Phonon Modes in Single-Walled Molybdenum Disulphide (MoS2) Nanotubes: Lattice Dynamics Calculation and Molecular Dynamics Simulation

We study the phonon modes in single-walled MoS$_{2}$ nanotubes via the lattice dynamics calculation and molecular dynamics simulation. The phonon spectra for tubes of arbitrary chiralities are calculated from the dynamical matrix constructed by the combination of an empirical potential with the conserved helical quantum numbers $(\kappa, n)$. In particular, we show that the frequency ($\omega$) of the radial breathing mode is inversely proportional to the tube diameter ($d$) as $\omega=665.3/d$ {cm$^{-1}$}. The eigen vectors of the first twenty lowest-frequency phonon modes are illustrated. Based on these eigen vectors, we demonstrate that the radial breathing oscillation is disturbed by phonon modes of three-fold symmetry initially, and the tube is squashed by the modes of two-fold symmetry eventually. Our study provides fundamental knowledge for further investigations of the thermal and mechanical properties of the MoS$_{2}$ nanotubes.Comment: Nanotechnology, publishe

Adaptive and Robust Fault-Tolerant Tracking Control of Contact force of Pantograph-Catenary for High-Speed Trains

Abstract This paper presents a modified multi-body dynamic model and a linear time-invariant model with actuator faults (loss of effectiveness faults, bias faults) and matched and unmatched uncertainties. Based on the fault model, a class of adaptive and robust tracking controllers are proposed which are adjusted online to tolerate the time-varying loss of effectiveness faults and bias faults, and compensate matched disturbances without the knowledge of bounds. For unmatched uncertainties, optimal control theory is added to the controller design processes. Simulations on a pantograph are shown to verify the efficiency of the proposed fault-tolerant design approach

First principle study of the thermal conductance in graphene nanoribbon with vacancy and substitutional silicon defect

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect (substitution of C by Si atom) is investigated by non-equilibrium Green's function (NEGF) formalism combined with first-principle calculations density-functional theory with local density approximation. An efficient correction to the force constant matrix is presented to solve the conflict between the long-range character of the {\it ab initio} approach and the first-nearest-neighboring character of the NEGF scheme. In nanoribbon with a vacancy defect, the thermal conductance is very sensitive to the position of the vacancy defect. A vacancy defect situated at the center of the nanoribbon generates a saddle-like surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance. For the Si defect, the position of the defect plays no role for the value of the thermal conductance, since the defective region is limited within a narrow area around the defect center.Comment: accepted by AP