Search

Sintered ceramic samples of Bi2Sr2-xBaxCaCu2Oy with nominal barium fraction 0=x=0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointTc of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesTc is little affected. The increase ofTc with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.

It was found that very carefully prepared Na crystals gave quite large-amplitude de Haas-van Alphen oscillations, as if little or none of the metal had undergone the martensitic transformation in cooling to liquid helium temperatures. Detailed study of the absolute amplitude and the harmonic content of the oscillations gave the ratio of the spin susceptibility to that of a free electron gas as ?/?0=1.632±0.007, the spin-splitting factor as g=2.636±0.024, and the parameterB0in Fermi liquid theory asB0=-0.241±0.007. These determinations are appreciably more accurate than earlier ones, and are consistent with them and agree well with recent theoretical estimates. In the analysis of the observed oscillations it was necessary to take account of strong magnetic interaction effects, particularly at 0.6 K, and the results provide some evidence for a modified from of the theory of magnetic interaction in the presence of significant phase smearing.

Conduction electrong-factors have been deduced from de Haas-van Alphen line shapes in the hexagonal group VIII 4d transition metal ruthenium and the electronically analogous 5d metal osmium. The values for orbits normal to [0001] are 1.8±0.1 and 1.3±0.1 for the ellipsoids centered on the lineLM in ruthenium and osmium, respectively, and 1.9±0.2 for the G-centered ellipsoid in ruthenium. The more marked suppression of theg-factor in osmium, where spin-orbit coupling is stronger, is consistent with recent theoretical studies of transition metalg-factor trends.

In the field modulation technique for magnetic measurements, the voltage signalV(H) from a pickup coil surrounding a metal cylinder of permeability µ=1+K(H) and conductivity s=s0[1+S(H)] in the presence of an alternating fieldh0 cos ?t superimposed on a steady fieldH (both parallel to the axis of the cylinder) provides a measure ofK(H). This technique is particularly useful for study of the de Haas-van Alphen effect, in whichK(H) oscillates withH, and the paper is mainly concerned with the effect of eddy currents on the relation betweenV(H) andK(H) and the practical implications for the use of the technique. It is also shown that if ? is high enough,S(H) (e.g., the oscillations of the Shubnikov-de Haas effect) and also information on the orientation dependence of s can be extracted from suitable measurements onV(H) and its orientation dependence. The calculations fall into two main parts, corresponding to weak and strong modulation, i.e., according ash0 is small or appreciable compared with the period of the field dependence ofK andS. For weak modulation, results are obtained for arbitrary values ofK andS over the whole range of ?. It is shown that ifK(H) is not small,V(H) no longer faithfully reproduces the form ofK(H) once ? is high enough to make the skin depth of the eddy currents comparable to the radius of the cylinder. In general,V(H) contains not only the fundamental frequency ?, but also its harmonics, i.e.,V(H)=?n=18Vn(H)eni?t, but it is only for strong modulation that the higher harmonics become appreciable. The variation of theVn with modulation current and ? is calculated under the special assumptions thatK andS « 1 and thatK andS are simple harmonic functions ofH; for these special assumptions the weak modulation results forV1 obtained earlier fall out as a limiting case of the results for arbitrary modulation. Some of the theoretical predictions were tested experimentally on sodium and potassium crystals. Good agreement was found at frequencies up to such that the skin depth became comparable to or smaller than the sample radius, when appreciable discrepancies appeared. Some of these could be attributed to helicon resonances, the possibility of which was ignored in the theory, and some to sample inhomogeneity, the effect of which are briefly discussed in an appendix.