Surface order dependent magnetic thin film growth: Fe on GaN(0 0 0 1)

X-ray photoemission spectroscopy and X-ray magnetic circular dichroism have been used to study the growth process, chemical composition and magnetic character of iron deposited on ordered and disordered GaN(0 0 0 1) surfaces. On the (1 × 1) ordered surface the Fe grows uniformly but with disruption to the substrate surface, subsequently nitrogen desorbs from the surface, some of which diffuses into the Fe overlayer. The film is magnetically fractured, with high magnetic coercivity and broad switching fields. Conversely, the gallium rich disordered surface protects the underlying substrate from any disruption and initially induces non-uniform growth, the Fe clusters coalesce at ∼12 Å, to produce a uniform film with desirable magnetic characteristics. Films beyond this point (>12 Å) indicate sharp hysteresis loops with low coercivities. For the resultant film (36 Å) we measure a magnetic moment of 2.02 μ_B, in agreement with bulk bcc iron (2.068 μ_B.)     

Attractive potential around a thermionically emitting microparticle

We present a simulation study of the charging of a dust grain immersed in a plasma, considering the effect of thermionic electron emission from the grain. It is shown that the orbit motion limited theory is no longer reliable when electron emission becomes large: screening can no longer be treated within the Debye-Huckel approach and an attractive potential well can form, leading to the possibility of attractive forces on other grains with the same polarity. We suggest to perform laboratory experiments where emitting dust grains could be used to create nonconventional dust crystals or macromolecules.     

Small clones and the projection property

In 1986, the second author classified the minimal clones on a finite universe into five types. We extend this classification to infinite universes and to multiclones. We show that every non-trivial clone contains a “small” clone of one of the five types. From it we deduce, in part, an earlier result, namely that if ${\mathcal{C}}In 1986, the second author classified the minimal clones on a finite universe into five types. We extend this classification to infinite universes and to multiclones. We show that every non-trivial clone contains a “small” clone of one of the five types. From it we deduce, in part, an earlier result, namely that if C{\mathcal{C}} is a clone on a universe A with at least two elements that contains all constant operations, then all binary idempotent operations are projections and some m-ary idempotent operation is not a projection for some m ≥ 3 if and only if there is a boolean group G on A for which C{\mathcal{C}} is the set of all operations f(x ₁, . . . , x _n ) of the form a + ?_{i ? I}x_ia + {\sum_{i \in I}x_{i}} for a ? A{a \in A} and I í {1, . . . ,n}{I \subseteq \{1,\,.\,.\,.\,,n\}}.     

On solving a primal geometric program by partial dual optimization

The several published methods for mapping a dual solution estimate to a primal solution estimate in posynomial geometric programming provide no criteria for deciding how much deviation from primal feasibility, or discrepancy between the primal and dual objective function values, should be permitted before the primal solution estimate is accepted by the designer. This paper presents a new and simple dual-to-primal conversion method that uses the cost coefficients to provide a sound economic criterion for determining when to accept a primal solution estimate. The primal solution estimate generated is the exact solution to a modified primal obtained from the given primal by modifying the cost coefficients, with the exponent matrix left unchanged. The method is shown to have desirable properties when coupled with a convergent dual algorithm.     

Telecommunications Network Case Study: Selecting a Data Network Architecture

This paper documents a model that was pivotal in deciding which of two architectures should be selected for a frame relay data communications network. The choices are either to continue using the current architecture, or to make a large incremental investment in new equipment which reduces the number of high speed inter-office trunks required to interconnect the switches. The analysis requires optimizing the mix of two types of customer port cards to determine the maximum customer port capacity of a switch. Simple approximations are used to estimate the number of inter-office trunks and trunk cards required. Based in large part on the costs computed by this model, an executive level decision was made to move to the new architecture.     

Two properties of fuzzy subquasigroups of a quasigroup

In a recent paper, Biswas (1996) studies two properties of fuzzy subrings of a ring. We extend them to quasigroups and to a certain generalization thereof. In its most basic form the first property is that a fuzzy subquasigroup R of a quasigroup satisfies R(a·a′)=min(R(a),R(a′)) whenever R(a)≠R(a′). The second property relates the cardinalities of level sets of R.     

Improved bounds for queues with delayed arrivals

Improved bounds are developed for a queue where arrivals are delayed by a fixed time. For moderate to heavy traffic, a simple improved upper bound is obtained which only uses the first two moments of the service time distribution. We show that our approach can be extended to obtain bounds for other types of delayed arrival queues. For very light traffic, asymptotically tight bounds can be obtained using more information about the service time distribution. While an improved upper bound can be obtained for light to moderate traffic it is not particularly easy to apply. This revised version was published online in June 2006 with corrections to the Cover Date.