A. Morrobel-Sosa

Interplay of magnetism and superconductivity in thulium and lutetium nickel-borocarbides

Abstract Implanted muon spectroscopy shows that magnetic order coexists with superconductivity in TmNi2B2C. The muon response indicates that large amplitude fluctuations of the internal field are superimposed on a relatively small static component. The static field exhibits an unusual temperature dependence which shows no interruption at the superconducting transition and may represent evolution of a staggered or spiral arrangement of Tm moments. The dynamic component changes its spectral density at the superconducting transition and the question arises as to whether this is associated with Ni moment formation and fluctuation. In LuNi2B2C, where the rare earth ion is nonmagnetic, no signature of static order is seen. An intriguing change in the muon response does occur below about 4 K in this material; this seems suggestive of Ni moment fluctuation, although the evidence is not entirely conclusive.

A triphenyl Bismuth-based solution route to Bi:Sr:Ca:Cu:O superconductors

Abstract A new solution route to bulk Bi-Sr-Ca-Cu-O superconductors using well-defined metallorganic precursors (triphenyl bismuth, strontium ethoxide, calcium ethoxide, and copper (II) 2-ethylhexanoate), all of which are either readily available or easily synthesized from common organic solvents, is described. The final synthetic procedure is straightforward and versatile, yielding a reproducible final product from several starting stoichiometries. The superconducting materials obtained from solution and solid-state materials are compared using four-probe (AC) resistance versus temperature, X-ray powder diffraction and scanning electron microscopy. The triphenyl bismuth-based solution route produces a superconducting material that is multiphased, with Bi 2 Sr 2 Ca 1 Cu 2 O x as the predominant phase.

Muon level-crossing resonance in Si∶Al

AbstractμLCR spectra from single-crystal Si∶Al have been taken at 30 K with the field along the 〈111〉 direction. The spectra show strong resonances near 12 and 50 mT, and weaker resonances near 38 and 42 mT. There is also indication of lines near 15 and 21 mT. The strong signals are due to the well-known zero crossings for Mu* in Si, and the weak signals may be associated with the quadrupole interaction of27Al, as predicted byab initio electronic structure calculations.

Development of a μLCR facility at LAMPF

It has long been recognized thatμLCR could profitably be done with the high intensity surface beam at LAMPF [1]. A spectrometer has been built that is matched to the LAMPF beam characteristics. The polarization information is obtained from a downstream array of counters while side counters, containing no polarization signal, monitor theμ+ beam. Degraders select higher energy e+, thereby reducing rates and required counter segmentation while maintaining information content. We apply a ramped longitudinal field in addition to the static one to average over instabilities in theμ+ beam. This field scan allows direct interpretation of data and does not require a prior estimate of the resonance structure of a sample. Flux coils monitor the applied ramp field and eddy-current induced fields. High average rate (2×107μ+/s). good stability, and the versatile field scan permitted useful data to be collected from Cu, Al(Cu), Al, Si(Al), and polycrystalline Si targets.

Muon level crossing resonance in niobium

A useful guide to detecting quadrupole level crossing resonance (QLCR) spectra is to search in the vicinity of BE\cdot(\gamman/\gamma_μ), where BE is the onset magnetic field for the decoupling of the quadrupole interaction, as seen in muon spin rotation linewidths. More detailed predictions of the positions and intensities of the resonances require numerical simulations taking account of the local geometry of the muon site. We present such simulations for muons adjacent to nuclei of spin 9/2 and demonstrate a pronounced dependence on the anisotropy of the quadrupole coupling tensor. Simulations for the specific cases of muons located at the octahedral and tetrahedral interstices in niobium metal are compared with the experimentally detected spectrum in a polycrystalline sample.

Muon level crossing resonance in aluminium

Cross relaxation between implanted positive muons and27Al nuclei in Al metal, lightly doped with Cu to impede the muon diffusion, shows weak resonances at 2.3 and 4.0 mT. Assignment of these to polarization transfervia the 1/2 ↔ 3/2 and 3/2 ↔ 5/2 transitions of the (J=5/2) spins leads to a quadrupole coupling constante2qQ/h=1.8(1) MHz,i.e. an electric field gradient on Al nuclei immediately adjacent to the muon siteq=0.048(3) a.u., with a small departure from axial symmetry.

Distinctive field dependence of the longitudinal muon polarization for Mu* in polycrystalline silicon

A distinctive longitudinal magnetic field dependence of the muon polarization for anomalous muonium in polycrystalline semiconductor targets has been predicted. The polarization exhibits a cusp,i.e., a discontinuous jump in the slope from negative to positive. Measurements of the longitudinal polarization for polycrystalline silicon in fields up to 0.5 T, and temperatures 53 and 200 K have been made at LAMPF. A cusp in the field dependence indeed occurs at 0.345 T, in excellent agreement with the prediction. No cusp is observed at 200 K because Mu* has been thermally ionized.

Zero resistance at 100 K in the superconductor GdBa2Cu3O7-z

Superconductivity in GdBa2Cu3Ox has been detected by AC and DC electrical resistance measurements. For both cases the onset temperature for the superconducting transition is 108 K, with a zero-resistance estimate of Tc=100 K for the DC measurement and 99 K for the AC measurement. X-ray diffraction analysis indicates that the sample is mostly single-phase.

Superconductivity in Rapidly Cooled Gd3Ba3Cu4Oz

Recently Bednorz and Mueller [1] initiated an almost explosive growth in the field of superconductivity by detecting indications that in the quaternary mixed-valence system LaxBa2−xCuO4−y superconductivity with critical temperatures Tc in excess of 30 K could be achieved. Since the previous record-holder was Nb3Ge at 23.3 K, the report generated immense interest, and within a few months it was found in laboratories around the world that there were several compounds with critical temperatures in the 90–100 K range, i.e. well above the boiling temperature of liquid nitrogen [2–7].

Effect of Oxidizing Atmosphere on Superconductivity in RBa2Cu3−xMxOz

The dependence of the superconducting properties of RBa 2 Cu 3−x M x O z (R = Y, Gd, M = Cr, Mn, Fe, Co, Ni, and x = 0.0, 0.025, 0.05, 0.1, 1.0, 3.0) on the oxidizing environment used during the processing (sintering/annealing) of these materials has been investigated. Samples of nominal composition, YBa 2 Cu 3 O z , were prepared from the same starting materials and subjected to two different controlled heat treatments, under both controlled oxygen and ozone atmospheres. Measurements of ac electrical resistance versus temperature indicate that the critical temperatures are increased, and the widths of the superconducting transitions are narrowed for identically heat-treated samples when processed under ozone atmosphere rather than under an oxygen environment. Thermogravimetric analysis data indicate this trend to be correlated with increased oxygen content and thermal stability. This effect is also observable for materials with metal substitution for Cu, although the measured T c s are generally lower for increased values of x. X-ray powder diffraction analysis showed the materials to be mostly single phase.