3,149 research outputs found
Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes
Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one
can generate and detect coherent phonons in chirality-specific semiconducting
single-walled carbon nanotubes. The signals are resonantly enhanced when the
pump photon energy coincides with an interband exciton resonance, and analysis
of such data provides a wealth of information on the chirality-dependence of
light absorption, phonon generation, and phonon-induced band structure
modulations. To explain our experimental results, we have developed a
microscopic theory for the generation and detection of coherent phonons in
single-walled carbon nanotubes using a tight-binding model for the electronic
states and a valence force field model for the phonons. We find that the
coherent phonon amplitudes satisfy a driven oscillator equation with the
driving term depending on photoexcited carrier density. We compared our
theoretical results with experimental results on mod 2 nanotubes and found that
our model provides satisfactory overall trends in the relative strengths of the
coherent phonon signal both within and between different mod 2 families. We
also find that the coherent phonon intensities are considerably weaker in mod 1
nanotubes in comparison with mod~2 nanotubes, which is also in excellent
agreement with experiment.Comment: 21 pages, 22 figure
Chirality dependence of the radial breathing phonon mode density in single wall carbon nanotubes
A mass and spring model is used to calculate the phonon mode dispersion for
single wall carbon nanotubes (SWNTs) of arbitrary chirality. The calculated
dispersions are used to determine the chirality dependence of the radial
breathing phonon mode (RBM) density. Van Hove singularities, usually discussed
in the context of the single particle electronic excitation spectrum, are found
in the RBM density of states with distinct qualitative differences for zig zag,
armchair and chiral SWNTs. The influence the phonon mode density has on the two
phonon resonant Raman scattering cross-section is discussed.Comment: 6 pages, 2 figures, submitted to Phys. Rev.
One-dimensional carbon nanostructures for terahertz electron-beam radiation
One-dimensional carbon nanostructures such as nanotubes and nanoribbons can feature near-ballistic electronic transport over micron-scale distances even at room temperature. As a result, these materials provide a uniquely suited solid-state platform for radiation mechanisms that so far have been the exclusive domain of electron beams in vacuum. Here we consider the generation of terahertz light based on two such mechanisms, namely, the emission of cyclotronlike radiation in a sinusoidally corrugated nanowire (where periodic angular motion is produced by the mechanical corrugation rather than an externally applied magnetic field), and the Smith-Purcell effect in a rectilinear nanowire over a dielectric grating. In both cases, the radiation properties of the individual charge carriers are investigated via full-wave electrodynamic simulations, including dephasing effects caused by carrier collisions. The overall light output is then computed with a standard model of charge transport for two particularly suitable types of carbon nanostructures, i.e., zigzag graphene nanoribbons and armchair single-wall nanotubes. Relatively sharp emission peaks at geometrically tunable terahertz frequencies are obtained in each case. The corresponding output powers are experimentally accessible even with individual nanowires, and can be scaled to technologically significant levels using array configurations. These radiation mechanisms therefore represent a promising paradigm for light emission in condensed matter, which may find important applications in nanoelectronics and terahertz photonics.DMR-1308659/National Science Foundationhttp://ultra.bu.edu/papers/Tantiwanichapan-2016-PRB-CNT-THz.pd
Induced fermionic current in toroidally compactified spacetimes with applications to cylindrical and toroidal nanotubes
The vacuum expectation value of the fermionic current is evaluated for a
massive spinor field in spacetimes with an arbitrary number of toroidally
compactified spatial dimensions in presence of a constant gauge field. By using
the Abel-Plana type summation formula and the zeta function technique we
present the fermionic current in two different forms. Non-trivial topology of
the background spacetime leads to the Aharonov-Bohm effect on the fermionic
current induced by the gauge field. The current is a periodic function of the
magnetic flux with the period equal to the flux quantum. In the absence of the
gauge field it vanishes for special cases of untwisted and twisted fields.
