Search

Behaviour of hot electrons under the influence of dc field in carbon nanotubes is theoretically considered. The study was done semi-classically by solving Boltzmann transport equation with the presence of the hot electrons source to derive the current densities. Plots of the normalized axial current density versus electric field strength of the chiral CNTs reveal a negative differential conductivity (NDC). Unlike achiral CNTs, the NDC occurs at a low field about $\mathrm{6\ kV/cm}$ for chiral CNT. We further observed that the switch from NDC to PDC occurs at lower dc field in chiral CNTs than achiral counterparts. Hence the suppression of the unwanted domain instability usually associated with NDC and a potential generation of terahertz radiations occurs at low electric field for chiral CNTs., Comment: 18 pages, 5 figures

Recent analyses of thermoelectric amplification of acoustic phonons in Free-Standing Graphene (FSG) $\Gamma_q^{grap}$ have prompted the theoretical study of the influence of external temperature gradient ($\nabla T$) on the acoustoelectric current $j_T^{(grap)}$ in FSG. Here, we calculated thermal field on open circuit ($j_T^{(grap)} = 0$) to be $(\nabla T)^g = 746.8Km^{-1}$. We then calculated acoustoelectric current ($j_T^{(grap)}$)to be $1.1mA\mu m^{-2}$ for $\nabla T = 750.0 Km^{-1}$, which is comparable to that obtained in semiconductors ($1.0mA\mu m^{-2}$), the thermal-voltage $(V_T)_0^g$ to be $6.6\mu V$ and the Seebeck coefficient $S$ as $8.8\mu V/K$. Graphs of the normalized $j_T^{(grap)}/j_0$ versus $\omega_q$, $T$ and $\nabla T/T$ were sketched. For $j_T^{(grap)}$ on $T$ for varying $\omega_q$, Negative Difference Conductivity (NDC) ($| \frac{\partial j}{\partial T}| < 0$) was observed in the material. This indicates graphene is a suitable material for developing thermal amplifiers and logic gates., Comment: 14 pages, 7 pages

We studied theoretically the absorption of acoustic phonons in the hypersound regime in Fluorine modified Carbon Nanotube (F-CNT) $\Gamma_q^{F-CNT}$ and compared it to that of undoped Single Walled Nanotube (SWNT) $\Gamma_q^{SWNT}$. Per the numerical analysis, the F-CNT showed less absorption to that of SWNT thus $\vert\Gamma_q^{F-CNT}\vert < \vert\Gamma_q^{SWNT}\vert $. This is due to the fact that Fluorine is highly electronegative and weakens the walls of the SWNT. Thus, the $\pi$-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 $T = 45K$ is $\frac{\Gamma_{(SWNT)}}{\Gamma_{(F-CNT)}}\approx 29$ whilst at $T = 55K$, is $\frac{\Gamma_{(SWNT)}}{\Gamma_{(F-CNT)}}\approx 9$ and at $T = 65K$, is $\frac{\Gamma_{(SWNT)}}{\Gamma_{(F-CNT)}}\approx 2$. Clearly, the ratio decreases as the temperature increases., Comment: 13 pages, 6 figures

Hypersound Absorption of acoustic phonons having $ql>>1$ in a degenerate Carbon Nanotube (CNT) with linear energy dispersion near the Fermi level was theoretically studied. The general expression for the absorption coefficient ($\Gamma$) under a non-quantizing electric field ($E$) with drift velocity ($V_D$)was obtained. At $T = 10K$ and scattering angle $\theta > 0$, the dependence of $\Gamma$ on acoustic wave number ($\vec{q}$), frequency ($\omega_q$), and $\gamma = 1-\frac{V_D}{V_s}$, ($V_s$ being the speed of sound) were analysed numerically at $n = 0, \pm 1, \pm 2$ (where $n$ represent the various harmonics) and presented graphically. In a $3D$ representation, when $\gamma < 0$, the maximum amplification was attained at $V_D = 1.1V_s$ which occurred at $E = 51.7Vcm^{-1}$. In the second harmonics, ($n =\pm 2$), the absorption obtained was compared to experimental measurement of acoustoelectric current via the Weinreich relation. From the graphs, the observed amplification of acoustic phonons caused by intraband transition shows CNT as a promising hypersound generator (SASER)., Comment: 18 pages, 13 figures

The Acoustomagnetoelectric Effect (AME) in Graphene Nanoribbon (GNR) was theoretically studied using the Boltzmann kinetic equation. On open circuit, the general formular for Surface Acoustomagnetoelectric field ($\vec{E}_{SAME}$) in GNR with energy dispersion $\varepsilon(p)$ near the Fermi point was calculated. The $E_{SAME}$ was found to depend on the magnetic strength ($\eta$), $\alpha$ = ${\hbar \omega_q}/{E_g}$ and the energy gap ($E_g$). The expression for $\vec{E}_{SAME}$ was analyzed numerically for varying width of GNR, magnetic strength ($\eta$) and $\alpha$ at different sub-bands indices ($p_i$). It was noted that the dependence of $\vec{E}_{SAME}$ on the width of GNR increased to a saturation point of approximately $15$Vcm$^{-1}$ and remained constant. For $E_{SAME}$ versus $\eta$, the $E_{SAME}$ increases rapidly to a maximum point and then decayed to a constant minimum value. The graph was modulated either by varying the width of GNR or the sub-band index $p_i$ with an inversion occurring at $p_i = 6$. The dependence of $E_{SAME}$ versus $\alpha$ was analyzed. The $E_{SAME}$ was constant up to a point and sharply increased asymptotically at approximately $\alpha = 1$. A $3$D graph of $\vec{E}_{SAME}$ with $\eta$ and width is also presented. This study is relevant for investigating the properties of GNR.