Annealing of tweed microstructure in high T c superconductors in the presence of impurities
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Annealing of tweed microstructure in high Tc superconductors in the presence of impurities K. Parlinski,a) Y. Watanabe, K. Ohno, and Y. Kawazoe Institute for Materials Research, Tohoku University, 2-1-1-Kitahira, Aoba-ku, Sendai 980, Japan (Received 16 August 1994; accepted 16 March 1995)
A two-dimensional model of oxygen-deficit layer of superconducting material YBa2Cu3O7 has been simulated by the molecular-dynamics technique in order to study the influence of the impurities in the site of copper on the low-temperature microstructure. The microstructure pattern arises as a result of quenching the system from a high-temperature tetragonal phase to the low-temperature orthorhombic one and subsequent annealing. The potential of the impurity is modified in such a way that it promotes occupation of opposite nearest-neighbor sites around impurity by an oxygen and vacancy simultaneously. The simulations of the annealing processes showed that the domain pattern becomes very tiny with increased concentration of randomly distributed impurities. Domains of larger sizes would appear if the impurities were able to diffuse to the domain walls. This is confirmed by annealing the sample containing linear chains of impurities. The tweed microstructure depends on the magnitude of the force constants of the elastic subsystem, and at too large coupling the randomly distributed impurities are not able to pin the stiff domain walls. The results resemble the electron-microscope photographs made for cobalt in YBa2(Cu 1 _ x Co x ) 3 O 7 -5.
I. INTRODUCTION The purpose of this computer simulation paper is to study the changes of the microstructure accompanying the structural phase transition from tetragonal to orthorhombic phase in YBa 2 Cu 3 07 when another metal element is substituted for Cu. Intensive studies with the long-range goal of improving superconducting properties have been performed with such substitution elements as Fe, Co, Al, and Ga.'~5 Transmission electron microscopy (TEM) observations in the orthorhombic phase revealed a fine-scale cross-hatched contrast in morphology, when 3% or more of Cu was replaced with Fe or Co. Also, streaks of diffuse scattering at each Bragg spot, reflecting the cross-hatched morphology, were seen in diffraction. At concentrations below 2% of Co, rather irregular stripes were observed,2 but even in pure YBa 2 Cu 3 07- 5 rare stripes still persist due to incomplete annealing. It seems to be obvious that the impurities are responsible for this change of morphology. Although grain boundaries present in each specimen are also influenced by the impurities, 67 this subject will not be considered in this paper. The morphology of the crystal is established during the production of the specimen. One of the controlled processes consists of initial quenching of the specimen
a)On
leave from the Institute of Nuclear Physics, ul.Radzikowskiego 152, 31-342 Cracow, and Academic Computer Centre, Cyfronet, ul.Nawojki 11, Cracow, Poland.
1864 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 8, Aug
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