Static disorder in the incommensurate structure of the high T _c superconductor Bi₂Sr₂CaCu₂O_8+δ

Abstract

The incommensurate modulated structure of the high Tc superconductor of nominal composition Bi2Sr2CaCu2O8 has been refined using an optimized data set of 1530 reflections (496 main reflections, 753 first-order and 281 second-order satellites) in superspace group Bbmb(Oβ1) using a Fourier expansion of the modulation functions up to second order. A large improvement of the R (wR) factor [global: 0.056 (0.051); main reflections: 0.049 (0.043); first-order satellites: 0.056 (0.058); second-order satellites: 0.078 (0.079)] is obtained by the introduction of a static modulated disorder on the Bi site without any thermal factor modulation and by considering occupational modulation functions. A detailed comparison with previous studies in modulated structure models or in supercell models is given, in particular for the description of the BiO layers.

Output from a Josephson stimulated terahertz amplified radiation emitter

“…There may be a number of sources for this non-uniformity, such as defects and thermal fluctuations, which may be enhanced by inhomogeneous heating effects [22]. Here it is assumed that the primary source of the inhomogeneities is thermodynamic fluctuations, but the model can also be employed to approximate smooth stoichiometry variations [29,30]. In the samples used in the experiments [26], the BSCCO crystals were prepared slowly over a very limited central region in gold-plated elliptical heating ovens, with highly controlled heating, producing samples that are probably much more uniform chemically (although still highly non-stoichiometric) than in those studies [31,32].…”

A five-dimensional structural investigation of the misfit layer compound [Bi0.87SrO2]2[CoO2]1.82

“…Note also that each [CoO 2 ] layer is inserted between two RSS, whose D zones are shifted by $7/4a 11 . Otherwise, in contrast with the Bi 2201 and 2212 modulated compounds with Cu, Fe and Co (see, for example, Leligny et al, 1992;Yamamoto et al, 1992;Tarascon et al, 1989;Jakubowicz et al, 1999;Grebille et al, 1996;Petr Ïõ Âc Ïek et al, 1990;Le Page et al, 1989;Pe  rez et al, 1997), no signi®cant puckering of the [BiO] and [SrO] layers along a 13 is observed in the composite crystal. This particular characteristic most probably originates from the fact that the Co atoms are only bound to the O3 atoms of the [CoO 2 ] layers, which appear as rigid structural units.…”

Imaging the impact on cuprate superconductivity of varying the interatomic distances within individual crystal unit cells

“…Fortunately, however, significant variations in cell dimensions and geometry occur naturally at the nanoscale in the Bi 2 Sr 2 Ca nϪ1 Cu n O 4ϩ2n (n ϭ 1, 2, 3) family. These variations take the form of a bulk incommensurate periodic modulation (12)(13)(14)(15)(16)(17) perturbing the atoms from their mean positions (18), as shown schematically in Fig. 1 C and D. This so-called crystal ''supermodulation'' is believed to originate from a misfit between the preferred bond lengths of the perovskite and rock-salt layers of the crystal.…”