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Pairing of Bi atoms in Bi_2Sr_2CaCu_2O_8+x high-temperature superconductor

Incommensurate modulations and "pairing" of Bi atoms are often observed in the crystals of Bi_2Sr_2CaCu_2O_8+x high-temperature superconductor (T_c ~ 85 K). We assumed that the pairing of Bi atoms along a axis is connected with the regular Bi 6sp lone-pair ordering along the b axis of the crystal. Using the Fourier transform of the photodensitometric traces of the electron micrographs, the shift of atoms from the average lattice-node position can be easily calculated by expanding the photodensitometric trace in the Fourier series and searching for the extrema of the chosen function. The amplitude ratio of the terms, which is given by the Fourier transform of the photodensitometric trace, is determined by the shifts of Bi atoms, so they can be calculated. This value (delta a =0.45 Angstrom) was used for redefining the atomic positions of Bi in the unit cell and calculating the intensities of the electron micrographs by the use of the multi-slice kinematic approximation. The preliminary results are shown in Fig. 1. The positions of atoms in the calculated images (left) are in good agreement with the experimental work (right), but the intensity distribution is to be explored in the future work.

HTCS; HREM; modulated structures

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