Applications of the general formulae to Kaluz-Klein type models and to
cylindrical and toroidal carbon nanotubes are given. In the absence of magnetic
flux the total fermionic current in carbon nanotubes vanishes, due to the
cancellation of contributions from two different sublattices of the graphene
hexagonal lattice.Comment: 18 pages, 5 figures, explicit regularization procedure adde
Systematic Determination of Absolute Absorption Cross-section of Individual Carbon Nanotubes
Determination of optical absorption cross-section is always among the central
importance of understanding a material. However its realization on individual
nanostructures, such as carbon nanotubes, is experimentally challenging due to
the small extinction signal using conventional transmission measurements. Here
we develop a technique based on polarization manipulation to enhance the
sensitivity of single-nanotube absorption spectroscopy by two-orders of
magnitude. We systematically determine absorption cross-section over broad
spectral range at single-tube level for more than 50 chirality-defined
single-walled nanotubes. Our data reveals chirality-dependent one-dimensional
photo-physics through the behaviours of exciton oscillator strength and
lifetime. We also establish an empirical formula to predict absorption spectrum
of any nanotube, which provides the foundation to determine quantum
efficiencies in important photoluminescence and photovoltaic processes
One-electron states and interband optical absorption in single-wall carbon nanotubes
Explicit expressions for the wave functions and dispersion equation for the
band p - electrons in single-wall carbon nanotubes are obtained within the
method of zero-range potentials. They are then used to investigate the
absorption spectrum of polarized light caused by direct interband transitions
in isolated nanotubes. It is shown that, at least, under the above
approximations, the circular dichroism is absent in chiral nanotubes for the
light wave propagating along the tube axis. The results obtained are compared
with those calculated in a similar way for a graphite plane.Comment: 16 pages, 8 figures, 1 tabl
Coherent Phonons in Carbon Nanotubes and Graphene
We review recent studies of coherent phonons (CPs) corresponding to the
radial breathing mode (RBM) and G-mode in single-wall carbon nanotubes (SWCNTs)
and graphene. Because of the bandgap-diameter relationship, RBM-CPs cause
bandgap oscillations in SWCNTs, modulating interband transitions at terahertz
frequencies. Interband resonances enhance CP signals, allowing for chirality
determination. Using pulse shaping, one can selectively excite
speci!c-chirality SWCNTs within an ensemble. G-mode CPs exhibit
temperature-dependent dephasing via interaction with RBM phonons. Our
microscopic theory derives a driven oscillator equation with a
density-dependent driving term, which correctly predicts CP trends within and
between (2n+m) families. We also find that the diameter can initially increase
or decrease. Finally, we theoretically study the radial breathing like mode in
graphene nanoribbons. For excitation near the absorption edge, the driving term
is much larger for zigzag nanoribbons. We also explain how the armchair
nanoribbon width changes in response to laser excitation.Comment: 48 pages, 41 figure
Kohn Anomaly in Raman Spectroscopy of Single Wall Carbon Nanotubes
Phonon softening phenomena of the point optical modes including the
longitudinal optical mode, transverse optical mode and radial breathing mode in
"metallic" single wall carbon nanotubes are reviewed from a theoretical point
of view. The effect of the curvature-induced mini-energy gap on the phonon
softening which depends on the Fermi energy and chirality of the nanotube is
the main subject of this article. We adopt an effective-mass model with a
deformation-induced gauge field which provides us with a unified way to discuss
the curvature effect and the electron-phonon interaction.Comment: 36 pages, 9 figure
Theory of coherent phonons in carbon nanotubes and graphene nanoribbons
We survey our recent theoretical studies on the generation and detection of coherent radial
breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial
breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic
theory for the electronic states, phonon modes, optical matrix elements and electronヨphonon
interaction matrix elements that allows us to calculate the coherent phonon spectrum. An
extended tight-binding (ETB) model has been used for the electronic structure and a valence
force field (VFF) model has been used for the phonon modes. The coherent phonon
amplitudes satisfy a driven oscillator equation with the driving term depending on the
photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on
the nanotube chirality and type, and also on the graphene nanoribbon mod number and class
(armchair versus zigzag). We compare these results with a simpler effective mass theory
where reasonable agreement with the main features of the coherent phonon spectrum is found.
In particular, the effective mass theory helps us to understand the initial phase of the coherent
phonon oscillations for a given nanotube chirality and type. We compare these results to two
different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube
films. In the case of graphene nanoribbons, there are no experimental observations to date. We
also discuss, based on the evaluation of the electronヨphonon interaction matrix elements, the
initial phase of the coherent phonon amplitude and its dependence on the chirality and type.
Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag
nanoribbons obtained using an effective mass theory
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