ausblenden:
Schlagwörter:
single crystal; Bi-2212; structure; superconductivity
Zusammenfassung:
The effect of annealing on the structure and superconductivity
of Bi2Sr2CaCU2O8-delta has been investigated on high-quality
and large single crystals grown by the traveling solvent
floating zone technique, Vacuum annealing was carried out on
crystals sealed in quartz ampoules under different oxygen
pressure ranging from 8 x 10(-1) to 5 x 10(-4) bar. The as-
grown and post-annealed crystals were explored by induction-
coupled plasma atomic emission spectroscopy, X-ray diffraction
(XRD). transmission electron microscopy (TEM) and magnetization
measurements. It was demonstrated that the oxygen homogeneity
of the as-grown crystals has a direct influence on the post-
annealed samples. The superconducting transition temperature.
T-c was found to be affected more strongly by the annealing
oxygen partial pressures. rho(O-2) than by the annealing
temperature, T-c decreases considerably from 92 to 29 K with
decreasing rho(O-2) from 1 down to 4 x 10(-2) bar. however.
increases surprisingly to 62 K with further decreasing rho(O-2)
to 5 x 10(-4) bar. The transitions remain relatively sharp for
the samples annealed under rho(O-2) = 8 x 10(-1) -4 x 10(-2)
bar, with no evidence of significant structural change by
powder XRD measurement. XRD studies indicate that the crystals
have orthorhombic symmetry. The refinement result for XRD data
shows that the c-axis expands. while both a- and b-axis
contract slightly with decreasing rho(O-2). The relation
between T-c and the c-axis lattice parameter suggests that the
change of T-c is correlated with a redistribution of holes
between the Bi-O layers and the Cu-O planes. Impurity phases
were found to segregate from samples annealed under rho(O-2) <
4 x 10(-2) bar. The one-dimensional incommensurate
superstructure along the b-axis was observed by TEM in the
crystal annealed at rho(O-2) = 1 x 10(-3) bar. The lattice
strain caused by the high-vacuum annealing possibly affect the
Cu-O-Cu angle and lead to an anomalous change in T-c (D 2002
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