Anthony Twum

External Electric Field Effect on Electrons Transport in Carbon Nanotubes

We consider a simple model of carbon nanotubes (CNTs) subject to external electric field E(t). Using a tight-binding approximation for the description of energy bands of CNTs, together with the standard Boltzmann transport equation and constant relaxation time, we predict the effect of self-induced transparency and absolute negative conductivity. The predicted effects may be useful in diagnostics of carbon nanotubes as well as in the amplification and efficiency conversion of electromagnetic signals.

Amplification of terahertz radiation in carbon nanotubes

We investigate theoretically the feasibility of amplification of terahertz radiation in aligned achiral carbon nanotubes, a zigzag (12,0) and an armchair (10,10) in comparison with a superlattice using a combination of a constant direct current (dc) and a high-frequency alternate current (ac) electric fields. The electric current density expression is derived using the semiclassical Boltzmann transport equation with a constant relaxation time. The electric field is applied along the nanotube axis. Analysis of the current density versus electric field characteristics reveals a negative differential conductivity behavior at high frequency, as well as photon assisted peaks. The photon assisted peaks are about an order of magnitude higher in the carbon nanotubes compared to the superlattice. These strong phenomena in carbon nanotubes can be used to obtain domainless amplification of terahertz radiation at room temperature.

Characterization of Potential Direct Recharge in the Gold Mining District of South-Western Ghana Using the Hydrus-1D Computer Code

The mean annual potential recharge rate to the ground water system in the Tarkwa gold mining district of South Western Ghana, has been estimated for the period 1977 to 2001 by analyzing precipitation recharge in response to rainfall pattern and distribution for the four main soil types; Huni, Tarkwa Phyllite, Banket and Kawere. Using the Hydrus-1D infiltration computer code model, simulation results of recharge-precipitation exhibited a linear relationship, and gave correlation coefficient values R2 ranging from 0.39 to 0.55 for the four soil types and 0.72 for the composite soil. Variability of annual recharge was assigned to various reasons, including the variation in the weather pattern (rainfall, temperature, evapotranspiration etc), topography and hydraulic properties of the aquifer system. The potential direct recharge rate was estimated to range from 269 mm/yr to 611 mm/yr, with an average value of 385 mm/yr. This is as a result of infiltration of about 18% to 36% (average 27%) of the mean annual precipitation. The high average value of 385 mm/yr is reasonable on the basis that it represents the potential maximum value of the actual recharge value of 299±72 mm/yr.