Clumping Data in 2-Way Tables: A User-Orientated Perspective

Following a critique of existing algorithms, an algorithm is presented which will re-organize a 2-way data table to bring like rows together, and like columns together. Extensions of the method are described, and justified, to accommodate distances measured in modular arithmetic, and with bipolar columns/rows, as in repertory grid analysis. One value of the algorithms is that the user can see relationships in the tables without the data in the cells themselves ever having been transformed. Thus, users will continue to feel they own their data.     

Refining the Diffusion Approximation for the <Emphasis Type="Italic">GI</Emphasis><Superscript><Emphasis Type="Italic">x</Emphasis></Superscript><Emphasis Type="Italic">/G/c</Emphasis> Queue

This paper deals with the GI ^x /G/c queueing system in a steady state. We refine a diffusion approximation method incorporating the constraint of traffic conservation for general queueing systems. An approximate expression for the distribution of the number of customers is obtained. Numerical results are presented to show that the refined model provides improved performance.     

Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

Anisotropic<Emphasis Type="Italic">s</Emphasis>-wave or<Emphasis Type="Italic">d</Emphasis>-wave pairing? Analysis of experiments on ion irradiation of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-x</Subscript> films

The effect of ion irradiation on the superconducting transition temperatureT _c and resistivityρ _ab (T) of YBa₂Cu₃O_7-x films with different oxygen content (initial temperatureT _c0≈90 K and 60 K) is studied experimentally. The dependenciesT _c /T _c0 on residual resistivityρ _o are obtained in very wide range 0.2<T _c /T _c0 <1 andρ _o μΩ·cm. The critical values ofρ _o , corresponding to the vanishing of superconductivity, are found to be an order of magnitude larger then those predicted by theory ford-wave pairing. At 0.5÷0.6<T _c /T _c0<1 the experimental data are in close agreement with theoretical dependencies, obtained for the anisotropics-wave superconductor within the BCS-framework.     

Investigations of the interlayer coupling in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> cuprates

Low-field magnetizationM(H) measurements can be used to probe the nature of the screening currents and the interlayer coupling in high-T _c cuprates. Here we compare theM(H) behaviour of single crystals of Bi₂Sr₂CaCu₂O₈ and fully oxygenated and oxygen reduced YBa₂Cu₃O_7??. In YBa₂Cu₃O₇, theM(H) behaviour is consistent with anisotropic 3D superconductivity whilst in Bi₂Sr₂CaCu₂O₈, the surface screening currents are strongly affected by the presence of vortices, implying that the CuO₂ planes are coupled via a weak Josephson interaction. In oxygen-deficient YBa₂Cu₃O_6.7 (T _c =63K), theM(H) behaviour at low temperatures is similar to that found for Bi₂Sr₂CaCu₂O₈, implying that the removal of oxygen from the chains has resulted in a dimensional crossover of the superconducting state in YBa₂Cu₃O_7??. As the temperature approachesT _c , the 3D behaviour is eventually restored as thec-axis coherence length ξ _c becomes comparable with the interlayer spacingd.