Anomalous magnetic hysteresis loops and smallH c1 values in highT c superconductors

We have studied the hysteresis loops of RBa₂Cu₃O₇ (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar and Chaddah based on an extension of Bean’s model. The anomalies indicate lowH _c1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these highT _c superconductors.     

Electrochemical transcarbonation: Hydroquinoxy carbonyl protecting group for alcohols and amines

An electrochemically labile protecting group for alcohols and amines is reported. It can be removed under acidic or basic conditions and at low temperatures.     

Computer-aided determination of annihilation and momentum transfer rates of slow positrons from argon gas atoms

Annihilation and momentum transfer rates of slow positrons in argon gas have been determined by an iterative computer-based analysis. Agreement of calculated positron annihilation decay constant with experimental measurements at various electric and temperature fields is very good.     

Computer calculation of positron drift velocities in He,Ne and Ar

The drift velocities of positrons in rare gases, He, Ne and Ar have been calculated at various temperatures. The drift velocity depends quite sensitively on the strength of the electric field and temperature.     

Complexes of amine cations with lasalocid A,a microbial ionophore

Complexes having the stoichiometry [R-NH₃⁺][lasalocid A^-] have been prepared and characterized, where R represents a series of organic substituents with diverse steric and electronic properties. The crystalline complexes dissolve readily in solvents of low polarity such as chloroform and are characterized in this solvent by molecular weight data and ¹³C nmr spectroscopy. NMR data indicate that the cations bind the lasalocid A anion $\text{via}$ hydrogen bonds to O₃, O₆, and O₈. Molecular weight data show that none of these complexes are appreciably dissociated in chloroform solution.     

A.C. susceptibility measurements in the presence of D.C. magnetic fields in CoGa alloys — A cluster spin glass system

Low field a.c. susceptibility measurements in the presence of large d.c. magnetic fields are reported on Co_xGa_1?x alloys (x = 0.585, 0.60 and 0.63), a cluster spin glass system, in the temperature range of 77 to 500 K. A peak in the a.c. susceptibility is observed in each case. The peak position shifts towards lower temperatures with increasing d.c. magnetic fields. In conjunction with the zero field NMR measurements reported earlier, these results may be taken to imply the existence of a continuous spectrum of freezing temperatures — cluster with highest anisotropy field having the lowest freezing temperature.     

Optical vortex coronagraph

We describe a method to observe dim exoplanets that eliminates light from the parent star across the entire exit pupil without sacrificing light from the planet by use of a vortex mask of topological charge m = 2.