Plasma mechanisms of resonant terahertz detection in two-dimensional electron channel with split gates

We analyze the operation of a resonant detector of terahertz (THz) radiation based on a two-dimensional electron gas (2DEG) channel with split gates. The side gates are used for the excitation of plasma oscillations by incoming THz radiation and control of the resonant plasma frequencies. The central gate provides the potential barrier separating the source and drain portions of the 2DEG channel. Two possible mechanisms of the detection are considered: (1) modulation of the ac potential drop across the barrier and (2) heating of the 2DEG due to the resonant plasma-assisted absorption of THz radiation followed by an increase in thermionic dc current through the barrier. Using the device model we calculate the frequency and temperature dependences of the detector responsivity associated with both dynamic and heating (bolometric) mechanisms. It is shown that the dynamic mechanisms dominates at elevated temperatures, whereas the heating mechanism provides larger contribution at low temperatures, T=35-40 K.Comment: 7 pages, 4 figure

Analytical device model for graphene bilayer field-effect transistors using weak nonlocality approximation

We develop an analytical device model for graphene bilayer field-effect transistors (GBL-FETs) with the back and top gates. The model is based on the Boltzmann equation for the electron transport and the Poisson equation in the weak nonlocality approximation for the potential in the GBL-FET channel. The potential distributions in the GBL-FET channel are found analytically. The source-drain current in GBL-FETs and their transconductance are expressed in terms of the geometrical parameters and applied voltages by analytical formulas in the most important limiting cases. These formulas explicitly account for the short-gate effect and the effect of drain-induced barrier lowering. The parameters characterizing the strength of these effects are derived. It is shown that the GBL-FET transconductance exhibits a pronounced maximum as a function of the top-gate voltage swing. The interplay of the short-gate effect and the electron collisions results in a nonmonotonic dependence of the transconductance on the top-gate length.Comment: 12 pages, 7 figure

Phase diagram of a frustrated mixed-spin ladder with diagonal exchange bonds

Using exact numerical diagonalization and the conformal field theory approach, we study the effect of magnetic frustrations due to diagonal exchange bonds in a system of two coupled mixed-spin $(1,{1/2})$ Heisenberg chains. It is established that relatively moderate frustrations are able to destroy the ferrimagnetic state and to stabilize the critical spin-liquid phase typical for half-integer-spin antiferromagnetic Heisenberg chains. Both phases are separated by a narrow but finite region occupied by a critical partially-polarized ferromagnetic phase.Comment: 5 PRB pages, 7 eps figures, to appear in Phys. Rev.

Factors affecting the commercialization of food legumes in North Cameroon

The study focused on factors affecting the commercialization of food legumes - cowpeas (Vigna unguiculata), bambara groundnuts (Vigna subterranea), groundnuts (Arachis hypogaea) and soybeans (Glycine max) in two departments (Benoué and Mayo-Louti) of the North Region of Cameroon. Qualitative and quantitative methods were used to collect data. Qualitative methods involved focus group discussions and direct observations. A structured interview guide was administered in major peri-urban and urban markets to get supportive quantitative data. The respondents were selected using purposive sampling technique. A content analysis was conducted and quantitative data was analyzed with the aid of XLSTAT 2007 software and Excel. A cross-section of the age and gender population was involved in the marketing of food legumes. Women constituted more than 80% in the retail sector on peri-urban markets. The study found that 100% of the vendors had never got any form of training on business management before engaging into the sector. Socio-cultural factors were determinant factors that impacted negatively in promoting an entrepreneurial culture among women. The marginalization of women, the restriction of land ownership by traditional tenure and poor distribution of land for agriculture were some of the main constraints faced by women. The impediments of women’s empowerment include their lack of access to the decisionmaking process, low participation in local governance and limited access to technology inputs and credit. Traditional practices forbid women and girls to have or manage large amounts of money. Such restrictions have serious consequences on agricultural production. Instead of exploiting the productive capacity of women at all levels, including food production and transformation, some of the traditions simply stifle their potential energies. In addition to socio-cultural factors, major bottlenecks faced in the production and commercialisation of food legumes includes social structural and institutional, agronomic, natural (biotic and abiotic stress) and climatic factors. Development of intervention strategies through exogenous innovation techniques might act as an incentive for smallholder farmers to invest more of their land and energy in cultivation of food legumes. More attention should be focused on market research and development with crop improvement programmes that are economically profitable, if food legumes are to continue making a greater contribution to the household income, nutrition and sustainable development in North Cameroon.&nbsp