Browsing by Author "Lin, MF"

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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.
Collective excitations in graphite layers
(1997-09-01)
Huang, CS
;
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
The pi-electronic excitations in graphite layers are studied within the self-consistent-field approach. The pi plasmon behaves as an optical plasmon, mainly due to the pi-band characteristics. The excitation spectra and the pi-plasmon frequencies are obviously enhanced by the interlayer Coulomb interactions and increase with the number of graphite layers. (C) 1997 Published by Elsevier Science Ltd.
Electron lifetime in armchair carbon nanotubes
(2004-11-01)
Chiu, CW
;
Shyu, FL
;
Chang, CP
;
Chuu, DS
;
Lin, MF
The excited conduction electrons in metallic armchair carbon nanotubes can decay by inelastic electron-electron scattering. The deexcitation channels include the single-particle and collective excitations of different angular momenta. The higher conduction subbands have more deexcitation channels, or shorter electron lifetimes. Each conduction subband has certain states with very long lifetimes, and a simple relationship between the inverse electron lifetime and the state energy is absent. Such results directly reflect the characteristics of the ID excitation spectra. The inverse electron lifetime contrasts sharply with those of semiconducting carbon nanotubes, graphite, and 2D and 3D metallic systems. Dimensionalities play an important role in the many-body effects. The predicted results could be verified by femtosecond time-resolved optical spectroscopy.
Electronic excitations of an electron-gas cylinder bundle
(1998-01-01)
Lin, MF
;
Huang, CS
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
We study the dielectric response function of an electron-gas cylinder bundle. The electron-electron interactions among different cylinders are taken into account. The electronic excitations are associated with those of an isolated cylinder. However, a cylinder bundle exhibits the three-dimensional characteristics, but a single cylinder the one-dimensional characteristics. The excitation properties are highly anisotropic. The plasma oscillations perpendicular and parallel to the axial direction quite differs from each other. The ratio between their oscillation frequencies is generally smaller than 1/root 2, which depends on the cylinder radius and the effective mass. (C) 1997 Elsevier Science Ltd.
Electronic excitations of an electron-gas cylinder bundle
(1998-01-01)
Lin, MF
;
Huang, CS
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
We study the dielectric response function of an electron-gas cylinder bundle. The electron-electron interactions among different cylinders are taken into account. The electronic excitations are associated with those of an isolated cylinder. However, a cylinder bundle exhibits the three-dimensional characteristics, but a single cylinder the one-dimensional characteristics. The excitation properties are highly anisotropic. The plasma oscillations perpendicular and parallel to the axial direction quite differs from each other. The ratio between their oscillation frequencies is generally smaller than 1/root 2, which depends on the cylinder radius and the effective mass. (C) 1997 Elsevier Science Ltd.
Electronic states of toroidal carbon nanotubes
(1998-01-01)
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
Toroidal carbon nanotubes (TCN's) are threaded by magnetic flux (phi). The magneto-electronic structures are studied within the tight-binding model. The curvature effect, which is due to the misorientation of the pi-electron orbitals, might play an important role on the low energy electronic states. The thin TCN's, with armchair-zigzag and zigzag-armchair structures, exhibit four (three) types of energy gaps in the presence (absence) of the curvature effect. The magnetoelec tronic structures clearly vary with phi except the thin zigzag-armchair TCN's. They could exhibit the periodical Aharonov-Bohm oscillations, since the Zeeman splitting is generally negligible except at large phi's.
Elementary excitations in a carbon nanotube
(1997-03-01)
Lin, MF
;
Chuu, DS
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
A carbon nanotube exhibits the decoupled pi plasmons of different angular momenta (L's). The magnetic Aux is expected to be useful in identifying them. All the L-decoupled plasmons have strong dispersion relations with momentum (q). The L not equal 0 plasmons belong to optical plasmons, while the L = 0 plasmon could not exist at q --> 0. The former, respectively, behave as the plasmons in three- and two-dimensional electron gases at small and large q's. The plasmon frequencies would be affected by radius, chiral angle, and intertube interaction. To obtain the actual plasmon frequencies of a multi-shell nanotube, the intertube interactions from the neighboring shells have to be taken into account. The calculated results are consistent with the experimental measurements. A detailed comparison between a carbon nanotube and a graphite layer is also made. These two related systems may exhibit similar pi plasmons, when L = 0 in a nanotube. Furthermore, the chirality effect of the former corresponds to the anisotropy effect of the latter. The predicted q-, L-, and geometry-dependence of the pi plasmons needs further experimental verifications.
