arXiv subjects

K. A. Dompreh

Publications and source records attributed to K. A. Dompreh.

At least 19 recordsLinked to original sources

arXiv2016

Behaviour of hot electrons under the dc field in chiral carbon nanotubes

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.

cond-mat.mes-hall↗

arXiv2016

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 $Γ^{CNT}$ on $\frac{V_d}{V_s}$ and $Ωτ$ were obtained where amplification ($Γ_{amp}^{CNT}$) $>>$ absorption ($Γ_{abs}^{CNT}$). The ratio, $\frac{\vert Γ_{amp}^{CNT}\vert}{\vertΓ_{abs}^{CNT}\vert}\approx 3.5$, at $V_d = 1.02V_s$, $ω_q = 3.0\ \mathrm{THz}$ and $T = 85\ K$ for scattering angle $θ> 0$ . A threshold field at which $Γ_{abs}^{CNT}$ switches over to $Γ_{amp}^{CNT}$ 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 $E_{BTO}^{dc} = 3.0\times 10^5\ \mathrm{V/m}$. The obtained $Γ_{amp}^{CNT}$ would enable the use of CNT for the production of SASER.

cond-mat.mes-hall↗

arXiv2016

Influence of external temperature gradient on acoustoelectric current in graphene

Recent analyses of thermoelectric amplification of acoustic phonons in Free-Standing Graphene (FSG) $Γ_q^{grap}$ 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 $1.1mAμm^{-2}$ for $\nabla T = 750.0 Km^{-1}$, which is comparable to that obtained in semiconductors ($1.0mAμm^{-2}$), the thermal-voltage $(V_T)_0^g$ to be $6.6μV$ and the Seebeck coefficient $S$ as $8.8μV/K$. Graphs of the normalized $j_T^{(grap)}/j_0$ versus $ω_q$, $T$ and $\nabla T/T$ were sketched. For $j_T^{(grap)}$ on $T$ for varying $ω_q$, 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.

cond-mat.mes-hall↗

arXiv2016

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) $Γ_q^{F-CNT}$ and compared it to that of undoped Single Walled Nanotube (SWNT) $Γ_q^{SWNT}$. Per the numerical analysis, the F-CNT showed less absorption to that of SWNT thus $\vertΓ_q^{F-CNT}\vert < \vertΓ_q^{SWNT}\vert $. This is due to the fact that Fluorine is highly electronegative and weakens the walls of the SWNT. Thus, the $π$-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 $\frac{Γ_{(SWNT)}}{Γ_{(F-CNT)}}\approx 29$ whilst at $T = 55K$, is $\frac{Γ_{(SWNT)}}{Γ_{(F-CNT)}}\approx 9$ and at $T = 65K$, is $\frac{Γ_{(SWNT)}}{Γ_{(F-CNT)}}\approx 2$. Clearly, the ratio decreases as the temperature increases.

cond-mat.mes-hall↗

arXiv2015

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.

cond-mat.mes-hall↗

arXiv2015

Hot electrons injection in carbon nanotubes under the influence of quasi-static ac-field

Hot electrons injection in carbon nanotubes (CNTs ) where in addition to applied dc field ($\mathbf{E}$), there exist simultaneously a quasi-static ac electric field (i.e. when the frequency $ω$ of ac field is much less than the scattering frequency $v$ ($ω\ll v$ or $ωτ\ll 1$, $v =τ^{-1}$, where $τ$ is relaxation time) is considered. The investigation is done theoritically by solving semiclassical Boltzmann transport equation with and without the presence of the hot electrons source to derive the current densities. Plots of the normalized current density versus dc field ($\mathbf{E}$) applied along the axis of the CNTs in the presence and absence of hot electrons reveal ohmic conductivity initially and finally negative differential conductivity (NDC) provided $ωτ\ll 1$ (i.e. quasi- static case). With strong enough axial injection of the hot electrons , there is a switch from NDC to positive differential conductivity (PDC) about $\mathbf{E} \geq 75 kV/cm$ and $\mathbf{E} \geq 140 kV/cm$ for a zigzag CNT and an armchair CNT respectively. Thus, the most important tough problem for NDC region which is the space charge instabilities can be suppressed due to the switch from the NDC behaviour to the PDC behaviour predicting a potential generation of terahertz radiations whose applications are relevance in current-day technology, industry, and research.

