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.     

Transport studies in boron-ion implanted type II<Emphasis Type="Italic">a</Emphasis> diamond

We have investigated low-temperature electrical transport mechanisms in the surface layer of a type IIa diamond which has been heavily implanted with boron-ions at low temperatures and then annealed at high temperatures. The boron atoms occupy substitutional sites giving rise to a heavily doped wide-bandgap semiconductor. The dc-conductivity results suggest that for the maximum boron doping that has been achieved, the diamond sample is close to the insulator-metal transition. A model to account for the observed increase in activated boron centres with ion dose is presented. On the insulating side of the transition, the data are interpreted in terms of variable-range hopping laws.     

Magnetic structure of graphite ribbon

Anoble mechanism of spin polarization is proposed for finite graphite sheet with edge. For graphite ribbon with zigzag edge, there appear peculiar ‘edge states’. These localized states comprise nearly flat band at the Fermi level, which easily causes magnetic instability. Magnetic structure is suggested from Hartree-Fock analysis of the Hubbard model, where huge magnetic moments are induced at around both of edges by weak HubbardU and are coupled antiferromagnetically with each other.     

Solutions of logarithmic type for elliptic and hypoelliptic equations

LetK be an algebraically closed field withp:=char (K)>0 andX a bielliptic curve over Spec(K) with genus≥6 and such that the canonical model ofX is ordinary. In this paper we determine all the possible gap sequences of Weierstrass points on such curves.     

Λ-Shaped Policies to Schedule Deteriorating Jobs

We study a problem of scheduling deteriorating jobs, i.e. jobs whose processing times are an increasing function of their starting times. We consider the case of a single machine and linear job-independent deterioration. The objective is to minimize the sum of weighted completion times, with weights proportional to the basic processing times. The optimal schedule is shown to be Λ-shaped, i.e. the sequence of the basic processing times has a single local maximum. Moreover, we show that the problem is solved in O(N log N) time. In the last section we test heuristics for the case of general weights.     

Structural and Spectroscopic Characterization of Iron(III) Dioxoporphodimethene Complexes and Their Autoreduction to an Iron(II) Complex in Pyridine

Three iron complexes of the meso-dioxo derivative of octaethylporphryin (trans-H(2)OEPO(2)) were characterized by X-ray diffraction. Green ClFe(III)(trans-OEPO(2)).1.5C(6)H(6) crystallizes in the monoclinic space group P2(1)/c with a = 13.766(3) ?, b = 19.075(3) ?, c = 15.217(3) ?, beta = 99.87(2) degrees at 123 K with Z = 4. Refinement of 2712 reflections with F > 6.0sigma(F) and 223 parameters yielded R = 0.0624, R(w) = 0.0596. The iron complex contains a domed dioxoporphodimethene macrocyclic ligand. The observation of a five-coordinate iron(III) ion with an axial Fe-Cl distance of 2.232(2) ? and in-plane Fe-N distances averaging 2.082 ? is consistent with its high-spin (S = (5)/(2)) character. This monomer is readily converted to the green {Fe(III)(trans-OEPO(2))}(2)O using aqueous hydroxide. {Fe(III)(trans-OEPO(2))}(2)O crystallizes in the monoclinic space group C2/c with a = 23.541(8) ?, b = 15.392(5) ?, c = 18.686(8) ?, and beta = 90.09(3) degrees at 294 K with Z = 8. Refinement of 3472 reflections with F > 6.0sigma(F) and 393 parameters yielded R = 0.0484, R(w) = 0.0527. The complex possesses a crystallographically imposed 2-fold symmetry axis that passes through the oxo ligand. The dioxoporphodimethene ligands within the molecule are roof-shaped and fold away from each other. The axial Fe-O distance is 1.749(1) ? with longer in-plane Fe-N distances (average 2.077 ?). The Fe-O-Fe angle of 165.4(2) degrees deviates significantly from linearity and is more acute than related porphyrin complexes. Pyridine solutions of either the iron(III) monomer or &mgr;-oxo dimer autoreduce over a period of days to give (py)(2)Fe(II)(trans-OEPO(2)). This red compound crystallizes in the space group P2(1) with a = 19.177(4) ?, b = 20.039(4) ?, c = 10.547(2) ?, and beta = 100.36(3) degrees at 130 K with Z = 2. Refinement of 5090 reflections with one restraint and 984 parameters yielded R1 = 0.0684, wR2 = 0.1763. The complex crystallizes with two distinct molecules in the asymmetric unit; each molecule contains a different degree of disorder with respect to the trans meso oxygen atoms (50/50, 71/29). Each independent molecule exhibits severe ruffling of the macrocycle. The six coordinate iron(II) center is diamagnetic. The axial Fe-N(pyridine) distances average 1.98 ?, and the in-plane Fe-N(pyrrole) distances average 1.95 ?. A common trend observed for the dioxoporphodimethene macrocycle in all of these structures is an elongation toward the trans oxidized meso carbons. These complexes were originally prepared as cis and trans isomeric mixtures that can be enriched in the trans isomer by fractional crystallization. This is evident in their distinctive (1)H NMR spectra. In addition, these compounds are characterized by electron impact mass spectrometry and UV-visible, ESR, and infrared spectroscopies.     

Analysis of Open Discrete Time Queueing Networks: A Refined Decomposition Approach

This paper deals with approximate analysis methods for open queueing networks. External and internal flows from and to the nodes are characterized by renewal processes with discrete time distributions of their interarrival times. Stationary distributions of the waiting time, the queue size and the interdeparture times are obtained using efficient discrete time algorithms for single server (GI/G/1) and multi-server (GI/D/c) nodes with deterministic service. The network analysis is extended to semi-Markovian representations of each flow among the nodes, which include parameters of the autocorrelation function.