Giant proximity effect in cuprate superconductors

Using an advanced molecular beam epitaxy system, we have reproducibly synthesized atomically smooth films of high-temperature superconductors and uniform trilayer junctions with virtually perfect interfaces. We found that supercurrent runs through very thick barriers. We can rule out pinholes and microshorts; this "giant proximity effect" (GPE) is intrinsic. It defies the conventional explanation; it might originate in resonant tunneling through pair states in an almost-superconducting barrier. GPE may also be significant for superconducting electronics, since thick barriers are easier to fabricate.     

Pyridine-dinucleotide models IV; stereoselective hydride transfer at a bridged pyridinium-ion

Two achiral bridged 1,4-dihydro-3,5-biscarboxamido pyridines are described. In one of these the bridging induces significant diastereotopy of the C-4 protons. This not only leads to magnetic anisochrony of these protons, but also allows for highly stereoselective (- 90%) substitution of one of them by deuterium via hydride exchange with a simple deuterated dihydropyridine.     

Pronounced solvatochromism of charge-transfer complexes in strongly hydrogen bonding solvents

A large bathochromic shift is reported for the charge-transfer (CT) transition in complexes of π-electron donors with electron acceptors containing CO groups in strongly H-bonding solvents such as trifluoroacetic acid and hexafluoroisopropanol. The results of ¹³C-NMR measurements and SCF-LCAOMO calculations indicate that this effect can be attributed to an increased electronegativity of the acceptor upon hydrogen bond formation with the solvent.     

A LEELS and auger study of the oxidation of liquid and solid tin

The oxidation of liquid and solid tin from 25 to 240°C has been investigated using 75 eV low energy electron loss spectroscopy (LEELS) and Auger spectroscopy over an oxygen exposure range from zero to 10⁷ L. LEELS was chosen for two reasons. First it can distinguish Sn, SnO and SnO₂ from each other. Second, we show that at 75 eV incident energy LEELS has a penetration depth of only one monolayer. As a result the continuity and stoichiometry of the oxide layer could be studied as a function of thickness from submonolayer to several monolayer thicknesses. Although unable to distinguish SnO from SnO₂ the larger penetration depth of the Auger technique complemented the LEELS study. From zero to one monolayer the oxide grows as islands containing both SnO and SnO₂. Above one monolayer coverage the oxide is continuous and free of metallic tin with its outer most surface enriched in SnO₂. Although oxide films grew more rapidly on polycrystalline tin than on single crystal tin the composition and continuity as a function of thickness remained unchanged. Very little change in oxide growth rate, continuity, or stoichiome- try was observed for solid tin up to temperatures near the melting point. However at 229°C, just 3°C below the melting point of tin, dissolution of oxygen into the metal was observed. A continuous, metal free solid oxide, primarily SnO, could be grown on liquid tin at 240°C than remained stable for 20 min after removal of the oxygen gas. Our model for the early stages of the oxidation of tin is different from that previously proposed on the basis of UPS, XPS, and 400 eV LEELS with respect to the continuity and relative ordering of the SnO and SnO₂ phases. Quantitative comparison of our results with those previously reported shows that the previous results are consistent with our model for the structure and stoichiometry of the initial oxide grown on tin.     

Lithium dynamics in LiMn2O4 probed directly by two-dimensional (7)Li NMR

LiMn2O4 has been studied using magic-angle-spinning nuclear magnetic resonance (MAS NMR). 1D MAS NMR shows three Li resonances assigned to different crystallographic sites. At low temperatures an extra peak appears, indicating charge ordering of Mn3+ and Mn4+. Direct observation of the lithium dynamics was possible using rotor-synchronized 2D exchange NMR. A millisecond time scale exchange of lithium starts around 285 K between the 8a and the 16c site. At 380 K lithium even starts to hop between more than two sites. The activation energies and Li jump rates are derived and are in agreement with those determined macroscopically.     

Size dependence of photocatalytic oxidation reactions of Rh nanoparticles dispersed on (Ga1-xZnx)(N1-xOx) support

Mixed Ga–Zn oxynitrides were synthesized using coprecipitation, wet‐precipitation, and sol‐id‐solution methods. The oxynitrides were used as supports for Rh nanoparticle catalysts in photo‐catalytic wa...     

An Efficient Synthesis of the Indole Acetic Acid Metabolite of MK-0462

A palladium-catalyzed coupling of iodoaniline 2 with bis-TES propargyl alcohol 3 gives indole-3-methanol 4b in 72% yield. Displacement of the hydroxy group of 4b by sodium cyanide followed by hydrolysis of the cyano group and desilylation provides the indole acetic acid metabolite, L-749,335 (1), of the 5-HT_1D receptor agonist MK-0462.