Parallel investigations have been developed on copper and nickel compounds with the simplest La2Cuo4 structure (synthesis, crystal growth, spectroscopic studies). Also, to discriminate between the role of phononic and magnetic interactions, the search for superconductivity in oxides which do not contain any magnetic ions has been undertaken.
The aims of the action were to :
- Acquire a precise knowledge of the phase diagram of the Y2O3-CuO-BaO system.
- Optimise the crystal growth of large good-quality samples.
- Measure complete pronon dispersion curves by inelastic neutron scattering.
- Understand the basic lattice dynamics of oxide superconductors, and search for the signature of highly-anharmonic motions and determine their role in the mechanism of superconductivity.
THE MECHANISMS UNDERLYING the superconductivity of oxides with a high critical temperature are investigated. Research focuses on the role played by atomic lattice vibrations. These phenomena are measured by inelastic neutron scattering, a technique requiring large (about 1 cm3) single crystals. to this end, the growth of large high-quality Y-Ba-cu-O single crystals is being improved through developing a new method pf growth in a non-polluting crucible.
APPROACH AND METHODS
The absence of congruent melting of Y-Ba-Cu-O requires a flux method for crystallisation using BaO-CuO melts as a solvent. The determination of the domain of primary crystallisation has been achieved. With the original growth method in the yttrium-oxide crucible, the starting material is the composition of the binary eutectic 7 BaO-18 CuO.
Y-Ba-Cu-O crystals display a sharp transition (2 degree wide) to the superconducting state at 90 K. The partners in this Action each provide their specific capabilities to join in the material characterisation. The crystals' resonance in the group of D. Jerome at Orsay and by infrared reflectivity spectroscopy at Orleans.
Inelastic neutron scattering measurements are performed with the German high-performance spectrometer located at the Orphee reactor at Saclay. Lattice dynamics calculations are done at KFK Karlsruhe with a shell model and anisotropic screening by mobile charge carriers.
PROGRESS AND RESULTS
Much has been learned about the phase diagram of YBaCuO and crystal growth. Several new ways have been suggested, but a lot of effort is still required. Crystals of YBaCuO as large as 7 x 5 mm were obtained, but their thickness along the direction of the c-axis remains sub-millimetric.Their small thickness, however, favoured oxygen isotope exchange (160 - 170) performed at CRPHT, an experiment preceding the NMR studies of 170 performed at Orsay, and the first report of temperature dependence of relaxation times of 170 in-plane oxygen with a single crystal.
Several crystals of La2NiO4, Pr2NiO4, La2CuO4 and YBaCuO (06 to 07) have been measured by inelastic neutron scattering by the KFA team at LLB, Saclay. It has been shown that there exits a tendency towards general lattice softening upon reaching electronic concentration known to enable the appearance of superconductivity.
A parallel approach towards the insular-metal transition in systems isostructural of La2CuO4, but not containing any magnetic ion, has partly succeeded with the discovery of a new system (Ba1-xKx)2Pb1-YBiyO4 which can be cycled from insulating to highly conducting states by a simple treatment with argon or oxygen.
One application of theses phase diagram studies deals with a way to improve critical current densities in textured ceramics of YBaCuO. This material remains the only superconductor at a temperature well above that the ebullition of nitrogen, though to allow the development of specific high-performance electronic devices at liquid nitrogen temperature.
The conclusion of the Kanazawa (Japan, July 1991) conference on superconductivity was that there is still no consensus about the mechanism of superconductivity in oxides. New families based on carbon have been discovered in the meantime, showing that the phenomenon might be much more general than anticipated. The essential argument which had been invoked to reject a mechanism based on pairing via phonons is the weakness of the isotope effect of oxygen, which now appears not to be small as originally claimed. The initial approach - to explore the lattice dynamics - which encountered scepticism when the Action began, has been shown to be a valuable one provided it is not restricted to the harmonic approximation. The apical oxygen, sitting in a sandwich position between the conducting and "reservoir" planes, is sensitive to an electric field which varies with electronic concentration, possibly leading to different regimes, including the highly anharmonic, consistent with an interpretation of X-ray absorption experiments in terms of double-well potential for apical oxygen.
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WC2N 5JT London