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The spontaneous decay process of an excited atom placed inside or outside a carbon nanotube is analyzed. Calculations have been performed for various achiral nanotubes. The effect of the nanotube surface is shown to increase the atomic spontaneous decay rate by up to 6 orders of magnitude compared with that of the same atom in vacuum. This increase is associated with nonradiative decay via surface excitations in the nanotube.

We present a novel, general, semiclassical theory of electron transport in a carbon nanotube exposed to an external electric field. The charge carriers are treated in the framework of the simplified tightbinding model. Simultaneous exposure to rapidly oscillating (ac) and constant (dc) electric fields is considered to exemplify our theory. Nonlinear and chiral effects are found, and their interaction is delineated. We predict the effect of an ac electric field on the magnitude and the direction of the total time-averaged current. [S0031-9007(97)03736-8] PACS numbers: 71.20.Tx, 61.48. + c, 72.20.Ht During the past several years, research on the properties of different carbon nanostructures ( fullerenes) has continued to grow unabated. An extremely attractive feature has emerged: The crystalline structure of a fullerene depends on the conditions of its preparation, with different structures possessing quite different physical response properties. We concentrate here on nanotubular fullerenes generally called carbon nanotubes (CNs). A CN is a fullerene rolled up into a ϳ20 100 Å diameter cylinder [1,2]. Not only do CNs possess very high strength-toweight ratios which makes them attractive mechanically [3], but they also have fascinating electromagnetic properties. Theoretical studies predicted that a CN can be a good conductor, a semiconductor, or an insulator, depending on its geometric parameters [4]; and very recent experiments verified this prediction [5].Semiconducting CNs are of special interest because of promising applications in solid state microelectronics [6]. However, a general analytical theory of electronic properties and transport in CN remains missing and only preliminary studies have been reported so far. Thus, Lou et al. [7] as well as Miyamoto et al. [8] examined the linear response of a CN immersed in a uniform electrostatic field, while Kasumov et al. [9] and Langer et al. [10] measured the electrical resistance of a CN in a uniform magnetostatic field.A CN can be approximately modeled as a structure where carbon atoms are situated along a base helix wrapped around the surface of a cylinder, the wrapping angle usually being a few degrees [11]. The spiral model is a phenomenological description of a carbon crystalline lattice rolled up into a cylinder. As it reflects the helicoidal symmetry of CNs and is pretty simple, it has been successfully applied for several theoretical studies of different effects in CNs: Kibis et al. investigated the motion of carriers in an external magnetic field [12], and researched electron-photon and electron-electron interactions [13][14][15]; while Lin-Chung and Rajagopal explored magnetoplasma oscillations [16]. Moreover, recent first-principles numerical simulations [8] proved that a small CN with complex (honeycomblike) lattice possesses chiral conductivity; in consequence, when a uniform electrostatic (dc) field is applied parallel to the tubular axis, the direction of total current coincides neither with the tubular axis nor with the base helix.In this Lette...

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We measure the conductivity spectra of thin films comprising bundled single-walled carbon nanotubes (CNTs) of different average lengths in the frequency range 0.3-1000 THz and temperature interval 10-530 K. The observed temperature-induced changes in the terahertz conductivity spectra are shown to depend strongly on the average CNT length, with a conductivity around 1 THz that increases/decreases as the temperature increases for short/long tubes. This behaviour originates from the temperature dependence of the electron scattering rate, which we obtain from Drude fits of the measured conductivity in the range 0.3-2 THz for 10 µm length CNTs. This increasing scattering rate with temperature results in a subsequent broadening of the observed THz conductivity peak at higher temperatures and a shift to lower frequencies for increasing CNT length. Finally, we show that the change in conductivity with temperature depends not only on tube length, but also varies with tube density. We record the effective conductivities of composite films comprising mixtures of WS 2 nanotubes and CNTs vs CNT density for frequencies in the range 0.3-1 THz, finding that the conductivity increases/decreases for low/high density films as the temperature increases. This effect arises due to the density dependence of the effective length of conducting pathways in the composite films, which again leads to a shift and temperature dependent broadening of the THz conductivity peak.

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