cond-mat.mes-hall↗

arXiv2015

High Frequency Conductivity of Hot Electrons in Carbon Nanotubes

High frequency conductivity of hot electrons in an undoped single walled achiral carbon nanotubes (CNTs) under the influence of ac-dc driven fields is considered. We investigated semiclassically by solving the Boltzmann's transport equation with and without the presence of the hot electrons source to derive the current densities. Plots of the normalized current density versus frequency of ac-field reveal an increase in both the minimum and maximum peaks of normalized current density at lower frequencies as a result of a strong enough injection of hot electrons . The applied ac-field plays twofold role of suppressing the space-charge instability in CNT and simultaneously pumping an energy for lower frequency generation and amplification of THz radiations which have enormous promising applications in very different areas of science and technology.

cond-mat.mes-hall↗

arXiv2015

Hypersound Absorption of Acoustic Phonons in a degenerate Carbon Nanotube

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 ($Γ$) under a non-quantizing electric field ($E$) with drift velocity ($V_D$)was obtained. At $T = 10K$ and scattering angle $θ> 0$, the dependence of $Γ$ on acoustic wave number ($\vec{q}$), frequency ($ω_q$), and $γ= 1-\frac{V_D}{V_s}$, ($V_s$ 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 $γ< 0$, 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).

cond-mat.mes-hall↗

arXiv2015

Acoustoelectric Effect in degenerate Carbon Nanotube

Acoustoelectric Effect $AE$ in degenerate Carbon Nanotube ($CNT$) was theoretically studied for hypersound in the regime $ql >> 1$. The dependence of acoustoelectric current $j^{ac}$ on the acoustic wave number $\vec{q}$ and frequency $ω_q$ at $T = 10K$ and scattering angle ($θ> 0$) was evaluated at various harmonics $n =\pm 1, 2, ...$ (where $n$ is an integer). In the first harmonics ($n = \pm 1$), the non-linear dependence of $j^{ac}$ on $ω_q$ and $\vec{q}$ were obtained. For $n = \pm 2$, the numerically evaluated $j^{ac}$ qualitatively agreed with an experimentally obtained result.

cond-mat.mes-hall↗

arXiv2015

Thermoelectric Amplification of Phonons in Graphene

Amplification of acoustic phonons due to an external temperature gredient ($\nabla T$) in Graphene was studied theoretically. The threshold temperature gradient $(\nabla T)_0^{g}$ at which absorption switches over to amplification in Graphene was evaluated at various frequencies $ω_q$ and temperatures $T$. For $T = 77K$ and frequency $ω_q = 12THz$, $(\nabla T)_0^{g} = 0.37Km^{-1}$. The calculation was done in the regime at $ql >> 1$. The dependence of the normalized ($Γ/Γ_0$) on the frequency $ω_q$ and the temperature gradient $(\nabla T/T)$ are evaluated numerically and presented graphically. The calculated $(\nabla T)_0^{g}$ for Graphene is lower than that obtained for homogeneous semiconductors ($n-InSb$) $(\nabla T)_0^{hom} \approx 10^3Kcm^{-1}$, Superlattices $(\nabla T)_0^{SL} = 384Kcm^{-1}$, Cylindrical Quantum Wire $(\nabla T)_0^{cqw} \approx 10^2Kcm^{-1}$. This makes Graphene a much better material for thermoelectric phonon amplifier.

cond-mat.mes-hall↗

arXiv2015

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.

cond-mat.mes-hall↗

arXiv2015

Acoustoelectric Effect in Graphene with degenerate Energy dispersion

The acoustoelectric effect $AE$ in Graphene with degenerate energy dispersion is theoretically studied for hypersound in the regime $ql >> 1$. At low temperatures ($k_βT <<1$), the non-linear dependence of Acoustoelectric current $j/j_0$ on the frequency $ω_q$ and temperature $T$ are numerically analysed. The obtained graph for $j/j_0$ against $ω_q$ qualitatively agreed with an experimentally obtained results. For $j/j_0$ versus $T$, the dependence of Acoustoelectric current in Graphene was found to manifest at low temperatures.

