localized in one spatial direction and delocalized in the other two directions. The lifetimes of the two-dimensional DBs are in the range of 5–14 ps, while the maximal lifetime of DBs excited on the basis of DNVM 7 is circa 28 ps. These DBs can accumulate vibrational energy, which is in the range of 0.1–0.5 eV per atom. The stable two-dimensional DBs are characterized by a hard type of nonlinearity. A comparison with analogous two-dimensional DBs in fcc metals are undertaken. The obtained results make a significant contribution to the study of DBs in metals and will be important for understanding the influence of intrinsic localized vibrational modes on the physical properties of materials.
Graphical abstract
•Pt3Al intermetallic alloy supports near-surface discrete breathers (DB).•Properies of DB depend on surface orientation and termination.•DB can localize energy about 1 eV.•DB can help to overcome ...potential barriers for structure transformations.
It is known that defect-free crystals can support spatially localized, large amplitude vibrational modes with frequencies outside the linear phonon spectrum. Such excitations are called discrete breathers (DB) or intrinsic localized modes. So far, for 3D crystals DB were considered only in the bulk. In the present molecular dynamics study, for the first time, we demonstrate that DB can be excited at the low Miller indices surfaces of the Pt3Al intermetallic alloy. It is shown that properties of the DB depend essentially on the surface orientation and termination, as well as on DB polarization. The study of DB at crystal surfaces is important because they can localize energy of order of 1 eV, which can reduce the potential barrier for local structure transformation or a chemical reaction.
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Delocalized nonlinear vibrational modes in fcc metals Shcherbinin, S.A.; Krylova, K.A.; Chechin, G.M. ...
Communications in nonlinear science & numerical simulation,
January 2022, 2022-01-00, 20220101, Letnik:
104
Journal Article
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Nonlinear lattices support delocalized nonlinear vibrational modes (DNVMs) that are exact solutions to the dynamical equations of motion dictated by the lattice symmetry. Since only lattice symmetry ...is taken into consideration for derivation of DNVMs, they exist regardless the type of interaction between lattice points, and for arbitrary large amplitude. Here, considering space symmetry group of the fcc lattice, we derive all one-component DNVMs, whose dynamics can be described by single equation of motion. Twelve such modes are found and their dynamics are analyzed for Cu, Ni, and Al based on ab initio and molecular dynamics simulations with the use of two different interatomic potentials. Time evolution of atomic displacements, kinetic and potential energy of atoms, and stress components are reported. Frequency–amplitude dependencies of DNVMs obtained in ab initio simulations are used to assess the accuracy of the interatomic potentials. Considered interatomic potentials (by Mendelev et al. and Zhou et al.) for Al are not as accurate as for Cu and Ni. Potentials by Mendelev can be used for relatively small vibration amplitudes, not exceeding 0.1 Å, while potentials by Zhou are valid for larger amplitudes. Overall, the presented family of exact solutions of the equations of atomic motion can be used to estimate the accuracy of the interatomic potentials of fcc metals at large displacements of atoms.
•Delocalized nonlinear vibrational modes (DNVMs) of the fcc lattice are presented.•Dynamics of DNVMs are analyzed numerically for Cu, Ni, and Al.•Comparison of ab initio and MD results helps to assess the interatomic potentials.
The analytical expressions for coherent and diffuse components of the integrated reflection coefficient are considered in the case of Bragg diffraction geometry for single crystals containing ...randomly distributed microdefects. These expressions are analyzed numerically for the cases when the instrumental integration of the diffracted X‐ray intensity is performed on one, two or three dimensions in the reciprocal‐lattice space. The influence of dynamical effects, i.e. primary extinction and anomalously weak and strong absorption, on the integrated intensities of X‐ray scattering is investigated in relation to the crystal structure imperfections.
A theoretical analysis is carried out of the influence of microdefects on the dynamical effects of primary extinction and anomalous absorption in the integrated intensities of coherent and diffuse scattering of X‐rays by imperfect crystals in Bragg diffraction geometry.
