The U.S.D.A. Veterinarian and Public Health

This discussion of the role of the veterinarian in agriculture in relation to public health is limited to the activities of veterinarians in the Department of Agriculture and for the most part, deals only with the work of those veterinarians who are in the Bureau of Animal Industry of the Department. The success of our work does, of course, depend greatly on the cooperation and the activities of veterinary colleges, experiment stations, state and city health departments, livestock sanitary officials, other scientists in the Department, other Federal agencies such as the Department of Defense, Public Health Service and Food and Drug Administration of the Federal Security Agency, and last but not least, the practitioner of veterinary medicine

Recurrent difficulties: solving quantitative problems

Investigating the process students use to solve quantitative problems using a think aloud strategy

Teaching and learning problem solving in science. Part II: Learning problem solving in a thermodynamics course

This paper examines important new aspects and results related to an experimental new course

Unexpected non-Wigner behavior in level-spacing distributions of next-nearest-neighbor coupled XXZ spin chains

The level-spacing distributions of XXZ spin chains with next-nearest-neighbor couplings are studied under periodic boundary conditions. We confirm that integrable XXZ spin chains mostly have the Poisson distribution as expected. On the contrary, the level-spacing distributions of next-nearest-neighbor coupled XXZ chains are given by non-Wigner distributions. It is against the expectations, since the models are nonintegrable.Comment: 4 pages, 4 figures, to be published in Physical Review

Fano Effect through Parallel-coupled Double Coulomb Islands

By means of the non-equilibrium Green function and equation of motion method, the electronic transport is theoretically studied through a parallel-coupled double quantum dots(DQD) in the presence of the on-dot Coulomb correlation, with an emphasis put on the quantum interference. It has been found that in the Coulomb blockage regime, the quantum interference between the bonding and antiboding DQD states or that between their Coulomb blockade counterparts may result in the Fano resonance in the conductance spectra, and the Fano peak doublet may be observed under certain non-equilibrium condition. The possibility of manipulating the Fano lineshape is predicted by tuning the dot-lead coupling and magnetic flux threading the ring connecting the dots and leads. Similar to the case without Coulomb interaction, the direction of the asymmetric tail of Fano lineshape can be flipped by the external field. Most importantly, by tuning the magnetic flux, the function of four relevant states can be interchanged, giving rise to the swap effect, which might play a key role as a qubit in the quantum computation.Comment: 7 pages, 5 figure

Resonant Tunneling through Multi-Level and Double Quantum Dots

We study resonant tunneling through quantum-dot systems in the presence of strong Coulomb repulsion and coupling to the metallic leads. Motivated by recent experiments we concentrate on (i) a single dot with two energy levels and (ii) a double dot with one level in each dot. Each level is twofold spin-degenerate. Depending on the level spacing these systems are physical realizations of different Kondo-type models. Using a real-time diagrammatic formulation we evaluate the spectral density and the non-linear conductance. The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure

Evidence for the involvement of tyrosine-69 in the control of stereospecificity of porcine pancreatic phospholipase A2

We have studied the role of Tyr-69 of porcine pancreatic phospholipase A2 in catalysis and substrate binding, using site-directed mutagenesis. A mutant was constructed containing Phe at position 69. Kinetic characterization revealed that the Phe-69 mutant has retained enzymatic activity on monomeric and micellar substrates, and that the mutation has only minor effects on kcat and Km. This shows that Tyr-69 plays no role in the true catalytic events during substrate hydrolysis. In contrast, the mutation has a profound influence on the stereospecificity of the enzyme. Whereas the wild-type phospholipase A2 is only able to catalyse the degradation of sn-3 phospholipids, the Phe-69 mutant hydrolyses both the sn-3 isomers and, at a low (1-2%) rate, the sn-1 isomers. Despite the fact that the stereospecificity of the mutant phospholipase has been altered, Phe-69 phospholipase still requires Ca2+ ions as a cofactor and also retains its specificity for the sn-2 ester bond. Our data suggest that in porcine pancreatic phospholipase A2 the hydroxyl group of Tyr-69 serves to fix and orient the phosphate group of phospholipid monomers by hydrogen bonding. Because no such interaction can occur between the Phe-69 side-chain and the phosphate moiety of the substrate monomer, the mutant enzyme loses part of its stereospecificity but not its positional specificity

Allosuppressor and allohelper T cells in acute and chronic graft-vs-host disease. V. F1 mice with secondary chronic GVHD contain F1-reactive allohelper but no allosuppressor T cells.

We studied the alloreactive properties of donor T cells obtained from F1 mice that had recovered from the allosuppression of acute graft-vs.-host disease (GVHD) and showed mild symptoms of chronic GVHD, i.e., so-called secondary chronic GVHD. To this end, we used (B10 x DBA/2)F1 mice that had been injected with 10(8) B10 spleen cells 100-150 d previously. Such GVHD F1 mice were repopulated by lympho-hematopoietic cells of donor (B10) origin, which exhibited split tolerance towards the host: Whereas F1-specific donor T helper (Th) cells as well as T cells proliferating in the mixed lymphocyte reaction were readily demonstrable, F1-specific T suppressor (Ts) and T killer (Tk) cells were not, or were hardly, detectable; responses against third-party alloantigens were normal. Upon adoptive transfer to nonirradiated secondary recipients, the B10 cells obtained from the repopulated GVH F1 mice induced F1-specific enlargement of the draining popliteal lymph node and enhancement of the IgG formation therein. B10 cells of the same kind were unable, however, to induce lethal GVHD upon transfer to 950 rad-irradiated secondary (B10 x DBA/2)F1 recipients. We conclude that alloactivated donor Ts/Tk cells disappear from the host at a relatively early stage of GVHD, i.e., at the end of acute GVHD , presumably because they are short-lived. By contrast, the longevity of alloactivated donor Th cells causes the symptoms of secondary chronic GVH

Josephson Current between Triplet and Singlet Superconductors

The Josephson effect between triplet and singlet superconductors is studied. Josephson current can flow between triplet and singlet superconductors due to the spin-orbit coupling in the spin-triplet superconductor but it is finite only when triplet superconductor has $L_z=-S_z=\pm 1$, where $L_z$ and $S_z$ are the perpendicular components of orbital angular momentum and spin angular momentum of the triplet Cooper pairs, respectively. The recently observed temperature and orientational dependence of the critical current through a Josephson junction between UPt$_3$ and Nb is investigated by considering a non-unitary triplet state.Comment: 4 pages, no figure