cond-mat.mes-hall↗

arXiv2015

Amplification of Acoustic Waves in Graphene Nanoribbon in the Presence of External Electric and Magnetic Field

Amplification of Acoustic Waves in Armchair Graphene Nanoribbon (AGNR) in the presence of an external Electric and Magnetic field was studied using the Boltzmann's kinetic equation. The general expression for the Amplification $(Γ_{\perp}/Γ_0)$ was obtained in the region $ql >> 1$ for the energy dispersion $\varepsilon(\vec{p})$ near the Fermi point. For various parameters of the quantized wave vector ($β$), the analysis of $Γ_{\perp}/Γ_0$ against the sub-bands index $(p_i)$; width of AGNR; and magnetic strength $(Ωτ)$, were numerically analyzed. The results showed a linear relation for $Γ_{\perp}/Γ_0$ with constant electric field $(\vec{E})$ but non-linear for $Γ_{\perp}/Γ_0$ with $q$ or $Ωτ$. Sound Amplification in AGNR is reported with an increase in Acoustic wave number $(\vec{q}) > 1.5\times 10^{7}cm^{-1}$. This can cause SASER in Armchair Graphene Nanoribbon (AGNR).

cond-mat.mes-hall↗

arXiv2015

Acoustomagnetoelectric Effect in Graphene Nanoribbon in the Presence of External Electric and Magnetic Field

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 ($η$), $α$ = ${\hbar ω_q}/{E_g}$ and the energy gap ($E_g$). The expression for $\vec{E}_{SAME}$ was analyzed numerically for varying width of GNR, magnetic strength ($η$) and $α$ 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 $η$, 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 $α$ was analyzed. The $E_{SAME}$ was constant up to a point and sharply increased asymptotically at approximately $α= 1$. A $3$D graph of $\vec{E}_{SAME}$ with $η$ and width is also presented. This study is relevant for investigating the properties of GNR.

cond-mat.mes-hall↗

arXiv2012

Laser Stimulated Thermal Conductivity in chiral carbon nanotube

An investigation of laser stimulated thermal conductivity in chiral CNT is presented. The thermal conductivity of a chiral CNT is calculated using a tractable analytical approach. This is done by solving the Boltzmann transport equation with energy dispersion relation obtained in the tight binding approximation. The electron thermal conductivity along the circumferential χ_c and axial χ_z are obtained. The results obtained are numerically analyzed and both χ_c and χ_z are found to oscillate in the presence of laser radiations. We have also noted that the laser source caused a drastic reduction in the both χ_c and χ_z values.

cond-mat.mtrl-sci↗

arXiv2012

Appearance of Large Amplitude Current Dynamics as a Result of Bragg Reflection in Carbon Nanotubes

We report on theoretical analysis of large amplitude current dynamics due to Bragg reflections in carbon nanotubes exposed to an external electric field. Using the kinetic equation with constant relaxation time, an analytical expression for the current density is obtained. Our results suggest that Bloch gain exists up to frequencies on the order of the Bloch frequency. We noted that due to the high density of states of conduction electrons in metallic carbon nanotubes and the specific dispersion law inherent in hexagonal crystalline structure result in a uniquely high frequency gain than the corresponding values for semiconducting ones. We suggest that this phenomenon can be used for domainless multiplication of the frequency of an electromagnetic signal at room temperature

cond-mat.mes-hall↗

arXiv2012

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.

cond-mat.mes-hall↗

arXiv2011

Effect of laser on thermopower of chiral carbon nanotube

An investigation of laser stimulated thermopower in chiral CNT is presented. The thermopower of a chiral CNT is calculated using a tractable analytical approach. This is done by solving the Boltzmann kinetic equation with energy dispersion relation obtained in the tight binding approximation to determine the electrical and thermal properties of chiral carbon nanotubes. The differential thermoelectric power α along the circumferential and axial axes are obtained. The results obtained are numerically analyzed and α is found to oscillate in the presence of laser radiations. We have also noted that Laser source above 4.6 x 107V/m lowered the thermopower otherwise there is no change. Varying delta s and delta z the thermopower changes from positive to negative.

cond-mat.mes-hall↗