J. W. Ding

Curvature and strain effects on electronic properties of single-wall carbon nanotubes

To describe accurately the electronic structures of carbon nanotubes, a semi-empirical tight-binding approach is presented in which the main intrinsic curvatures have been fully taken into account. The calculated electronic structures and band gaps are consistent with experimental measurements. Studies of the relative importance of various intrinsic curvatures show that each curvature has a contribution of varying importance to the curvature-induced band gap. Additionally, under both uniaxial and torsional strain, semiconductor–metal–semiconductor phase transitions have been observed for primary metallic carbon nanotubes. The critical stress of the transition and the gaps sensitivity with stress are dependent on both the diameter and chirality of nanotubes, which is at variance with previous predictions.

Conductance growth in metallic bilayer graphene nanoribbons with disorder and contact scattering

By using a decomposition elimination method for Greens function matrix, we explore the effects of both disorder and contact scattering on electronic transport in metallic bilayer graphene nanoribbons (BGNRs) and related structures, in the limit of phase-coherent transport. Due to the inter-layer interaction, a conductance gap is observed at Fermi energy in primary metallic zigzag BGNRs. It is found that the fashion of the conductance variations with disorder depends strongly on the type of disorder and contact scattering. In the edge disordered BGNR, the conductance decreases monotonically with the disorder increasing and finally tends to disappear, while a nonmonotonic behavior is obtained in the single-layer disordered BGNR, first decreasing then increasing. In the presence of contact scattering, especially, an abnormal growth of the conductance appears at much lower disorder in both edge and single-layer disordered BGNRs, which may be due to the destruction of coherence by the introduction of disorder.

Band structures of carbon nanotubes: the sp3s* tight-binding model

We present a simple sp3s* tight-binding model for use in calculating the band structures of single-walled carbon nanotubes. The 2s, 2px, 2py, 2pz, and s* orbitals of each carbon atom are used as the basis set for expressing the tight-binding model, and the interatom interaction between neighbouring sites is fully taken into account. The elements of the Hamiltonian matrix and related parameters are obtained by adjusting the model to fit the primary reflectivity and photoemission band-structure data. We have employed this tight-binding model in investigating [n,0] (n = 6,7,8,9) carbon nanotubes. Our band-structure and band-gap results show that [6,0] and [9,0] tubes are narrow-gap semiconductors rather than metallic ones, which is at variance with the findings of previous work.

Visible photon-avalanche upconversion in Ho 3+ singly doped β−Na(Y 1.5 Na 0.5 )F 6 under 980 nm excitation

Visible IR-to-green photon-avalanche upconversion is reported in an Ho3+ singly doped β−Na(Y1.5Na0.5)F6 crystal under 980 nm excitation. Upconverted green, red, and IR emissions are observed at 540, 645, and 751 nm, respectively. Temporal evolution and excitation power dependent upconversion intensity are measured, suggesting that a photon-avalanche mechanism is responsible for the upconversion process. It is believed that an efficient cross relaxation (5S2,5I8)→(5I6,5I6) mainly performs the population of 5I6 excited state, resulting in the intense photon-avalanche upconversion emission in the synthesized samples.

Three-phonon Umklapp process in zigzag single-walled carbon nanotubes

The rate of relaxation of zigzag single-walled carbon nanotubes is calculated by consideration of three-phonon Umklapp process. The results show that the relaxation rate increases exponentially with phonon frequency at low frequency. The linear dependence of the relaxation rate on temperature is obtained. It is shown that the value of the phonon mean free path reaches a few micrometres, which is consistent with the estimated experimental result.

Luminescence properties of rare earth doped YF3 and LuF3 nanoparticles

Eu3+ doped and Yb3+/Ho3+ codoped LuF3 and YF3 nanoparticles with a size distribution of 200–300 nm have been prepared by adopting a combustion-fluorization method. The luminescence spectra of Eu3+ and Ho3+ ions in the YF3 and LuF3 nanoparticles have been investigated through comparison. It is found that the Eu3+ and Ho3+ ions in the two hosts show different luminescence properties although the sites occupied by the rare earth (RE) ions in the LuF3 and YF3 hosts are of the same symmetry. The different luminescence properties may be ascribed to the difference in the RE-F bond nature in the YF3 and LuF3 hosts.

Formation and interaction characteristics of two-component spatial weak-light soliton in a four-level double-Λ type system

By using the multiple-scale method, we study analytically the formation and stability of two-component spatial optical solitons in a cold, lifetime-broadened resonant four-level double-Λ type atomic system via electromagnetically induced transparency. It is shown that stable two-component (1+1) dimension spatial optical solitons with extremely weak light intensity can occur, which is different from the passive ones with photorefractive and planar waveguides. Furthermore, the interaction characteristics between two solitons are studied by numerical simulations. We find that the collisional dynamics and the energy transfer of the two solitons are closely correlated with their relative phase shift. Our results may provide a good idea to obtain useful spatial optical solitons for application in optical soliton communications.

Giant orbital paramagnetism in toroidal carbon nanopeapods

The electronic structure and the magnetic response of toroidal carbon nanopeapods (TCNPs) are investigated within the sp(3) tight-binding formalism. It is found that in the presence of mirror symmetry, there exists a level crossing at the Fermi level in the energy spectrum of a TCNP, leading to giant orbital paramagnetism (GOP), in spite of the curvatures and hybridizations of the outer toroidal carbon nanotube (TCN). When the mirror symmetry is broken by rotating the inner C(60)s, however, two level crossings appear at the Fermi level, and the GOP changes into a very small diamagnetic response. The results reveal the GOP in a filled TCN, depending on the characteristics of the filling materials and temperature.

Nonperiodic Oscillation of Bright Solitons in Condensates with a Periodically Oscillating Harmonic Potential 10.5560/ZNA.2012-0085

Considering a periodically oscillating harmonic potential, we explored the dynamic properties of bright solitons in a Bose-Einstein condensate by using Darboux transformation. It is found that the soliton movement exhibits a nonperiodic oscillation under a slow oscillating potential, while it is hardly affected under a fast oscillating potential. Furthermore, the head-on and/or ‘chase’ collisions of two solitons have been obtained, which could be controlled by the oscillation frequency of the potential.