Commensuration and Interlayer Coherence in Twisted Bilayer Graphene

The low energy electronic spectra of rotationally faulted graphene bilayers are studied using a long wavelength theory applicable to general commensurate fault angles. Lattice commensuration requires low energy electronic coherence across a fault and preempts massless Dirac behavior near the neutrality point. Sublattice exchange symmetry distinguishes two families of commensurate faults that have distinct low energy spectra which can be interpreted as energy-renormalized forms of the spectra for the limiting Bernal and AA stacked structures. Sublattice-symmetric faults are generically fully gapped systems due to a pseudospin-orbit coupling appearing in their effective low energy Hamiltonians.Comment: 4 pages RevTeX, 3 jpg figure

Continuum theory for the piezoelectric response of chiral nanotubes under uniaxial and torsional stresses

We develop and solve a continuum theory for the piezoelectric response of nanotubes under applied uniaxial and torsional stresses. We find that the piezoelectric response is controlled by the chiral angle, the aspect ratio, and two dimensionless parameters specifying the ratio of the strengths of the electrostatic and elastic energies. The model is solved in two limiting cases and the solutions are discussed. These systems are found to have several unexpected physical effects not seen in conventional bulk systems, including a strong stretch-twist coupling and the development of a significant bound charge density in addition to a surface charge density. The model is applied to estimate the piezoelectric response of a boron-nitride nanotube under uniform tensile stress.Comment: 8 pages, 4 figures. Submitted to Physical Review

Electron Propagation in Orientationally Disordered Fullerides

We study the electronic spectrum for doped electronic states in the orientationally disordered M3C60 fullerides. Momentum-resolved Green's functions are calculated within a cluster-Bethe-lattice model, and compared with results from calculations on periodically repeated supercells containing quenched orientational disorder. Despite the relatively strong scattering from orientational fluctuations, the electronic states near the Fermi energy are well described by propagating states characterized by an effective Bloch wave vector, and a mean free path of approximately 20 Angstroms. The effective Fermi surface is calculated in this model. This differs from that previously calculated for the orientationally ordered crystal, but is relatively well described within a disorder-averaged virtual-crystal Hamiltonian, which we derive.Comment: 22 pages + 7 postscript figures (by request), REVTeX 3.0, UP-31415

A radiatively improved fermiophobic Higgs boson scenario

The naive fermiophobic scenario is unstable under radiative corrections, due to the chiral-symmetry breaking induced by fermion mass terms. In a recent study, the problem of including the radiative corrections has been tackled via an effective field theory approach. The renormalized Yukawa couplings are assumed to vanish at a high energy scale $\Lambda$, and their values at the electroweak scale are computed via modified Renormalization Group Equations. We show that, in case a fermiophobic Higgs scenario shows up at the LHC, a linear collider program will be needed to accurately measure the radiative Yukawa structure, and consequently constrain the $\Lambda$ scale.Comment: 7 pages, 3 figures, Proceedings of the 2011 International Workshop on Future Linear Colliders (LCWS11), Granada (Spain), 26-30 September 201

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

Looking for anomalous gamma-gamma-H and Z-gamma-H couplings at future linear collider

We consider the possibility of studying anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices through the process e-gamma--> e-H at future e-gamma linear colliders, with Sqrt(S)=500-1500 GeV. We make a model independent analysis based on SU(2)xU(1) invariant effective operators of dim=6 added to the standard model lagrangian. We consider a light Higgs boson (mostly decaying in bar(b)-b pairs), and include all the relevant backgrounds. Initial e-beam polarization effects are also analyzed. We find that the process e-gamma--> e-H provides an excellent opportunity to strongly constrain both the CP-even and the CP-odd anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices.Comment: LaTeX, 33 pages, 16 eps figures, extended section