RESUMEN
We study theoretically the plasmon scattering at the intersection of two metallic carbon nanotubes. We demonstrate that, for a small angle of crossing theta<<1, the transmission coefficient is an oscillatory function of lambda/theta, where lambda is the interaction parameter of the Luttinger liquid in an individual nanotube. We calculate the tunnel density of states nu(omega,x) as a function of energy omega and distance x from the intersection. In contrast with a single nanotube, we find that, in the geometry of crossed nanotubes, conventional "rapid" oscillations in nu(omega,x) due to the plasmon scattering acquire an aperiodic "slow-breathing" envelope which has lambda/theta nodes.
RESUMEN
Quantum theory predicts that Bose-Einstein condensation of a spatially homogeneous gas with attractive interactions is precluded by a conventional phase transition into either a liquid or solid. When confined to a trap, however, such a condensate can form, provided that its occupation number does not exceed a limiting value. The stability limit is determined by a balance between the self-attractive forces and a repulsion that arises from position-momentum uncertainty under conditions of spatial confinement. Near the stability limit, self-attraction can overwhelm the repulsion, causing the condensate to collapse. Growth of the condensate is therefore punctuated by intermittent collapses that are triggered by either macroscopic quantum tunnelling or thermal fluctuation. Previous observations of growth and collapse dynamics have been hampered by the stochastic nature of these mechanisms. Here we report direct observations of the growth and subsequent collapse of a 7Li condensate with attractive interactions, using phase-contrast imaging. The success of the measurement lies in our ability to reduce the stochasticity in the dynamics by controlling the initial number of condensate atoms using a two-photon transition to a diatomic molecular state.