Model of Subgrain Structure Formation in HTS Cuprates and Ferropnictides
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Model of Subgrain Structure Formation in HTS Cuprates and Ferropnictides Constantin G. Tretiatchenko1, Vassily L. Svetchnikov1, and Harold Wiesmann2 1 2
G. V. Kurdyumov Institute for Metal Physics, 36 Vernadsky Blvd., Kyiv 03142, Ukraine Brookhaven National Laboratory, 76 Cornell Avenue, Upton, NY 11973, U.S.A.
ABSTRACT We have modified the model of rotational relaxation of stresses at mismatched interface by taking into account elastic strains of the growing film. This extended the model validity range to a wider class of compounds including pnictides. The model describes formation of low angle boundaries consisting of threading edge dislocations. Calculated interface energy shows that rotational relaxation occurs due to finite size of clusters and to non-equilibrium effect of the film growth. Subgrain size and expected angle of domain rotation depending on the lattice mismatch have been estimated. Unusual effect of increasing angle between the film subgrains at reduction of the deposition rate is predicted. The computed parameters of subgrains are consistent with the observed film nanostructure. INTRODUCTION It is well known that critical properties of superconducting films in a great extent are determined by their grain and subgrain structure. Relatively high-angle grain boundaries serve as easy motion channels for Abrikosov – Josephson vortices thus limiting critical current density. In a contrary, low-angle boundaries between columnar domains (subgrains) consist of well separated edge dislocations and provide strong pinning of Abrikosov vortices [1-3]. Therefore, the nature of such a subgrain structure and mechanisms of its formation are extremely important for comprehending and development of new superconducting materials. It has been established recently that properties of new Fe-based superconductors are also strongly affected by the boundaries. Mismatch between film and substrate crystal lattices is a principal reason of subgrain structure formation. The free energy of mismatched interface can be reduced either by longitudinal displacement of atoms leading to appearance of misfit dislocations in the interface plane or by transverse displacement, at which relaxation of stresses occurs by rotation of the film lattice around the c-axis and results in the film fragmentation and formation of threading edge dislocations. The mechanism of rotational relaxation of interface stresses is common for the film growth on substrates with a certain mismatch of crystal lattices. The effect was explained by that smallangle rotation of growing film nanovolumes turns out to be much more energy beneficial than compression or extension, because the compression elastic modulus is always much higher than the shear modulus [4, 5]. Recently we have suggested an alternative approach and developed an analytical model for a rigid film [6]. Conditions, under which one or another mechanism takes place, can be predicted. The calculations have shown that the free energy of the film–substrate interface is oscillating with the cluster size
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