Estimation of the exchange constants in the Fe sublattice of the Y2Fe17 and Y2Fe17N3−δ compounds from the analysis of Mössbauer measurements

From the analysis of Mössbauer data for Y₂Fe₁₇ and Y₂Fe₁₇N_3−δ at various temperatures the hyperfine fields for 4f, 6g, 12j, 12k iron sites were estimated as a function of temperature. The reduced magnetizations calculated from the values of the hyperfine fields are fitted with a mean field model for four interacting sublattices using a computer program. The estimated exchange interaction from the fitting procedure between the 4f sites is found strongly negative (antiferromagnetic) in Y₂Fe₁₇ whereas in Y₂Fe₁₇N_3−δ it increases and becomes weak negative following a modified Slater-Néel curve. The rest of the exchange interactions are found positive or weak negative depending on the distances between the Fe atoms.     

Television by the bit

The various types of advanced television (ATV) are defined, and the most advanced type, high-definition TV (HDTV), is discussed. The present status of HDTV development in the US, Japan, and Europe is examined. Signal processing requirements for HDTV are briefly considered, and the benefits of and prospects for all-digital HDTV are explored. Video compression techniques, implementation issues, and the future of HDTV are also discussed     

Phonon growth in a many valley model semiconductor at low lattice temperatures

It is shown how the traditional method of neglecting the energy of acoustic phonons and approximating their distribution by the equipartition law leads to significant errors in the phonon growth rate in a many-valley model semiconductor when the lattice temperature is low.     

Trefftz,collocation, and other boundary methods—A comparison

In this article we survey the Trefftz method (TM), the collocation method (CM), and the collocation Trefftz method (CTM). We also review the coupling techniques for the interzonal conditions, which include the indirect Trefftz method, the original Trefftz method, the penalty plus hybrid Trefftz method, and the direct Trefftz method. Other boundary methods are also briefly described. Key issues in these algorithms, including the error analysis, are addressed. New numerical results are reported. Comparisons among TMs and other numerical methods are made. It is concluded that the CTM is the simplest algorithm and provides the most accurate solution with the best numerical stability. © 2006 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007     

Semidefinite optimization models for limit and shakedown analysis problems involving matrix spreads

Limit and shakedown analysis problems of Computational Mechanics lead to convex optimization problems, characterized by linear objective functions, linear equality constraints and constraints expressing the restrictions imposed by the material strength. It is shown that two important strength criteria, the Mohr–Coulomb and the Tresca criterion, can be represented as systems of semidefinite constraints, leading this way to semidefinite programming problems.     

High bit rate digital subscriber line echo cancellation

The authors explore the issues of complexity, precision, and dynamic range and practical implementation issues such as nonlinearities in high bit rate digital subscriber line (HDSL) echo cancelers. The approach taken is to study these issues analytically using computer simulations, and then to verify simulation results with laboratory prototyping. One of the objectives was to determine whether HDSL echo cancellation will be practical and cost effective within the next several years, consistent with the desire to develop and deploy a repeaterless T1 capability by 1993. Thus, attention is given to practical design issues     

A VLSI priority packet queue with inheritance and overwrite

Reliable priority-based flow-control is essential for real-time applications involving hard-deadlines. However, the use of first-in-first-out (FIFO) queues in such systems introduces priority inversion resulting in unbounded transmission delays. For this reason, a priority transmission queue is critical for multimedia and mission-critical systems. Yet very few priority queue implementations exist. This paper presents the design of a novel VLSI priority packet queue (PPQ), implemented in 1.2 μm CMOS technology. It achieves fast operation by manipulating its contents in terms of packet segments, rather than individual words. Similar to paged memory, this new segmented architecture greatly reduces implementation cost by reusing segments and avoiding storage area fragmentation. By distributing the computationally intensive priority comparison operation over the access time for an entire segment, the PPQ achieves 96% of the speed of a high-speed packet FIFO. The PPQ can either perform priority inheritance or overwrite lower priority packets to minimize the impact of queue overflow. In addition, it is suitable as a general computer network interface buffer, since it robustly handles asynchronous read and write clocks of greatly disparate frequencies. Our initial implementation achieves well over twice the speed of a nonpipelined approach with minimal additional overhead. Furthermore, scaling this design to larger capacities and more priority levels results in an even greater improvement in speed over conventional approaches     

Laser nanosources based on planar photonic crystals as new platforms for nanophotonic devices

Two-dimensional photonic crystal lasers have been fabricated on III–V semiconductor slabs. Tuning of the spontaneous emission in micro and nanocavities has been achieved by accurate control of the slab thickness. Different structures, some of them of new application to photonic crystal lasers, have been fabricated like the Suzuki-phase or the coupled-cavity ring-like resonators. Laser emission has been obtained by pulsed optical pumping. Optical characterization of the lasing modes have been performed showing one or more laser peaks centred around 1.55 μm. Far field characterization of the emission pattern has been realized showing different patterns depending on the geometrical shape of the structures. These kinds of devices may be used as efficient nanolaser sources for optical communications or optical sensors.     

Synthesis and thermal behaviour of salicylaldimine-based liquid crystalline symmetrical dimers

A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C₈) while the number of methylene units in the central spacer was varied from C₃ to C₁₁. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dimers containing 4, 6 and 8 methylene units in the central spacer, a unique filament growth pattern has been observed in the smectic A phase while cooling from the isotropic phase. The dimers containing of C₉ to C₁₁ methylene groups exhibit the nematic phase in addition to smectic modifications. This observation indicates that when the terminal chains are shorter than the spacer, the tendency to form smectic phases is not fully extinguished but is perhaps reduced.