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We have recently improved a two level model of a quantum cascade laser. The two level model is simpler than the 3-level model reported earlier. We calculated the intensity modulation of a laser emitting light of wavelength 5μm. As the laser considered is rather old, we consider a more recent quantum cascade laser and examine its intensity modulation response. [ABSTRACT FROM AUTHOR]

An analytical model of a possible DNA sensor based on a carbon nanotube network that functions as a selective detector of DNA molecules is presented. The ability to implement label-free electronic detection using a DNA sensor based on a carbon nanotube network constitutes an important step towards low-cost, highly sensitive, simple and accurate molecular diagnostics. In particular, there is an urgent need for a simple method of detection of DNA molecules as this will provided a new and efficient way to diagnosis genetic or pathogenic diseases. Bio-compatibility and high sensitivity towards environmental perturbations make graphene nanomaterials a good choice for a sensing layer in an electronic DNA sensor. In this study, a conductance model of a DNA sensor based on a carbon nanotube network is suggested and the performance of the model is evaluated by calculating current-voltage characteristics. [ABSTRACT FROM AUTHOR]

We present trilayer graphene nanoribbon carrier statistics in the degenerate and the nondegenerate limits. Within zero to 3kBT from the conduction or valence band edgers high concentrations of carriers sensitively depend on a normalized Fermi energy which is independent of temperature. The effect of different stacking orders of graphene multilayers on the electric field induced band gap is studied. The gap for trilayer graphene with the ABC stacking is much larger than the corresponding gap for the ABA trilayer. The gap for the different types of stacking is much larger as compared to the case of Bernal stacking. A non-monotonic dependence of the true energy gap in trilayer graphene on the charge density is investigated along with the electronic low-energy band structure of ABC stacked multilayer graphene. The band structure of trilayer graphene systems in the presence of a perpendicular electric field is obtained using a tight-binding approach. [ABSTRACT FROM AUTHOR]

In this paper we investigate the band structure of graphene nano-ribbons and the current density. A square root approximation shows that the band energy is parabolic in the low energy limit. In this parabolic region charge transport is controlled by the saturation velocity. [ABSTRACT FROM AUTHOR]

In this work, fundamental results on carrier statistics in a carbon nanotube treated as a one-dimensional material are presented. Also the effect of degeneracy on the capacitance of the carbon nanotube channel in a carbon nan-otube field effect transistor is discussed. A quantum capacitance as well as a classical capacitance is revealed. Furthermore it is shown that for low gate voltage, the total capacitance is equivalent to the classical capacitance but for high gate voltage it is equivalent to the quantum capacitance. We predict that in the nondegenerate regime, the total capacitance is equivalent to the classical capacitance and that the quantum capacitance can be neglected, whereas only quantum capacitance needs to be taken into account in the calculation of the total capacitance in the degenerate regime. [ABSTRACT FROM AUTHOR]

This paper focuses on the electronic transport properties of bilayer Graphene nanoribbons (BGNRs). The electronic transport of two dimensional AB stacked BGNRs with widths less than 10 nm is investigated. Due to the small width, the BGNR can be assumed to operate as a one dimensional device. A mathematical model of carrier concentration in BGNRs in the nondegenerate regime is presented. The model shows that in this regime the carrier statistics of BGNRs can be expressed by an exponential function of the normalized Fermi energy η, which is particularly useful in the lower carrier concentration regions. [ABSTRACT FROM AUTHOR]

In this paper we discuss the energy band structure of bilayer graphene nanoribbons (BGNRs) near the Fermi level between zero and 3kBT away from the conduction and valence bands. BGNRs can be used as the channel in field effect transistors (FETs). A FET can be created using graphene bilayers with the gate voltage perpendicular to the layers. We focus on carrier statistics in the degenerate regime and the density of states, and consider them to be fundamental properties of BGNRs. The model presented indicates that the normalized Fermi energy in the degenerate regime strongly depends on carrier concentration and is independent of temperature. [ABSTRACT FROM AUTHOR]

