Optic neuritis - more than a loss of vision

Optic neuritis (ON) is the presence of an acute inflammation of the optic nerve that results in painful loss of vision. It is the most commonly encountered optic neuropathy in general practice, and is often associated with multiple sclerosis (MS). Studies show that in about 15–20% of MS cases, ON was the presenting symptom and more than half of people with MS experience at least one episode of ON during their disease. The risk of developing MS can be stratified by appropriate imaging investigations at the diagnosis of ON. Therefore, early recognition is important to ensure timely referral, investigation and treatment; prompt treatment can hasten visual recovery

Electron Spin Resonance In Argon-ion-implanted Silicon

A new paramagnetic center with g = 2.0029 is observed, in both n- and p-type silicon after they are heavily implanted (higher than 1017 ions/cm2) with 150-keV argon ions. © 1973 American Institute of Physics

2-[N-(4-{4-[(E)-(2-Hydroxybenzyl­idene)amino]phenoxy}phenyl)carbox­imidoyl]phenol

The mol­ecular structure of the title Schiff base compound, C26H20N2O3, shows that respective methyl­idene residues are almost coplanar with the adjacent terminal benzene ring owing to the presence of intra­molecular O—H⋯N hydrogen bonds [the N—C—C—C torsion angles are −6.6 (7) and −6.7 (7)°]. However, twists are exhibited about each methyl­idene and respective benzene ring connected to the central O atom; the dihedral angles formed between the two inner and two outer benzene rings are 54.6 (2) and 45.6 (3)°, respectively. The conformation about each of the C=N bonds [1.285 (5) and 1.295 (5) Å] is E. In the crystal, extensive C—H⋯π contacts involving all benzene rings results in the formation of layers in the ac plane

Implications of the ALEPH tau-Lepton Decay Data for Perturbative and Non-Perturbative QCD

We use ALEPH data on hadronic $\tau$ decays in order to calculate Euclidean coordinate space correlation functions in the vector and axial-vector channels. The linear combination $V-A$ receives no perturbative contribution and is quantitatively reproduced by the instanton liquid model. In the case of $V+A$ the instanton calculation is in good agreement with the data once perturbative corrections are included. These corrections clearly show the evolution of $\alpha_s$. We also analyze the range of validity of the Operator Product Expansion (OPE). In the $V-A$ channel we find a dimension $d=6$ contribution which is comparable to the original SVZ estimate, but the instanton model provides a different non-singular term of the same magnitude. In the $V+A$ case both the OPE and the instanton model predict the same $d=4$ power correction induced by the gluon condensate, but it is masked by much larger perturbative contributions. We conclude that the range of validity of the OPE is limited to x\lsim0.3 fm, whereas the instanton model describes the data over the entire range.Comment: 4 pages, 6 figure

A Mean Field Approach To The Instanton-Induced Effect Close To The QCD Phase Transition

In the instanton models the chiral phase transition is driven by a transition from random instanton-antiinstanton liquid and correlated instanton-antiinstanton molecules. So far this phenomenon was studied by numerical simulations, while we develop alternative semi-analytic approach. For two massless quark flavors, both instantons and ``molecules" generate specific 4-fermion effective interactions. After those are derived, we determine the temperature dependence of the thermodynamic quantities, the quark condensate and the fraction of molecules using standard mean field method. Using Bethe-Salpeter equation, we calculate T-dependence of mesonic correlation functions.Comment: 26 pages, LaTeX, 6 postscript files of 6 figures in additio

Steric control of supramolecular association in structures of Zn(S2COR)2 with N,N′-bis(pyridin-4-ylmethyl)oxalamide

The crystal and molecular structures of the one-dimensional coordination polymer [Zn(S2COEt)2(4LH2)]n (1) and binuclear [Zn(S2COCy)2]2(4LH2) (2) are described, where 4LH2 is N,N'-bis(pyridin4-ylmethyl)ethanediamide. In 1, the Zn(S2COEt)2 entities are linked by bidentate bridging 4 LH2 ligands through the pyridyl-N atoms to generate a twisted supramolecular chain. As a result of monodentate xanthate ligands, the N2S4 donor set defines a distorted tetrahedral coordination geometry and, crucially, allows the participation of the non-coordinating sulphur atoms in supramolecular association. Thus, in the crystal amide-N‒ H…O(amide) and amide-N‒H…S(thione) hydrogen bonds link chains into a three-dimensional architecture. The substitution of the ethyl group in the xanthate ligand with a cyclohexyl group results in very different structural outcomes. In 2, a binuclear molecule is observed with the coordination geometry for zinc being defined by chelating xanthate ligands and a pyridyl-N atom with the NS4 donor set defining a highly distorted geometry. In the molecular packing, amide-N‒H…S(thione) hydrogen bonds stabilise a supramolecular chain along the a-axis and these are connected into a three-dimensional arrangement by methylene-C‒H…O and methylene-C‒H…π(pyridyl) interactions. The relative importance of the specified intermolecular interactions and weaker, contributing contacts has been revealed by an analysis of the Hirshfeld surfaces of 1 and 2

The Case for Dynamic Models of Learners' Ontologies in Physics

In a series of well-known papers, Chi and Slotta (Chi, 1992; Chi & Slotta, 1993; Chi, Slotta & de Leeuw, 1994; Slotta, Chi & Joram, 1995; Chi, 2005; Slotta & Chi, 2006) have contended that a reason for students' difficulties in learning physics is that they think about concepts as things rather than as processes, and that there is a significant barrier between these two ontological categories. We contest this view, arguing that expert and novice reasoning often and productively traverses ontological categories. We cite examples from everyday, classroom, and professional contexts to illustrate this. We agree with Chi and Slotta that instruction should attend to learners' ontologies; but we find these ontologies are better understood as dynamic and context-dependent, rather than as static constraints. To promote one ontological description in physics instruction, as suggested by Slotta and Chi, could undermine novices' access to productive cognitive resources they bring to their studies and inhibit their transition to the dynamic ontological flexibility required of experts.Comment: The Journal of the Learning Sciences (In Press

Evidence Against Instanton Dominance of Topological Charge Fluctuations in QCD

The low-lying eigenmodes of the Dirac operator associated with typical gauge field configurations in QCD encode, among other low-energy properties, the physics behind the solution to the $U_A(1)$ problem (i.e. the origin of the $\eta'$ mass), the nature of spontaneous chiral symmetry breaking, and the physics of string-breaking, quark-antiquark pair production, and the OZI rule. Moreover, the space-time chiral structure of these eigenmodes reflects the space-time topological structure of the underlying gauge field. We present evidence from lattice QCD on the local chiral structure of low Dirac eigenmodes leading to the conclusion that topological charge fluctuations of the QCD vacuum are not instanton-dominated. The result supports Witten's arguments that topological charge is produced by confinement-related gauge fluctuations rather than instantons.Comment: 35 pages, 11 figure

meso-1-{[2-(Propyl-1-sulfin­yl)eth­yl]sulfin­yl}propane

The title mol­ecule, C8H18O2S2, is disposed about a centre of inversion implying an anti-disposition of the sulfinyl-O atoms; the terminal n-propyl group has an extended conformation. The crystal packing is dominated by C—H⋯O inter­actions, which lead to the formation of supra­molecular arrays in the bc plane