Browsing by Author "Shung, KWK"

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Collective excitations in a single-layer carbon nanotube
(1996-06-15)
Lin, MF
;
Chuu, DS
;
Huang, CS
;
Lin, YK
;
Shung, KWK
The inter-pi-band response function of a single-layer carbon nanotube is calculated within the random-phase approximation. Such a cylindrical system exhibits rich inter-pi-band excitations. The decoupled pi plasmons;of different angular momenta (L's) strongly depend on the transferred momenta (q's). Furthermore, they exhibit a dimensionality crossover as the nanotube radius is increased. For small q's and L's, the pi plasmon frequency is similar to 6-8 eV, which is insensitive to the nanotube geometry such as the radius and chiral angle. Comparison with recent experiments is discussed.
Low-frequency plasmons in metallic carbon nanotubes
(1997-07-15)
Lin, MF
;
Chuu, DS
;
Shung, KWK
;
電子物理學系
;
物理研究所
A metallic carbon nanotube could exhibit a low-frequency plasmon, while a semiconducting carbon nanotube or a graphite layer could not. This plasmon is due to the free carders in the linear subbands intersecting at the Fermi level. The low-frequency plasmon, which corresponds to the vanishing transferred angular momentum, belongs to an acoustic plasmon. For a smaller metallic nanotube, it could exist at larger transferred momenta, and its frequency is higher. Such a plasmon behaves as that in a one-dimensional electron gas (EGS). However, it is very different from the pi plasmons in all carbon nanotubes. Intertube Coulomb interactions in a metallic multishell nanotube and a metallic nanotube bundle have been included. They have a strong effect on the low-frequency plasmon. The intertube coupling among coaxial nanotubes markedly modifies the acoustic plasmons in separate metallic nanotubes. When metallic carbon nanotubes are packed in the bundle form, the low-frequency plasmon would change into an optical plasmon, and behave like that in a three-dimensional EGS. Experimental measurements could be used to distinguish metallic and semiconducting carbon nanotubes.
Low-frequency plasmons in metallic carbon nanotubes
(1997-07-15)
Lin, MF
;
Chuu, DS
;
Shung, KWK
;
電子物理學系
;
物理研究所
A metallic carbon nanotube could exhibit a low-frequency plasmon, while a semiconducting carbon nanotube or a graphite layer could not. This plasmon is due to the free carders in the linear subbands intersecting at the Fermi level. The low-frequency plasmon, which corresponds to the vanishing transferred angular momentum, belongs to an acoustic plasmon. For a smaller metallic nanotube, it could exist at larger transferred momenta, and its frequency is higher. Such a plasmon behaves as that in a one-dimensional electron gas (EGS). However, it is very different from the pi plasmons in all carbon nanotubes. Intertube Coulomb interactions in a metallic multishell nanotube and a metallic nanotube bundle have been included. They have a strong effect on the low-frequency plasmon. The intertube coupling among coaxial nanotubes markedly modifies the acoustic plasmons in separate metallic nanotubes. When metallic carbon nanotubes are packed in the bundle form, the low-frequency plasmon would change into an optical plasmon, and behave like that in a three-dimensional EGS. Experimental measurements could be used to distinguish metallic and semiconducting carbon nanotubes.
Thermal conductance and the Peltier coefficient of carbon nanotubes
(1996-04-15)
Lin, MF
;
Chuu, DS
;
Shung, KWK
;
電子物理學系
;
Department of Electrophysics
The pi-band structure of one-dimensional carbon nanotubes is very special. Their ballistic transport properties are studied theoretically and are found to exhibit rich magnetic-flux-dependent structures. The thermal conductance kappa(phi) has many step structures caused by the Zeeman splitting; a similar effect has been found in the electrical conductance G(phi). The Peltier coefficient Pi(phi) vanishes in the zero-voltage limit at any magnetic flux due to the symmetric pi-band structure about the chemical potential mu=0. However, a finite Peltier effect could be observed by applying a finite voltage or by doping carbon nanotubes. Doping also causes peak structures with quantized maxima in Pi(phi), as well as more step structures in kappa(phi). Both the quantized peaks and the steps should be observable at T<1 K. These structures and also the validity of Wiedemann-Franz law kappa(phi)approximate to pi(2)k(B)(2)TG(phi)/3e(2) are found to depend upon the temperature, the chemical potential, the pi-band property, and the Zeeman effect.