5,545 research outputs found
An Rotor Model for Rotational Bands of Superdeformed Nuclei
A nonrigid rotor model is developed from the two-parameter quantum algebra
. [This model presents the 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 , 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
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
HIV-1 Evolutionary Patterns Associated with Metastatic Kaposi's Sarcoma during AIDS.
Kaposi's sarcoma (KS) in HIV-infected individuals can have a wide range of clinical outcomes, from indolent skin tumors to a life-threatening visceral cancer. KS tumors contain endothelial-related cells and inflammatory cells that may be HIV-infected. In this study we tested if HIV evolutionary patterns distinguish KS tumor relatedness and progression. Multisite autopsies from participants who died from HIV-AIDS with KS prior to the availability of antiretroviral therapy were identified at the AIDS and Cancer Specimen Resource (ACSR). Two patients (KS1 and KS2) died predominantly from non-KS-associated disease and KS3 died due to aggressive and metastatic KS within one month of diagnosis. Skin and visceral tumor and nontumor autopsy tissues were obtained (n = 12). Single genome sequencing was used to amplify HIV RNA and DNA, which was present in all tumors. Independent HIV tumor clades in phylogenies differentiated KS1 and KS2 from KS3, whose sequences were interrelated by both phylogeny and selection. HIV compartmentalization was confirmed in KS1 and KS2 tumors; however, in KS3, no compartmentalization was observed among sampled tissues. While the sample size is small, the HIV evolutionary patterns observed in all patients suggest an interplay between tumor cells and HIV-infected cells which provides a selective advantage and could promote KS progression
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