Elementary excitations in a carbon nanotube
(1997-03-01)
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
A carbon nanotube exhibits the decoupled pi plasmons of different angular momenta (L's). The magnetic Aux is expected to be useful in identifying them. All the L-decoupled plasmons have strong dispersion relations with momentum (q). The L not equal 0 plasmons belong to optical plasmons, while the L = 0 plasmon could not exist at q --> 0. The former, respectively, behave as the plasmons in three- and two-dimensional electron gases at small and large q's. The plasmon frequencies would be affected by radius, chiral angle, and intertube interaction. To obtain the actual plasmon frequencies of a multi-shell nanotube, the intertube interactions from the neighboring shells have to be taken into account. The calculated results are consistent with the experimental measurements. A detailed comparison between a carbon nanotube and a graphite layer is also made. These two related systems may exhibit similar pi plasmons, when L = 0 in a nanotube. Furthermore, the chirality effect of the former corresponds to the anisotropy effect of the latter. The predicted q-, L-, and geometry-dependence of the pi plasmons needs further experimental verifications.
Elementary excitations in cylinder bundles
(1998-07-01)
Huang, CS
;
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
Elementary excitations in a cylinder bundle, which is formed by packing the same hollow tubules into a two-dimensional triangular lattice, are studied. The dielectric response function is calculated within the linear response approximation. The electronic excitations of the cylinder bundle are the superposition of those of all tubules. Due to the intertube Coulomb interactions, their characteristics quite differ from those of an isolated tubule. The plasmon, which corresponds to the coherent oscillations of all carriers, could exhibit a prominent peak in excitation spectrum. The excitation properties strongly depend on the tubular radius, the effective electron mass, and the magnitude and the direction of the momentum transfer (q). At small momentum, there exists a simple relation between excitation properties and polar angles (theta's). Both intensity of excitation spectrum and plasmon frequency decrease as theta increases. The collective plasma oscillations parallel and perpendicular to the tubular axis contrast sharply with each other, which clearly illustrates the anisotropic behavior. The ratio between these two plasmon frequencies is found to be smaller than root 2.
Holographic polarization-selective elements in optical network applications
(1996)
Huang, YT
;
Lin, MF
;
Deng, JS
;
Fan, KT
;
Chen, MJ
Impurity screening in carbon nanotubes
(1997-08-15)
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
Nanotube geometry determines electronic structure and thus impurity screening. A metallic carbon nanotube could effectively screen a charged impurity, while a semiconducting carbon nanotube could not. The ability to screen a long-range Coulomb field is mainly determined by whether there are: Free carriers in the subbands nearest the Fermi level. The detailed screening propel-ties are sensitive to the impurity position, and the tubular structure (such as radius and chiral angle). Strong, short-wavelength Friedel oscillations at long distances are found to exist only in metallic armchair nanotubes. They are relatively obvious for a smaller armchair nanotube, and could survive at room temperature.
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.
Magnetization of finite carbon nanotubes
(2005-05-01)
Chen, RB
;
Chang, CP
;
Hwang, JS
;
Chuu, DS
;
Lin, MF
Magnetoelectronic properties of finite carbon nanotubes (CNs) are studied for an arbitrary field direction. They are strongly affected by the nanotube geometry (length, radius; boundary structure), the magnitude and direction of the magnetic field, the Zeeman effect, and the temperature. Geometric structures determine electronic structures and magnetic properties, which thus leads to three types of energy gaps and induced magnetic fields. The critical angle, which corresponds to the change of magnetism, exists in armchair CNs, but not in zigzag CNs. It also depends on the length and the radius of CNs. Finite CNs are very different from infinite CNs. Zeeman splitting could induce complete energy-gap modulation, a drastic change in magnetization, and a gigantic paramagnetic response for all zigzag CNs. The predicted results are observable even at room temperature.
