S. Y. Mensah

Amplification of acoustic phonons in a degenerate semiconductor superlattice

Amplification of acoustic phonons propagating along the axis of a semiconductor superlattice (SL) is investigated using the microscopic theory. A non-quantizing electric field is applied to the SL to produce a drift velocity vD of the charge carriers and whenever vD exceeds vs (sound velocity) amplification occurs. The threshold field E0 at which absorption switches over to amplification depends on the SL parameters and vs. It is noted that E0 in SL is by far lower than that in bulk semiconductors and that there exists the possibility of finding a field E∗ such that −Γ(E∗)⩾Γ(−E∗). This allows, in principle, the use of SL as hypersound generator.

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

Abstract The theory of hot electrons injection in carbon nanotubes (CNTs) where both dc electric field (Ez), and a quasi-static ac field exist simultaneously (i.e. when the frequency ω of ac field is much less than the scattering frequency v ( ω ⪡ v or ω τ ⪡ 1 , v = τ − 1 ) where τ is relaxation time) is studied. The investigation is done theoretically by solving semi-classical 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 (Ez) 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 ω τ ⪡ 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 E z ≥ 75 kV / cm and E z ≥ 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.

High frequency conductivity of hot electrons in carbon nanotubes

Abstract High frequency conductivity of hot electrons in undoped single walled achiral Carbon Nanotubes (CNTs) under the influence of ac–dc driven fields was considered. We investigated semi-classically Boltzmanns transport equation with and without the presence of the hot electrons’ source by deriving the current densities in CNTs. Plots of the normalized current density versus frequency of ac-field revealed an increase in both the minimum and maximum peaks of normalized current density at lower frequencies as a result of a strong injection of hot electrons. The applied ac-field plays a twofold role of suppressing the space-charge instability in CNTs and simultaneously pumping an energy for lower frequency generation and amplification of THz radiations. These have enormous promising applications in very different areas of science and technology.

General Scattering Mechanism and Transport in Graphene

Using quasi-time dependent semi-classical transport theory in RTA, we obtained coupled current equations in the presence of time varying field and based on general scattering mechanism

External Electric Field Effect on Electrons Transport in Carbon Nanotubes

\tau \propto \mathcal{E}^{\beta}

Terahertz generation and amplification in graphene nanoribbons in multi-frequency electric fields

. We find that close to the Dirac point, the characteristic exponent

Amplification of terahertz radiation in carbon nanotubes

\beta = +2

Hydrodynamic study of edge spin-vortex excitations of fractional quantum Hall fluid

corresponds to acoustic phonon scattering.

Appearance of Negative Differential Conductivity in Graphene Nanoribbons at High-Harmonics

\beta = +1

Laser induced amplification of hypersound in nondegenerate semiconductor superlattices

long-range Coulomb scattering mechanism.