Single-layer and bilayer graphene superlattices: collimation, additional Dirac points and Dirac lines

We review the energy spectrum and transport properties of several types of one- dimensional superlattices (SLs) on single-layer and bilayer graphene. In single-layer graphene, for certain SL parameters an electron beam incident on a SL is highly collimated. On the other hand there are extra Dirac points generated for other SL parameters. Using rectangular barriers allows us to find analytic expressions for the location of new Dirac points in the spectrum and for the renormalization of the electron velocities. The influence of these extra Dirac points on the conductivity is investigated. In the limit of {\delta}-function barriers, the transmission T through, conductance G of a finite number of barriers as well as the energy spectra of SLs are periodic functions of the dimensionless strength P of the barriers, P{\delta}(x) ~ V (x). For a Kronig-Penney SL with alternating sign of the height of the barriers the Dirac point becomes a Dirac line for P = {\pi}/2 + n{\pi} with n an integer. In bilayer graphene, with an appropriate bias applied to the barriers and wells, we show that several new types of SLs are produced and two of them are similar to type I and type II semiconductor SLs. Similar as in single-layer graphene extra "Dirac" points are found. Non-ballistic transport is also considered.Comment: 26 pages, 17 figure

Extra Dirac points in the energy spectrum for superlattices on single-layer graphene

We investigate the emergence of extra Dirac points in the electronic structure of a periodically spaced barrier system, i.e., a superlattice, on single-layer graphene, using a Dirac-type Hamiltonian. Using square barriers allows us to find analytic expressions for the occurrence and location of these new Dirac points in k-space and for the renormalization of the electron velocity near them in the low-energy range. In the general case of unequal barrier and well widths the new Dirac points move away from the Fermi level and for given heights of the potential barriers there is a minimum and maximum barrier width outside of which the new Dirac points disappear. The effect of these extra Dirac points on the density of states and on the conductivity is investigated.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.

Dirac electrons in a Kronig-Penney potential: dispersion relation and transmission periodic in the strength of the barriers

The transmission T and conductance G through one or multiple one-dimensional, delta-function barriers of two-dimensional fermions with a linear energy spectrum are studied. T and G are periodic functions of the strength P of the delta-function barrier V(x,y) / hbar v_F = P delta(x). The dispersion relation of a Kronig-Penney (KP) model of a superlattice is also a periodic function of P and causes collimation of an incident electron beam for P = 2 pi n and n integer. For a KP superlattice with alternating sign of the height of the barriers the Dirac point becomes a Dirac line for P = (n + 1/2) pi.Comment: 5 pages, 6 figure

KINEMATIC AND DYNAMIC ANALYSIS OF THE ROWER'S GESTURE ON CONCEPT II ERGOMETER

INTRODUCTION : Biomechanics Studies of rowing, remain most of the time global and consider the gesture as an indivisible whole (classification in style(DAL MONTE 89), coefficient of efficiency(ZATSIORSKY 91), peak of force on the handle (HARTMANN 93)). We plan to consider the gesture as the result of an elementary movements succession(movement of legs, movement of the trunk, movement of arms). Therefore the evaluation of the gesture efficiency depends on the study organization of these movements. The method used was the morphological analysis of kinematic and dynamic variable. An original experimental device has been elaborated. It consists of an optoelectronic system and a Concept II ergometer with of force and torque transducers. The population was a group of three rowers : a beginner, a regional level rower and a female rower of French team. After a period of warming of few minutes, the experimentation consisted in rowing during 20 minutes. The order was to row the furthest possible. The acquisition has been carried out for the first 5minutes.RESULTS : The first results show, for the three subjects, that the developed force on the handle cancels each other out before the end of the propulsion. This corresponds to a inefficiency phase of the gesture of the rower. A thorough morphological analysis shows that this phase is synchronized with a fall of the speed of the handle. Nevertheless, during this phase, the elbow angular speed is maximal. Consequently. During this phase, the contribution of arm is inefficient. The rower does not manageto increase the speed of the handle anymore. In addition, a comparative analysis between the three rowers is presented. It is based on inter-limb angular variable study and on effort delivered by the feet and the hands. The angular variable analysis shows a movement stereotyped for skilled rower. This confirmed that the expert's gestures are an automatism. Moreover, the increase of the force, applied on the feet strechers, carried out by the female rower, during the recovery, was delated, comparatively with the others rowers. The female rower controls her recovery. As this force does not make the boat further, the analysis of this variable shows as inefficient phase for the beginner and the regional rower. CONCLUSION : Kinematic and dynamic analysis of the rower gesture allowed to find 2 ineffective phases : the first during the end of the propulsion and the second during the end of the recovery. REFERENCES :DAL MONTE 89 : Dal Monte A,, Komor A.,Rowing and Sculling Mechanics, Article, Biomechanics of sport, Vaughan C.L.,ISBN : 0-8493-6820-0, 1989ZATSIORSKY 91 : Zatsiorsky V., YakuninN., Mechanics and Biomechanics of Rowing : TO review, International Newspaper of sport biomechanics, p229-281, 1991HARTMANN 93 : Hartmann U., Mader A.,Wasser K., Klauer I., Peak Forces,Velocity, and Power During Five and Maximal Ten Rowing Ergometer Strokesby World Class Female and Pain Rowers, Int J. Sport Med, Flight 14, Supl.1, p 42-545,199