Persistent currents in toroidal carbon nanotubes
(1998-03-15)
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
The geometric structure of the toroidal carbon nanotubes (TCN's) determines the electronic structure and thus the characteristics of the persistent current. Such current is caused by the magnetic flux phi through TCN's. The semiconducting TCN's exhibit diamagnetism at small phi, which is in great contrast with paramagnetism of the metallic TCN's. The induced magnetic moment is proportional to the toroid radius, but independent of the toroid width. The magnetic response is weak, while it is much stronger than that of a mesoscopic semiconductor or metal ring. The persistent current is a linearly periodical function of phi, with a period phi(0)(hc/e). Such an oscillation is the manifestation of the Aharonov-Bohm (AB) effect. Temperature (T) does not destroy the periodical AB oscillation, although it would significantly reduce the persistent currents. The Zeeman splitting may lead to the destruction of the periodicity at very large phi. A larger TCN at lower T and phi is relatively suitable for verifying the AB effect.
pi plasmons in carbon nanotube bundles
(1998-04-15)
Lin, MF
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
The pi-electronic collective excitations (pi plasmons) in carbon nanotube bundles are studied within the linear-response approximation. The intertube Coulomb interactions significantly affect the electronic excitations, so that the pi plasmons in nanotube bundles are quite different from those in a single nanotube. The pi plasmons strongly depend on the direction and magnitude of the transferred momentum. They clearly exhibit the anisotropic behavior. Moreover, the pi plasmon frequencies are lower for bundles made up of larger nanotubes.
Plasmons in graphite and stage-1 graphite intercalation compounds
(1997-05-15)
Lin, MF
;
Huang, CS
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
The pi-electronic excitations df graphite layers are studied within the random-phase approximation. They principally reflect the pi-band characteristics, the strong wave-vector dependence, the anisotropic behavior, and the special symmetry. The pi plasmons in graphite have strong dispersion relations with the transferred momentum (q). They behave as an optical plasmon in a three-dimensional electron gas at small q. Moreover, the anisotropic behavior at the plane is apparent at large q. For a single graphite layer, the pi plasmons would disappear at very small q, and their frequencies are obviously reduced. The absence of interlayer Coulomb interactions is the main reason for this. The stage-1 graphite intercalation compounds (GIC's), as compared with graphite, exhibit the richer excitation spectra and the lower pi-plasmon frequencies. They have the intraband plasmon as well as the interband pi plasmon. These two kinds of plasmons are quite different from each other in certain respects, e.g., the cause of the plasmon. The enhanced interlayer distances could effectively reduce the pi-plasmon frequency, but not the transferred charges. The calculated plasmon frequencies are consistent with the experimental measurements on graphite and stage-1 GIC's.
Plasmons in graphite and stage-1 graphite intercalation compounds
(1997-05-15)
Lin, MF
;
Huang, CS
;
Chuu, DS
;
交大名義發表
;
電子物理學系
The pi-electronic excitations df graphite layers are studied within the random-phase approximation. They principally reflect the pi-band characteristics, the strong wave-vector dependence, the anisotropic behavior, and the special symmetry. The pi plasmons in graphite have strong dispersion relations with the transferred momentum (q). They behave as an optical plasmon in a three-dimensional electron gas at small q. Moreover, the anisotropic behavior at the plane is apparent at large q. For a single graphite layer, the pi plasmons would disappear at very small q, and their frequencies are obviously reduced. The absence of interlayer Coulomb interactions is the main reason for this. The stage-1 graphite intercalation compounds (GIC's), as compared with graphite, exhibit the richer excitation spectra and the lower pi-plasmon frequencies. They have the intraband plasmon as well as the interband pi plasmon. These two kinds of plasmons are quite different from each other in certain respects, e.g., the cause of the plasmon. The enhanced interlayer distances could effectively reduce the pi-plasmon frequency, but not the transferred charges. The calculated plasmon frequencies are consistent with the experimental measurements on graphite and stage-1 GIC's.
Temperature-dependent carrier dynamics in metallic carbon nanotubes
(2005-10-31)
Chiu, CW
;
Chiu, YH
;
Shyu, FL
;
Chang, CP
;
Chuu, DS
The low energy states in metallic armchair carbon nanotubes can decay by the electron-electron Coulomb interactions with the intraband and the interband excitations. The inelastic scattering rate is very sensitive to the changes in the electronic distribution and the state energy (or the wavevector). The temperature dependence is linear for the Fermi-momentum state. Such dependence is hardly affected by the nanotube radius and the Fermi energy. The electron-electron interactions are more efficient in electronic deexcitations compared with the electron-phonon interactions. The calculated results are roughly consistent with the experimental measurements from the time-resolved photoemission spectroscopy. (c) 2005 Elsevier B.V. All rights reserved.
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.