An Uqp(u2)U_{qp}(u_2) Rotor Model for Rotational Bands of Superdeformed Nuclei

A nonrigid rotor model is developed from the two-parameter quantum algebra $U_{qp}({\rm u}_2)$. [This model presents the $U_{qp}({\rm u}_2)$ symmetry and shall be referred to as the qp-rotor model.] A rotational energy formula as well as a qp-deformation of E2 reduced transition probabilities are derived. The qp-rotor model is applied (through fitting procedures) to twenty rotational bands of superdeformed nuclei in the $A \sim 130$, 150 and 190 mass regions. Systematic comparisons between the qp-rotor model and the q-rotor model of Raychev, Roussev and Smirnov, on one hand, and a basic three-parameter model, on the other hand, are performed on energy spectra, on dynamical moments of inertia and on B(E2) values. The physical signification of the deformation parameters q and p is discussed.Comment: 24 pages, Latex File, to appear in IJMP

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

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

EXPERIMENTAL DEVICE FOR THE ANALYSIS OF THE CYCLER'S MOVEMENT

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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

Circumstellar environment of RX Puppis

The symbiotic Mira, RX Pup, shows long-term variations in its mean light level due to variable obscuration by circumstellar dust. The last increase in extinction towards the Mira, between 1995 and 2000, has been accompanied by large changes in the degree of polarization in the optical and red spectral range. The lack of any obvious associated changes in the position angle may indicate the polarization variations are driven by changes in the properties of the dust grains (e.g. variable quantity of dust and variable particle size distribution, due to dust grain formation and growth) rather than changes in the viewing geometry of the scattering region(s), e.g. due to the binary rotation.Comment: Paper presented at Torun 2000 conference on Post-AGB objects as a phase of stellar evolution; 8 pages, 3 figure

The euro crisis: Where do we stand?

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