Fundamental Limits of Low-Density Spreading NOMA with Fading

Spectral efficiency of low-density spreading non-orthogonal multiple access channels in the presence of fading is derived for linear detection with independent decoding as well as optimum decoding. The large system limit, where both the number of users and number of signal dimensions grow with fixed ratio, called load, is considered. In the case of optimum decoding, it is found that low-density spreading underperforms dense spreading for all loads. Conversely, linear detection is characterized by different behaviors in the underloaded vs. overloaded regimes. In particular, it is shown that spectral efficiency changes smoothly as load increases. However, in the overloaded regime, the spectral efficiency of low- density spreading is higher than that of dense spreading

Are the Perseus-Pisces chain and the Pavo-Indus wall connected?

A significant empty region was found between the southern Pavo- Indus (PI) wall and the northern Perseus-Pisces (PP) chain. This survey tests the reality of this void which may simply reflect previous poor sampling of the galaxies in this region. Redshifts for a magnitude selected sample of 379 galaxies were obtained covering the four UKST/SERC survey fields with Bt <= 17.0. All redshifts were obtained with the FLAIR multi-object spectroscopy system on the 1.2 m U.K. Schmidt Telescope at Siding Spring, Australia. Two highly significant density enhancements were found in the galaxy distribution at 133 Mpc and 200 Mpc (Ho=75 km/s/Mpc). We claim that no connexion exists between PP and PI. However, a southern extension of PP was detected and makes the total length of this chain of more than 150 Mpc.Comment: 14 pages, postscript including tables and figures

Field emission from single multi-wall carbon nanotubes

Electron field emission characteristics of individual multiwalled carbon nanotubes have been investigated by a piezoelectric nanomanipulation system operating inside a scanning electron microscopy chamber. The experimental setup ensures a high control capability on the geometric parameters of the field emission system (CNT length, diameter and anode-cathode distance). For several multiwalled carbon nanotubes, reproducible and quite stable emission current behaviour has been obtained with a dependence on the applied voltage well described by a series resistance modified Fowler-Nordheim model. A turn-on field of about 30 V/um and a field enhancement factor of around 100 at a cathode-anode distance of the order of 1 um have been evaluated. Finally, the effect of selective electron beam irradiation on the nanotube field emission capabilities has been extensively investigated.Comment: 16 pages, 5 figure

The Entanglement Entropy of Solvable Lattice Models

We consider the spin k/2 analogue of the XXZ quantum spin chain. We compute the entanglement entropy S associated with splitting the infinite chain into two semi-infinite pieces. In the scaling limit, we find S ~ c_k/6 (ln(xi))+ln(g)+... . Here xi is the correlation length and c_k=3k/(k+2) is the central charge associated with the sl_2 WZW model at level k. ln(g) is the boundary entropy of the WZW model. Our result extends previous observations and suggests that this is a simple and perhaps rather general way both of extracting the central charge of the ultraviolet CFT associated with the scaling limit of a solvable lattice model, and of matching lattice and CFT boundary conditions.Comment: 6 pages; connection with boundary entropy of Affleck and Ludwig added in revised version and notation slightly change

Light Quasiparticles Dominate Electronic Transport in Molecular Crystal Field-Effect Transistors

We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic field-effect transistors based on rubrene single crystals. Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m* comparable to free electron mass. Furthermore, the m* values inferred from our experiments are in agreement with those determined from band structure calculations. These findings reveal no evidence for prominent polaronic effects, which is at variance with the common beliefs of polaron formation in molecular solids.Comment: 4 pages, 4 figure

On the Performance Prediction of BLAS-based Tensor Contractions

Tensor operations are surging as the computational building blocks for a variety of scientific simulations and the development of high-performance kernels for such operations is known to be a challenging task. While for operations on one- and two-dimensional tensors there exist standardized interfaces and highly-optimized libraries (BLAS), for higher dimensional tensors neither standards nor highly-tuned implementations exist yet. In this paper, we consider contractions between two tensors of arbitrary dimensionality and take on the challenge of generating high-performance implementations by resorting to sequences of BLAS kernels. The approach consists in breaking the contraction down into operations that only involve matrices or vectors. Since in general there are many alternative ways of decomposing a contraction, we are able to methodically derive a large family of algorithms. The main contribution of this paper is a systematic methodology to accurately identify the fastest algorithms in the bunch, without executing them. The goal is instead accomplished with the help of a set of cache-aware micro-benchmarks for the underlying BLAS kernels. The predictions we construct from such benchmarks allow us to reliably single out the best-performing algorithms in a tiny fraction of the time taken by the direct execution of the algorithms.Comment: Submitted to PMBS1

Spatially embedded random networks

Many real-world networks analyzed in modern network theory have a natural spatial element; e.g., the Internet, social networks, neural networks, etc. Yet, aside from a comparatively small number of somewhat specialized and domain-specific studies, the spatial element is mostly ignored and, in particular, its relation to network structure disregarded. In this paper we introduce a model framework to analyze the mediation of network structure by spatial embedding; specifically, we model connectivity as dependent on the distance between network nodes. Our spatially embedded random networks construction is not primarily intended as an accurate model of any specific class of real-world networks, but rather to gain intuition for the effects of spatial embedding on network structure; nevertheless we are able to demonstrate, in a quite general setting, some constraints of spatial embedding on connectivity such as the effects of spatial symmetry, conditions for scale free degree distributions and the existence of small-world spatial networks. We also derive some standard structural statistics for spatially embedded networks and illustrate the application of our model framework with concrete examples

H_2D^+ in the High-mass Star-forming Region Cygnus X

H_2D^+ is a primary ion that dominates the gas-phase chemistry of cold dense gas. Therefore, it is hailed as a unique tool in probing the earliest, prestellar phase of star formation. Observationally, its abundance and distribution is, however, just beginning to be understood in low-mass prestellar and cluster-forming cores. In high-mass star-forming regions, H_2D^+ has been detected only in two cores, and its spatial distribution remains unknown. Here, we present the first map of the ortho-H_2D^+J_(k^+,k^-) = 1_(1,0) → 1_(1,1) and N_2H^+ 4-3 transition in the DR21 filament of Cygnus X with the James Clerk Maxwell Telescope, and N_2D^+ 3-2 and dust continuum with the Submillimeter Array. We have discovered five very extended (≤34, 000 AU diameter) weak structures in H2D+ in the vicinity of, but distinctly offset from, embedded protostars. More surprisingly, the H_2D^+ peak is not associated with either a dust continuum or N_2D^+ peak. We have therefore uncovered extended massive cold dense gas that was undetected with previous molecular line and dust continuum surveys of the region. This work also shows that our picture of the structure of cores is too simplistic for cluster-forming cores and needs to be refined: neither dust continuum with existing capabilities nor emission in tracers like N_2D^+ can provide a complete census of the total prestellar gas in such regions. Sensitive H_2D^+ mapping of the entire DR21 filament is likely to discover more of such cold quiescent gas reservoirs in an otherwise active high-mass star-forming region