In this paper, a circuit model is devised to analyze nonlinear behaviour of quantum cascade lasers. Such nonlinear behavior influences the light output when the laser is driven by currents comparable to the average (DC) current. The simplified 2-level rate equations are first improved. Next, the circuit model is obtained following the approach for interband lasers. The difference between the circuit models of quantum cascade lasers and interband lasers is pointed out. The circuit model is simpler compared to that obtained from a 3-level model. Unlike a circuit model derived earlier from the 2-level model, the present model can handle both small and large current drives. [ABSTRACT FROM AUTHOR]

The band structure of a carbon nanotube (CNT) near to the minimum band energy is parabolic. However it is not parabolic in other parts of the band energy. In the parabolic part, based on the confinement effect, we present an analytical model that captures the essence of the physical processes relevant to the operation of a carbon nanotube field effect transistor (CNTFET). The model covers seamlessly the whole range of transport from drift-diffusion to ballistic. It has been clarified that the intrinsic speed of CNTs is governed by the transit time of electrons. Although the transit time is more dependent on the saturation velocity than on the weak-field mobility, the feature of high-electron mobility is beneficial in the sense that the drift velocity is always maintained closer to the saturation velocity, at least at the drain end of the transistor where the electric field is necessarily high and controls the saturation current. The results obtained are applied to the modeling of the current-voltage characteristics of a CNTFET. The channel-length modulation is shown to arise from the drain velocity becoming closer to the ultimate saturation velocity as the drain voltage is increased. [ABSTRACT FROM AUTHOR]

Landau-Devonshire theory is a phenomenological theory of ferroelectrics. In this paper a detailed discussion of the application of this theory to ferroelectric films is given; also the interaction of the film with incident electromagnetic radiation is discussed via the Landau-Khalatnikov equations of motion. In particular the role of optimization in the theory is emphasized. It is shown that two important areas to which optimization is relevant are the minimization of the free-energy density, an essential part of the Landau-Devonshire theory, and to the way in which the equations of motion are solved. For the latter it turns out that it is better to work in real variables rather than the complex variables that are convenient to introduce into the formalism prior to the numerical work when dealing with harmonic waves. [ABSTRACT FROM AUTHOR]

This paper shows how a Landau thermodynamic theory can be utilized to study size effects in antiferroelectric thin films on metal substrates via reflectivity measurements in the terahertz region. A Landau free energy that includes a gradient term to describe the size effects is minimized. Landau Khalatnikov equations together with Maxwell’s equations can then be solved to describe how the radiation interacts with the film. From this reflectivity curves can be calculated and related to experimental studies. The metal substrate is expected to influence the reflectivity curves compared to free standing films without substrates. This is significant since ceramic thin films are of technological importance, and films on metal substrates such as electrodes are relevant to applications such as memory devices. The main expectation is that terahertz wave measurements provide a sensitive probe of size effects and substrate influence. [ABSTRACT FROM AUTHOR]

Landau-Devonshire theory is used to find analytical expressions for the polarization profile in a ferroelectric film by minimizing the free energy functional for the film. Particular attention is paid to how the profile can be calculated from the boundary conditions, which ultimately requires the solution of transcendental equations. A numerical scheme for dealing with this is discussed. [ABSTRACT FROM AUTHOR]

In this paper, the carrier concentration for bilayer graphene is modelled. The effect of an applied interlayer voltage V on the bandgap and carrier concentration is analysed using a new analytical approach. We begin by modelling of the density of states followed by carrier concentration as a function of V. It is found that as V increases, carrier concentration and the bandgap also increase. [ABSTRACT FROM AUTHOR]

We study the high-field transport behavior of a graphene nanoribbon in the degenerate and nondegenerate regimes by incorporating a previous low field mobility calculation modified for a high electric field in both regimes. The drift velocity model presented predicts that the transport behaviour is different in each regime when subject to an electric field. Our theoretical results are comparable with experimental data. The Fermi velocity (for the degenerate regime) and thermal velocity (for the nondegenerate regime) coupled with optical phonon emission have a large influence on the drift velocity leading to a velocity saturation preceded by a monotonic increase with increasing electric field. [ABSTRACT FROM AUTHOR]