Granularity, microstructure, and flux creep in TlBa2Ca2Cu3O9+δ superconducting powder and silver-sheated tapes

Silver-sheathed TlBa₂Ca₂Cu₃O_9+δ (Tl-1223) tapes, with a transport critical current density, J_ct, of 6200 A/cm² at 75 K under zero magnetic field, were fabricated by the oxide-powder-in-tube (OPIT) method and characterized using an X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and superconducting quantum interference device (SQUID) magnetometer. The results of the magnetization and SEM study indicate that, in these tapes, individual grains are distributed randomly in orientation and weak links exist. However, strongly linked percolative current paths within the tape persisting in increasing fields, accompanied by strong intrinsic interplanar coupling, sustain a significant J_ct at high fields and lead to the plateau in the J_ct-H curve. Dislocation networks, which may act as effective pinning centers, are the common features observed in the tapes. To investigate the effects on flux pinning due to thermomechanical processing, magnetic relaxation at 1 and 2 T over 5–50 K was measured. The tape shows slightly lower normalized relaxation rates (S=-(1/M_o)dM/ dln t) than the cauliflower-like precursor powder. Analyses of the relaxation data obtained from the tape, after incorporating temperature dependence and field scaling, yield an expression for the effective pinning energy U_eff(J,T,H) = (U₁/H^0.54)[1-(T/ 72.5)²]⁴(J/J_i)^μ. This result was compared with the prediction of the collective flux-creep model, which suggests that Tl-1223 has a three-dimension-like (3D-like) vortex lattice. Presumably, a more plate-like powder morphology may result in improved texturing by the OPIT process. Tl_0.5Bi_0.5Sr₂CaCu₂O_7+δ ((Tl,Bi)-1212) powder with this morphology was therefore synthesized for comparison.     

Intrinsic pinning of vortices as a direct probe of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell state in layered superconductors

Previously the search for the modulated superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state was performed by means of measurements which do not give direct information on spatial modulation of the superconducting state. We propose to measure interlayer conductivity in Josephson-coupled layered superconductors as a function of the strength and the orientation of the parallel magnetic field. We show that interlayer critical current and the conductivity have peaks when the magnetic field is perpendicular to the in-plane wave vector of the LOFF state and when the period of the Josephson vortex lattice induced by the magnetic field is commensurate with the LOFF period.     

New superconducting phases in field-induced organic superconductor lambda-(BETS)2FeCl4

We derive the parallel upper critical field, Hc2, as a function of the temperature T in quasi-2D organic compound lambda-(BETS)2FeCl4, accounting for the formation of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state. To further check the 2D LOFF model, we propose to study the Hc2(T) curve at low T in tilted fields, where the vortex state is described by the high Landau level functions characterized by the index n. We predict a cascade of first-order transitions between vortex phases with different n, between phases with different types of the symmetry at given n and the change of the superconducting transition from the second order to the first order as FeCl4 ions are replaced partly by GaCl4 ions.     

C-axis Josephson plasma resonance observed in Tl(2)Ba(2)CaCu(2)O(8) superconducting thin films by use of terahertz time-domain spectroscopy

We have unambiguously observed the c -axis Josephson plasma resonance (JPR) in high-critical-temperature (T(c)) cuprate (Tl(2)Ba(2)CaCu(2)O(8)) superconducting thin films, employing terahertz time-domain spectroscopy in transmission as a function of temperature in zero magnetic field. These are believed to be the first measurements of the JPR temperature dependence of a high-T(c) material in transmission. With increasing temperature, the JPR shifts from 705 GHz at 10 K to ~170 GHz at 98 K, corresponding to an increase in c-axis penetration depth from 22.4+/-0.6mum to 94+/-9mum . The linewidth of the JPR peak increases with temperature, which indicates an increase in the quasi-particle scattering rate. We have probed the onset of the c -axis phase coherence to ~0.95T(c) . The JPR vanishes above T(c) as expected.     

Spectroscopic Analysis of Folate Binding to Thymidylate Synthase Active Site

The fluorescence properties of folate binding to thymidylate synthase (TS) were analyzed. Two antifolates with different binding modes to the TS active site were the ligands. Intrinsic tryptophan fluorescence was used to evaluate the binding of both antifolates to the wild-type TS and a mutant Escherichia coli TS (K48Q) that is impaired in folate binding. During titration of wild-type TS with PDDF, tryptophan fluorescence was quenched at 330 nm, which was accompanied by an increase in emission at 379 nm, suggesting an energy transfer process from a tryptophan in the TS active site to the folate analogue. Energy transfer was not observed with the mutant TS, as expected. Tryptophan emission is a very useful tool to test for substrate-like inhibitors with biological activity.     

Data Collection for the Sloan Digital Sky Survey—A Network-Flow Heuristic

This paper describes an NP-hard combinatorial optimization problem arising in the Sloan Digital Sky Survey and a practical approximation algorithm that has been implemented and will be used in the survey. The algorithm is based on network flow theory and Lagrangian relaxation.     

High current densities at 20T in long (20m) tapes of CVD-prepared Nb3Ge

Critical current densities J_c as a function of magnetic field have been measured on tapes of chemically vapor deposited (CVD) Nb₃Ge that are up to 20m long. Values of J_c exceed 5 × 10⁴ A/cm² at 20T. Such high critical current densities are encouraging for use of this material in producing a 20T superconducting magnet.