Interactions between Membrane Inclusions on Fluctuating Membranes

We model membrane proteins as anisotropic objects characterized by symmetric-traceless tensors and determine the coupling between these order-parameters and membrane curvature. We consider the interactions between transmembrane proteins that respect up-down (reflection) symmetry of bilayer membranes and that have circular or non-circular cross-sectional areas in the tangent-plane of membranes. Using a field theoretic approach, we find non-entropic $1/R^{4}$ interactions between reflection-symmetry-breaking transmembrane proteins with circular cross-sectional area and entropic $1/R^{4}$ interactions between transmembrane proteins with circular cross-section that do not break up-down symmetry in agreement with previous calculations. We also find anisotropic $1/R^{4}$ interactions between reflection-symmetry-conserving transmembrane proteins with non-circular cross-section, anisotropic $1/R^{2}$ interactions between reflection-symmetry-breaking transmembrane proteins with non-circular cross-section, and non-entropic $1/R^{4}$ many-particle interactions among non-transmembrane proteins. For large $R$, these interactions might provide the dominant force inducing aggregation of the membrane proteins.Comment: REVTEX, 29 pages with 4 postscript figures compressed using uufiles. Introduction and Discussion sections revised. To appear in J. Phys. France I (September

Disclination Asymmetry in Deformable Hexatic Membranes and the Kosterlitz-Thouless Transitions

A disclination in a hexatic membrane favors the development of Gaussian curvature localized near its core. The resulting global structure of the membrane has mean curvature, which is disfavored by curvature energy. Thus a membrane with an isolated disclination undergoes a buckling transition from a flat to a buckled state as the ratio $\kappa/K_{A}$ of the bending rigidity $\kappa$ to the hexatic rigidity $K_{A}$ is decreased. In this paper we calculate the buckling transition and the energy of both a positive and a negative disclination. A negative disclination has a larger energy and a smaller critical value of $\kappa/K_{A}$ at buckling than does a positive disclination. We use our results to obtain a crude estimate of the Kosterlitz-Thouless transition temperature in a membrane. This estimate is higher than the transition temperature recently obtained by the authors in a renormalization calculation.Comment: REVTEX, 16 pages with 5 postscript figures compressed using uufiles. Accepted for publication in J. Phys. France

Sample-specific and Ensemble-averaged Magnetoconductance of Individual Single-Wall Carbon Nanotubes

We discuss magnetotransport measurements on individual single-wall carbon nanotubes with low contact resistance, performed as a function of temperature and gate voltage. We find that the application of a magnetic field perpendicular to the tube axis results in a large magnetoconductance of the order of e^2/h at low temperature. We demonstrate that this magnetoconductance consists of a sample-specific and of an ensemble-averaged contribution, both of which decrease with increasing temperature. The observed behavior resembles very closely the behavior of more conventional multi-channel mesoscopic wires, exhibiting universal conductance fluctuations and weak localization. A theoretical analysis of our experiments will enable to reach a deeper understanding of phase-coherent one-dimensional electronic motion in SWNTs.Comment: Replaced with published version. Minor changes in tex

Controlling spin in an electronic interferometer with spin-active interfaces

We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire.Comment: 5 pages, 3 figures. to be published in Europhysics Letter

Spin-dependent Quantum Interference in Single-Wall Carbon Nanotubes with Ferromagnetic Contacts

We report the experimental observation of spin-induced magnetoresistance in single-wall carbon nanotubes contacted with high-transparency ferromagnetic electrodes. In the linear regime the spin-induced magnetoresistance oscillates with gate voltage in quantitative agreement with calculations based on a Landauer-Buttiker model for independent electrons. Consistent with this interpretation, we find evidence for bias-induced oscillation in the spin-induced magnetoresistance signal on the scale of the level spacing in the nanotube. At higher bias, the spin-induced magnetoresistance disappears because of a sharp decrease in the effective spin-polarization injected from the ferromagnetic electrodes.Comment: Replaced with published versio