STRAIN FIELDS AND UNIT-CELL DEFORMATION IN HIGH-T-C SUPERCONDUCTOR HETEROSTRUCTURES

In this work the lattice deformation of arbitrarily oriented high-T-c superconductor heterostructures of orthorhombic symmetry are calculate d within the framework of elasticity theory by using the appropriate b oundary condition at the heterointerfaces. We consider the case of ver y thin films grown coherently on thick substrate crystals and that of freestanding superlattices with coherent heterointerfaces. As an examp le, the lattice deformation of thin YBa2Cu3O7-delta films grown on SrT iO3 substrates and of freestanding YBa2Cu3O7-delta/PrBa2Cu3O7-delta' a nd YBa2Cu3O4/La1.85Sr0.15CuO4 superconductor superlattices is investig ated. We determine the symmetry of the distorted unit cell and the str ain fields as a function of the orientation of the surface on which th e epitaxial growth is performed. In the case of a freestanding superla ttice, i.e. in lattice-relaxed conditions, the dependence of the strai n tensor components on the thickness of the constituent layers is disc ussed in detail. Our theoretical model and the relations derived in th is work can be applied to a large variety of material systems, in part icular to all strained material systems with orthorhombic and higher c rystallographic symmetry, i.e. materials of tetragonal crystal structu re of classes 4mm, (4) over bar 2m, 422 and 4/mmm, and for all the cla sses of the cubic crystal structure. (C) 1997 Elsevier Science S.A.