Energy transfer from the translational degrees of freedom to phonon modes is studied for isolated systems of two coaxial carbon nanotubes, which may serve as a nearly frictionless nano-oscillator. It is found that for oscillators with short nanotubes (less than 30 A) a rocking motion, occurring when the inner tube is pulled about 1/3 out of the outer tube, is responsible for significant phonon energy acquisitions. For oscillators with long nanotubes translational energies are mainly dissipated via a wavy deformation in the outer tube undergoing radial vibrations. Frictional forces between 10(-17) and 10(-14) N per atom are found for various dissipative mechanisms.

The green emission band of ZnO has been investigated by both experimental and theoretical means. Two sets of equally separated fine structures with the same periodicity (close to the longitudinal optical (LO) phonon energy of ZnO) are well resolved in the low-temperature broad green emission spectra. As the temperature increases, the fine structures gradually fade out and the whole green emission band becomes smooth at room temperature. An attempt to quantitatively reproduce the variable-temperature green emission spectra using the underdamped multimode Brownian oscillator model taking into account the quantum dissipation effect of the phonon bath is done. Results show that the two electronic transitions strongly coupled to lattice vibrations of ZnO lead to the observed broad emission band with fine structures. Excellent agreement between theory and experiment for the entire temperature range enables us to determine the dimensionless Huang-Rhys factor characterizing the strength of electron-LO phonon coupling and the coupling coefficient of the LO and bath modes.Historically, zinc oxide (ZnO) is a technologically important material thanks to its piezoelectric characteristics and other unique properties such as its transparency up to the near ultraviolet (UV). It is also known that ZnO is a semiconductor with a wide band gap (∼3.37 eV) and an extremely large exciton binding energy (as high as 60 meV). 1 Recently, it has attracted renewed research interest due to its newly-found application potential in exciton-type short-wavelength optoelectronic devices that are functional at room temperature or above. 1-3 Despite a long history of industrial applications, a clear understanding of some fundamental properties of ZnO still remains elusive. 1,2,4-7 For example, contention still surrounds the microstructural origin. 4,8 To date, very different defect origins, such as the substitutional Cu 2+ on the zinc site, 9 oxygen vacancy (V O ), 10 zinc vacancy (V Zn ), 11 and interstitial zinc (Zn i ), 12 have been suggested to be responsible for the green band of ZnO. Among them, the substitutional Cu 2+ model proposed first by Dingle 9 has received much attention due to the distinct spectral features of a sharp zero-phonon line (ZPL) and a broad longitudinal optical (LO) phonon sideband at low temperature. [13][14][15] Taking into account only the coupling between one LO phonon mode and one electronic transition, Kuhnert and Helbig 13 employed a Poission distribution, I n ) S n e -S /n!, to fit the line shape of the green emission band and then obtained a Huang-Rhys factor of S ) 6.5. It is well-known that the Poission distribution simply gives only a backbone of the absorption or luminescence line shape of the electron-LO phonon coupling system. Broadening due to acoustic-phonon-bath dissipation and the temperature effect cannot be accounted for in the model. Moreover, in addition to the first set of structures, the second set of structures with the same periodicity was also observed but its origin is not yet understood. 9,...

We reported in a previous study (Zhao et al 2003 Phys. Rev. Lett. 91 75504) that energy transfer from the orderly intertube translational oscillation to intratube vibrational modes for an isolated system of two coaxial carbon nanotubes at low temperatures takes place primarily via two distinct types of collective motion of the carbon nanotubes, i.e., off-axial rocking motion of the inner tube and radial wavy motion of the outer tube, and that these types of motion may or may not occur for such a system, depending upon the amount of the initial extrusion of the inner tube out of the outer tube. Our present study, using microcanonical molecular dynamics (MD), indicates the existence of an energy threshold, largely independent of system sizes and configurations, for a double-walled nano-oscillator to deviate from the intertube translational oscillation and thus to encounter significant intertube friction. The frictional forces associated with several distinct dissipative mechanisms are all found to exhibit no proportional dependence upon the normal force between the two surfaces in relative sliding, contrary to the conventional understanding resulting from tribological studies of macroscopic systems. Furthermore, simulation has been performed at different initial temperatures, revealing a strong temperature dependence of friction in the early phase of oscillation. Finally, our studies of three-walled nano-oscillators show that an initial extrusion of the middle tube can cause inner-tube offaxial instabilities, leading to strong frictional effects.

