Photogalvanic Effects in Heteropolar Nanotubes

We show that an electrical shift current is generated when electrons are photoexcited from the valence to conduction bands on a BN nanotube. This photocurrent follows the light pulse envelope and its symmetry is controlled by the atomic structure of the nanotube. We find that the shift current has an intrinsic quantum mechanical signature in which the chiral index of the tube determines the direction of the current along the tube axis. We identify discrete lattice effects in the tangent plane of the tube that lead to an azimuthal component of the shift current. The nanotube shift current can lead to ultrafast opto-electronic and opto-mechanical applications.Comment: 4 pages in RevTeX cotaining 2 epsf color figure

Band Symmetries and Singularities in Twisted Multilayer Graphene

The electronic spectra of rotationally faulted graphene bilayers are calculated using a continuum formulation for small fault angles that identifies two distinct electronic states of the coupled system. The low energy spectra of one state features a Fermi velocity reduction which ultimately leads to pairwise annihilation and regeneration of its low energy Dirac nodes. The physics in the complementary state is controlled by pseudospin selection rules that prevent a Fermi velocity renormalization and produce second generation symmetry-protected Dirac singularities in the spectrum. These results are compared with previous theoretical analyses and with experimental data.Comment: 5 pages, 3 figure

Carbon Nanotubes in Helically Modulated Potentials

We calculate effects of an applied helically symmetric potential on the low energy electronic spectrum of a carbon nanotube in the continuum approximation. The spectrum depends on the strength of this potential and on a dimensionless geometrical parameter, P, which is the ratio of the circumference of the nanotube to the pitch of the helix. We find that the minimum band gap of a semiconducting nanotube is reduced by an arbitrarily weak helical potential, and for a given field strength there is an optimal P which produces the biggest change in the band gap. For metallic nanotubes the Fermi velocity is reduced by this potential and for strong fields two small gaps appear at the Fermi surface in addition to the gapless Dirac point. A simple model is developed to estimate the magnitude of the field strength and its effect on DNA-CNT complexes in an aqueous solution. We find that under typical experimental conditions the predicted effects of a helical potential are likely to be small and we discuss several methods for increasing the size of these effects.Comment: 12 pages, 10 figures. Accepted for publication in Physical Review B. Image quality reduced to comply with arxiv size limitation

Semiconductor Surface Studies

Contains reports on two research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400)U.S. Navy-Office of Naval Research (Contract N00014-77-C-0132

Ultrahigh energy neutrino scattering onto relic light neutrinos in galactic halo as a possible source of highest energy extragalactic cosmic rays

The diffuse relic neutrinos with light mass are transparent to Ultrahigh energy (UHE) neutrinos at thousands EeV, born by photoproduction of pions by UHE protons on relic 2.73 K BBR radiation and originated in AGNs at cosmic distances. However these UHE $\nu$s may interact with those (mainly heaviest $\nu_{\mu_r}$, $\nu_{\tau_r}$ and respective antineutrinos) clustered into HDM galactic halos. UHE photons or protons, secondaries of $\nu\nu_r$ scattering, might be the final observed signature of such high-energy chain reactions and may be responsible of the highest extragalactic cosmic-ray (CR) events. The chain-reactions conversion efficiency, ramifications and energetics are considered for the October 1991 CR event at 320 EeV observed by the Fly's Eye detector in Utah. These quantities seem compatible with the distance, direction and power (observed at MeV gamma energies) of the Seyfert galaxy MCG 8-11-11. The $\nu\nu_r$ interaction probability is favoured by at least three order of magnitude with respect to a direct $\nu$ scattering onto the Earth atmosphere. Therefore, it may better explain the extragalactic origin of the puzzling 320 EeV event, while offering indirect evidence of a hot dark galactic halo of light (i.e., $m_\nu\sim$ tens eV) neutrinos, probably of tau flavour.Comment: 25 pages, 1 figure minor corrections, updated references. In press in AP

Neutral top-pion and the rare top decays t→cliljt\to c l_{i} l_{j}

We study the rare top decays $t\to c l_{i} l_{j}(l=\tau,\mu,or e)$ in the framework of topcolor-assisted technicolor($TC2$) models. We find that the neutral top-pion $\pi_{t}^{0}$ can produce significant contributions to these processes via the flavor changing couplings $\pi_{t}^{0} \bar{t} c$ and $\pi_{t}^{0} l_{i} l_{j}$. For the $\pi_{t}^{0}$ mass $m_{\pi_{t}}=150 GeV$ and the parameter $\epsilon=0.08$, the branching ratio $Br$(t$\to c \tau \tau)$ can reach $7.1\times10^{-7}$. Taking into account the constraints of the present experimental limit of the process $\mu\to e \gamma$ on the free parameters of $TC2$ models, we find that the value of $Br$($t\to c \tau \mu$)$\approx$$Br$($t\to c \tau e$) is in the range of 1.8$\times10^{-10}\sim1.7\times10^{-8}$.Comment: To be published in Phys.

Mechanical properties of polyethylene terephthalate under selected conditions and methods of preparation Semiannual progress report, period ending 31 Oct. 1967

Mechanical properties of polyethylene terephthalate under selected conditions and methods of preparatio