Dirac and Klein-Gordon particles in one-dimensional periodic potentials

We evaluate the dispersion relation for massless fermions, described by the Dirac equation, and for zero-spin bosons, described by the Klein-Gordon equation, moving in two dimensions and in the presence of a one-dimensional periodic potential. For massless fermions the dispersion relation shows a zero gap for carriers with zero momentum in the direction parallel to the barriers in agreement with the well-known "Klein paradox". Numerical results for the energy spectrum and the density of states are presented. Those for fermions are appropriate to graphene in which carriers behave relativistically with the "light speed" replaced by the Fermi velocity. In addition, we evaluate the transmission through a finite number of barriers for fermions and zero-spin bosons and relate it with that through a superlattice.Comment: 9 pages, 12 figure

Does Land Degradation Increase Poverty in Developing Countries?

Additional data sets can be found at these links and in the supplemental file: http://www.edwardbbarbier.com/Projects/ELD/Economics_of_Land_Degradation_data_2000DAL.html, http://www.edwardbbarbier.com/Projects/ELD/Economics_of_Land_Degradation_data_2010DAL.html, http://www.edwardbbarbier.com/Projects/ELD/Economics_of_Land_Degradation_data_2000_to_2010DAL.htmlLand degradation is a global problem that particularly impacts the poor rural inhabitants of low and middle-income countries. We improve upon existing literature by estimating the extent of rural populations in 2000 and 2010 globally on degrading and improving agricultural land, taking into account the role of market access, and analyzing the resulting impacts on poverty. Using a variety of spatially referenced datasets, we estimate that 1.33 billion people worldwide in 2000 were located on degrading agricultural land (DAL), of which 1.26 billion were in developing countries. Almost all the world’s 200 million people on remote DAL were in developing countries, which is about 6% of their rural population. There were also 1.54 billion rural people on improving agricultural land (IAL), with 1.34 billion in developing countries. We find that a lower share of people in 2000 on DAL, or a higher share on IAL, lowers significantly how much overall economic growth reduces poverty from 2000 to 2012 across 83 developing countries. As the population on DAL and IAL in developing countries grew by 13% and 15% respectively from 2000 to 2010, these changing spatial distributions of rural populations could impact significantly future poverty in developing countries.ECU Open Access Publishing Support Fun

Normal ranges of 2D speckle tracking left ventricular segmental longitudinal, radial and circumferential maximum strain post-systolic time delays used for dyssynchrony analysis

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On the determination of Poisson's ratio of stressed monolayer and bilayer submicron thick films

In this paper, the bulge test is used to determine the mechanical properties of very thin dielectric membranes. Commonly, this experimental method permits to determine the residual stress (s0) and biaxial Young's modulus (E/(1-u)). Associating square and rectangular membranes with different length to width ratios, the Poisson's ratio (u) can also be determined. LPCVD Si3N4 monolayer and Si3N4/SiO2 bilayer membranes, with thicknesses down to 100 nm, have been characterized giving results in agreement with literature for Si3N4, E = 212 $\pm$ 14 GPa, s0 = 420 $\pm$ 8 and u = 0.29.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/16838