We report a novel phenomenon in carbon nanotube (CNT) based devices, the transphonon effects, which resemble the transonic effects in aerodynamics. It is caused by dissipative resonance of nanotube phonons similar to the radial breathing mode, and subsequent drastic surge of the dragging force on the sliding tube, and multiple phonon barriers are encountered as the intertube sliding velocity reaches critical values. It is found that the transphonon effects can be tuned by applying geometric constraints or varying chirality combinations of the nanotubes.It was widely perceived prior to World War II that supersonic flights were prohibited by the sound barrier due to catastrophes that occurred when flying vessels approached the sound speed. Thanks to von Karman and many other pioneers, great progress has been made to understand the transonic effect which had caused drastic reductions of the plane-lifting forces in the catastrophic events. Consequently, supersonic flights have become reality 1 . Figure 1A depicts a US navy aircraft flying at or near the speed of sound. A condensation cloud was generated around the aircraft due to the transonic effect. Now imagine a nanoscale train travelling inside a nanoscale tunnel. Will similar speed barriers be encountered by the superfast nano-train? We have performed a molecular dynamics study of a fasting moving carbon nanotube inside a

Real-time, local basis-set implementation of time-dependent density functional theory for excited state dynamics simulations A model study of quantum dot polarizability calculations using time-dependent density functional methods Basing on the earlier works on the hierarchical equations of motion for quantum transport, we present in this paper a first principles scheme for time-dependent quantum transport by combining timedependent density functional theory (TDDFT) and Keldysh's non-equilibrium Green's function formalism. This scheme is beyond the wide band limit approximation and is directly applicable to the case of non-orthogonal basis without the need of basis transformation. The overlap between the basis in the lead and the device region is treated properly by including it in the self-energy and it can be shown that this approach is equivalent to a lead-device orthogonalization. This scheme has been implemented at both TDDFT and density functional tight-binding level. Simulation results are presented to demonstrate our method and comparison with wide band limit approximation is made. Finally, the sparsity of the matrices and computational complexity of this method are analyzed.

We present analytical results on the transient current response of noninteracting open electronic systems under time-dependent external voltages in both linear- and nonlinear-response regimes. The derivations are based on an equation of motion formalism for the system reduced single-electron density matrix (Zheng et al 2007 Phys. Rev. B 75 195127). Dissipative interactions between the system and leads are treated by the nonequilibrium Green's function approach. The linear-response dynamics is characterized by the analytical admittance spectrum of the open system, through which the quantum coherent transport properties are mapped to equivalent classical circuits. The nonlinear-response current spectrum not only resolves the intrinsic energetic configuration of the system, but also reflects the unique dynamical features due to the transient characteristics of the applied voltages.

Using microcanonical molecular dynamics, we investigate effects of single defects on the performance of a nanoscale oscillator composed of coaxial double-walled carbon nanotubes. It is found that at low temperatures a single defect placed on the outer nanotube can significantly reduces axial oscillation energy leakage by impeding intertube rotational modes, and therefore mitigates the frictional effects between sliding nanotubes.

A system of two harmonic oscillators is placed in an Agarwal bath. The resulting quantum master equations are studied with the help of quantum characteristic functions. The density matrix positivity is investigated in view of the recent interest in searching for a sound quantum dissipation theory. An analytical criterion is derived for density matrix negativity for two uncoupled oscillators. It is found that, for an initial two-oscillator squeezed state with a real squeezing parameter s, density matrix negativity occurs for two uncoupled oscillators at temperatures lower than Planck's over 2 pi omega/(k(B) ln coth/s/) with omega the oscillator frequency and k(B) the Boltzmann factor. As a by-product an analytical expression is also obtained for determining the quantum separability of two uncoupled oscillators. The effects of interoscillator coupling on density matrix negativity are discussed.