A systematic ab initio study of the optical and electronic properties of BN nanotubes within density functional theory in the local density approximation is performed. Specifically, the optical dielectric function and the band structure of the single-walled zigzag ͓͑5,

A systematic ab initio study of the second-order nonlinear optical properties of BN nanotubes within density functional theory in the local density approximation has been performed. Highly accurate full-potential projector augmented-wave method was used. Specifically, the second-harmonic generation (χ (2) abc ) and linear electro-optical (r abc ) coefficients of a large number of the singlewalled zigzag, armchair and chiral BN nanotubes (BN-NT) as well as the double-walled zigzag (12,0)@(20,0) BN nanotube and the single-walled zigzag (12,0) BN-NT bundle have been calculated. Importantly, unlike carbon nanotubes, both the zigzag and chiral BN-NTs are found to exhibit large second-order nonlinear optical behavior with the χ (2) abc and r abc coefficients being up to thirty times larger than that of bulk BN in both zinc-blende and wurtzite structures, indicating that BN-NTs are promising materials for nonlinear optical and opto-electric applications. Though the interwall interaction in the double-walled BN-NTs is found to reduce the second-order nonlinear optical coefficients significantly, the interwall interaction in the single-walled BN-NT bundle has essentially no effect on the nonlinear optical properties. The prominant features in the spectra of χ (2) abc (−2ω, ω, ω) of the BN-NTs are successfully correlated with the features in the linear optical dielectric function ε(ω) in terms of single-photon and two-photon resonances.

We have performed low-temperature transport measurements on a GaAs two-dimensional electron system at low magnetic fields. Multiple temperature-independent points and accompanying oscillations are observed in the longitudinal resistivity between the low-field insulator and the quantum Hall (QH) liquid. Our results support the existence of an intermediate regime, where the amplitudes of magneto-oscillations can be well described by conventional Shubnikov-de Haas theory, between the low-field insulator and QH liquid.Comment: Magneto-oscillations governed by Shubnikov-de Haas theory are observed between the low-field insulator and quantum Hall liqui

A systematic investigation of the electronic states and magnetism in quantum dots and quantum rings within current-spin density-functional theory ͑CSDFT͒ has been performed. CSDFT allows for studying the combined effects of confinement, Coulomb interaction, spin polarization, and magnetic field for a realistic dot or ring system. The dot and ring systems are considered to be three dimensional and the screening due to the gate electrodes is included. The chemical potential, addition energy, and spontaneous magnetization for a quantum dot and a quantum ring in zero magnetic field have been calculated as a function of electron number. A number of spin-polarized ground states in both the quantum dot and the quantum ring, which generally obey Hund's rule, are found. The chemical potential, spin magnetization, orbital angular momentum, and persistent current in the quantum systems under a magnetic field have also been calculated. With increasing magnetic field, a rich variety of transitions between the ground states with different orbital angular momentum and spin configuration are predicted. The calculated addition energy spectrum of the quantum dot reproduces the observed spectrum very well, especially, the charging energy, exchange energy, and shell structure. The features in the calculated chemical potential versus magnetic-field spectra of the quantum dot, which reflect the many-electron state transitions, agree well with those observed in the experiments, indicating that CSDFT is a useful tool for studying the correlated electrons in quantum dots and quantum rings.

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