The rapid development of new technologies is often associated with the realization of nonequilibrium states in materials, in which new mechanisms of structure evolution, different from the ...traditional ones, can arise. One example is the formation of crowdions, that is, interstitial atoms located in close-packed atomic rows. Crowdions can move at subsonic or supersonic speeds. It has previously been demonstrated that supersonic crowdion clusters are much more efficient at transferring mass than classic supersonic crowdions. This work presents an analysis of the propagation of supersonic crowdion clusters in parallel close-packed atomic rows in an fcc Morse crystal. Supersonic 2-crowdions are excited in four close-packed atomic rows, between which there is one close-packed row, which is not initially excited. The counterintuitive formation of a vacancy in the inner atomic row was observed rather far from the point of excitation. The distance between the vacancy in the inner row and the initiation point depends on the initiation energy. The mechanism of vacancy formation is described. The results obtained can be useful for analyzing the rearrangement and accumulation of defects in materials under extreme conditions.
Graphic abstract
The localization of energy on chaotic discrete breathers (DBs) arising in a two- dimensional triangular lattice due to the modulation instability of delocalized nonlinear vibrational modes (DNVMs) is ...analyzed. Three DNVMs with frequencies above the phonon band and demonstrating hard-type anharmonicity (an increase in the vibration frequency with amplitude) are considered. Chaotic DBs have long lifetime, slowly radiate their energy and eventually disappear. The evolution of the macroscopic characteristics of the lattice is observed during the transition from the regime with chaotic DBs to thermal equilibrium. It is established that chaotic DBs with a hard type of anharmonicity reduce the ratio of the total energy to the kinetic energy (and, consequently, reduce the heat capacity). They also reduce lattice pressure at constant area (and therefore reduce thermal expansion). The tensile rigidity of the lattice also decreases due to DBs with a hard type of anharmonicity. The most sensitive to the presence of DBs is the pressure, which in the presence of DBs is approximately 30% less than in thermal equilibrium. The ratio of the total energy to the kinetic energy in the regime of chaotic DBs decreases by about 3%, and the tensile rigidity by only 0.1%.
•Dynamics of triangular beta-Fermi–Pasta–Ulam lattice is investigated.•Discrete breathers appear as a result of instability of delocalized vibrational modes.•Discrete breathers affect macroscopic properties of the lattice.
•A spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target is developed.•The circularly polarized cathodoluminescence (CL) technique is used to ...study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells.•Quantitative measurement of spin polarization in photoelectron emission spectroscopy.•Semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free electron spin polarization.
The circularly polarized cathodoluminescence (CL) technique has been used to study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells (QWs). A polarized electron beam was created by the emission of optically oriented electrons from the p-GaAs(Cs,O) negative electron affinity (NEA) photocathode. The prepared beam was injected in a semiconductor QW target, which was activated by cesium and oxygen to reduce the work function. To study the spin-dependent injection, we developed a spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target placed parallel to each other on the opposite ends of the vacuum tube (photodiode). The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range of 0.5-5.0 eV and temperature in the range of 90-300 K was studied. The CL was polarized to 2 % by the injection of 20 % spin-polarized electron beam with the energy of 0.5 eV at room temperature. The asymmetry (Sherman function) of spin detection was estimated. It was shown that the dependence of the CL polarization degree on the injected electron energy is satisfactory described by the model that considers the electron spin relaxation in the heterostructure matrix and QWs. The results demonstrate that semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free-electron spin polarization.
An interstitial atom placed in a close-packed atomic row of a crystal is called crowdion. Such defects are highly mobile; they can move along the row, transferring mass and energy. We generalize the ...concept of a classical supersonic crowdion to an
N
-crowdion in which not one but
N
atoms move simultaneously with a high velocity. Using molecular dynamics simulations for a close-packed two-dimensional Morse crystal, we show that
N
-crowdions transfer mass much more efficiently, because they are capable of covering large distances while having a lower total energy than that of a classical 1-crowdion.
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