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

Approximate Message-Passing Decoder and Capacity Achieving Sparse Superposition Codes

We study the approximate message-passing decoder for sparse superposition coding on the additive white Gaussian noise channel and extend our preliminary work [1]. We use heuristic statistical-physics-based tools such as the cavity and the replica methods for the statistical analysis of the scheme. While superposition codes asymptotically reach the Shannon capacity, we show that our iterative decoder is limited by a phase transition similar to the one that happens in Low Density Parity check codes. We consider two solutions to this problem, that both allow to reach the Shannon capacity: i) a power allocation strategy and ii) the use of spatial coupling, a novelty for these codes that appears to be promising. We present in particular simulations suggesting that spatial coupling is more robust and allows for better reconstruction at finite code lengths. Finally, we show empirically that the use of a fast Hadamard-based operator allows for an efficient reconstruction, both in terms of computational time and memory, and the ability to deal with very large messages.Comment: 40 pages, 18 figure

EXPERIMENTAL DEVICE FOR THE ANALYSIS OF THE CYCLER'S MOVEMENT

Ni

EXPERIMENTAL DEVICE FOR THE ANALYSIS OF THE CYCLER'S MOVEMENT

INTRODUCTION This study deals with the characterization of the cycler's movement in its kinematic and dynamic aspects. The article presents the instrumentation of the cycle to characterize the differences between the clipless fixed and the clipless float pedals. METHODS A bicycle used in cycling sport competition and a home trainer constituted the experimental devices. The bicycle is fixed by its front fork and this device allows to simulate the seated position of a cyclist on the road. On the one hand, the system SAGA3 equipped with 4 CCD 50 Hz cameras allows the acquisition of kinematic variables. Cameras are placed according to the frontal and sagittal plane. The 2 cameras placed in the frontal plane allow the idenfication of known positions of markers on the lateral and medial femoral epicondyle and the lateral and medial malleolus. These 2 cameras contributed to increase the accuracy of the determination of articulate centers (Fig l).The accuracy is of 1.9% for distances and 1.82 for angles. On the other hand, a mini platform is used to acquire pedal loads (Fig 2). Its measurement range was 200 daN for Fz, 50 daN for Fx and Fy, 6 daN*m for Mz and 5 daN*m for Mx and My. The theorical accuracy is 1 % of the measurement range for the different components. To locate this force plate in the global reference system Ro(xo,yo,zo), a tripod was used and fixed under the pedal (Fig 1). In addition, a goniometer was used to determine the position of the crank arm at each moment. Experimental data were collected from the right leg of 1 male subject during seated cycling at 90 rpm and 200 W at a sampling rate of 50 Hz for 30 s. Prior data collection, subjects cycled at least 15 min at 85 rpm and a reduced power level of about 120 W to familiarize themselves with each pedals design. The data were averaged over 40 cycles. Fig 1: The cycle instrumented Fig 2: The adiptation of the Fig 3: The effective transmission mini platform at the pedal from the shoe to the pedal RESULTS AND CONCLUSION From a qualitative point of view, it is often said that in cycling the lateral movement of the clipless float pedals reduces the effective transmission of force from the shoe to the pedal. This torque is a function of the crank angle, of the forces Fx, Fz as stated in the Ro (xo,yo,zo) reference system. The two curves (Fig 3) are quite similar, which means that there is no effective loss of mechanical torque transmission from the shoe to the pedal when the floating pedal systems are used. This fact emphasizes Wheeler 's results (1995). In addition, for the clipless float pedals, the applied Mz moment at the pedal is weaker. This result is an indicator for the reduction of the axial moment realized at the knee (Ruby, 1992). This article describes the instrumentation to measure with optoelectronics systems and force platforms the cycler's movement. The experimental device was test with 1 male subject. With this subject, clipless float pedals reduce the applied Mz moment at the pedal without compromising power transmitted to the bike. This study is supported by DECATHLON and la Region Nord Pas de Calais - France

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

Rainbow Vectors for Broad-Range Bacterial Fluorescence Labeling

Since their discovery, fluorescent proteins have been widely used to study protein function, localization or interaction, promoter activity and regulation, drug discovery or for non-invasive imaging. They have been extensively modified to improve brightness, stability, and oligomerization state. However, only a few studies have focused on understanding the dynamics of fluorescent proteins expression in bacteria. In this work, we developed a set plasmids encoding 12 fluorescent proteins for bacterial labeling to facilitate the study of pathogen-host interactions. These broad-spectrum plasmids can be used with a wide variety of Gram-negative microorganisms including Escherichia coli, Pseudomonas aeruginosa, Burkholderia cepacia, Bordetella bronchiseptica, Shigella flexneri or Klebsiella pneumoniae. For comparison, fluorescent protein expression and physical characteristics in Escherichia coli were analyzed using fluorescence microscopy, flow cytometry and in vivo imaging. Fluorescent proteins derived from the Aequorea Victoria family showed high photobleaching, while proteins form the Discosoma sp. and the Fungia coccina family were more photostable for microscopy applications. Only E2-Crimson, mCherry and mKeima were successfully detected for in vivo applications. Overall, E2-Crimson was the fastest maturing protein tested in E. coli with the best overall performance in the study parameters. This study provides a unified comparison and comprehensive characterization of fluorescent protein photostability, maturation and toxicity, and offers general recommendations on the optimal fluorescent proteins for in vitro and in vivo applications