1,913 research outputs found
Acoustoelectric Effect in Graphene with degenerate Energy dispersion
The acoustoelectric effect in Graphene with degenerate energy dispersion
is theoretically studied for hypersound in the regime . At low
temperatures (), the non-linear dependence of Acoustoelectric
current on the frequency and temperature are numerically
analysed. The obtained graph for against qualitatively
agreed with an experimentally obtained results. For versus , the
dependence of Acoustoelectric current in Graphene was found to manifest at low
temperatures.Comment: three figur
Acoustoelectric Effect in degenerate Carbon Nanotube
Acoustoelectric Effect in degenerate Carbon Nanotube () was
theoretically studied for hypersound in the regime . The dependence of
acoustoelectric current on the acoustic wave number and
frequency at and scattering angle () was
evaluated at various harmonics (where is an integer). In
the first harmonics (), the non-linear dependence of on
and were obtained. For , the numerically
evaluated qualitatively agreed with an experimentally obtained result.Comment: 6 figure
Negative Differential Conductivity in Carbon Nanotubes in the Presence of an External Electric Field
We study theoretically the electron transport properties in carbon nanotubes
under the influence of an external electric field E(t) using Boltzmann's
equation. The current-density equation is derived. Negative differential
conductivity is predicted when {\omega}{\tau}<<1 (quasi-static case). We
observed this in the neighbourhood where the constant electric field E_o is
equal to the amplitude of the AC electric field E_1 and the peak decreases with
increasing E_1. This phenomenon can also be used for the generation of
terahertz radiation without electric current instability.Comment: 6 pages, 4 figure
On the amplification of acoustic phonons in carbon nanotube
We present a theoretical study of acoustic phonons amplification in Carbon
Nanotubes (CNT). The phenomenon is via Cerenkov emission (CE) of acoustic
phonons using intraband transitions proposed by Mensah et. al.,~\cite{1} in
Semiconductor Superlattices (SSL) and confirmed in ~\cite{2}. From this, an
asymmetric graph of on and were
obtained where amplification () absorption
(). The ratio, , at , and for scattering angle
. A threshold field at which switches over to
was calculated to be $E_{z}^{dc} = 6.2\times 10^3\
\mathrm{V/m}. This field is far less than that deduced using Bloch-Type
Oscillation (BTO)~\cite{3} which is E_{BTO}^{dc} = 3.0\times 10^5\
\mathrm{V/m}\Gamma_{amp}^{CNT}$ would enable the use of CNT
for the production of SASER.Comment: 12 pages, 2 figure
Generation of Terahertz Radiation by Wave Mixing in Armchair Carbon Nanotubes
Using semiclassical Boltzmann equation we have studied theoretically an
effect of a direct current (DC) generation in undoped armchair carbon nanotube
(CN) by mixing two coherent electromagnetic waves with commensurate frequencies
i.e omega and 2 omega . We compared the results of the armchair with that of
the zigzag carbon nanotubes for the same conditions (i.e. when the normalized
current is plotted against the amplitude). Quantitatively they agree with each
other except that the absolute value of the peaks of the current for zigzag is
about 1.1 times that of the armchair. We noticed that the current is negative
similar to that observed in zigzag CNs describing the same effect. However it
is interesting to note that graph of normalized current against omega tau
showed that the armchair is greater than that of the zigzag for about 1.1 times
which is opposite. We also observed that when the phase shift theta lies
between pi divided by 2 and 3 pi divided by 2 there is an inversion and the
current becomes positive. We suggest the use of this approach for the
generation of terahertz radiation and also for the determination of the
relaxation time of electrons in carbon nanotubes.Comment: 6pages,5figure
Generation of Terahertz Radiation by Wave Mixing in Zigzag Carbon Nanotubes
With the use of the semiclassical Boltzmann equation we have calculated a
direct current (d.c) in undoped zigzag carbon nanotube (CN) by mixing two
coherent electromagnetic waves with commensurate frequencies i.e and . This
effect is attributed to the nonparabolicity of the electron energy band which
is very strong in carbon nanotubes. We observed that the current is negative
similar to that observed in superlattice. However if the phase shift lies
between and there is an inversion and the current becomes positive. It is
interesting to note that exhibit negative differential conductivity as expected
for d.c through carbon nanotubes. This method can be used to generate terahertz
radiation in carbon nanotubes. It can also be used in determining the
relaxation time of electrons in carbon nanotubesComment: 12pages,5figure
Generation and Amplification of Terahertz Radiation in Carbon Nanotubes
We investigate theoretically the feasibility of generation and amplification
of terahertz radiation in aligned achiral carbon nanotubes (zigzag and
armchair) in comparison with a superlattice in the presence of a constant (dc)
and high-frequency (ac) electric fields. The electric current density
expression is derived using the semiclassical Boltzmann transport equation with
a constant relaxation time with the electric field applied along the nanotube
axis. Our analysis on the current density versus electric field characteristics
demonstrates negative differential conductivity at high frequency as well as
photon assisted peaks. The characteristic peaks are about an order of magnitude
styronger in the carbon nanotubes compared to superlattice. These strong
phenomena in carbon nanotubes can be used to obtain domainless amplification of
terahertz radiation in carbon nanotubes at room temperature.Comment: 11 pages, 3 figure
Nonlinear Conductivity in Graphene
We consider the tight-binding approximation for the description of energy
bands of graphene, together with the standard Boltzmann's transport equation
and constant relaxation time, an expression for the conductivity was obtained.
We predicted strong nonlinear effects in graphene which may be useful for high
frequency generation
Stark-cyclotron Resonance in an Array of Carbon Nanotubes
Using the kinetic approach based on the semiclassical Boltzmann transport
equation with constant relaxation time, we theoretically studied the
Stark-cyclotron resonance in an array of Carbon Nanotubes (CNs). Exact
expression for the current density was obtained. We noted that Stark-cyclotron
resonance occurs when the Larmor frequency coincides with the Stark frequency.
A coincidence of these frequencies produce resonance.Comment: 1 figur
Absorption of acoustic phonons in Fluorinated Carbon Nanotubes with non-parabolic, double periodic band
We studied theoretically the absorption of acoustic phonons in the hypersound
regime in Fluorine modified Carbon Nanotube (F-CNT) and
compared it to that of undoped Single Walled Nanotube (SWNT) .
Per the numerical analysis, the F-CNT showed less absorption to that of SWNT
thus . This is due to
the fact that Fluorine is highly electronegative and weakens the walls of the
SWNT. Thus, the -electrons associated with to the Fluorine which causes
less free charge carriers to interact with the phonons and hence changing the
metallic properties of the SWNT to semiconductor by the doping process. From
the graphs obtained, the ratio of hypersound absorption in SWNT to F-CNT at is whilst at , is and at ,
is . Clearly, the ratio
decreases as the temperature increases.Comment: 13 pages, 6 figure
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