Nonlinear elastic properties in high-Tc superconductor Nd1.85Ce0.15CuO4 under pressure

The elasticity of high temperature superconductor Nd1.85Ce0.15CuO4 (NCCO) under the application of pressure has been studied theoretically using finite strain elasticity theory. The pressure derivatives of the second-order elastic constants of NCCO are calculated from the knowledge of its second- and third-order elastic constants. We have used the deformation theory for the evaluation of the second- and third-order elastic constants. The strain energy density phi is estimated by taking into account the interactions of nine nearest neighbors of each atom in the unit cell of NCCO. The energy density phi thus obtained is compared with the strain-dependent lattice energy derived from continuum model approximation. The second-order elastic constants of NCCO show that the elastic stiffness within the ab-layers is large compared with the binding forces between the layers, which is expected of a layer-like material. C333 is larger than C111, which means that the anisotropy along the c-axis is more pronounced than that along the basal plane of the crystal. The expressions for the effective second-order elastic constants of NCCO system have been derived in its strained state in terms of the natural state second- and third-order elastic constants. These expressions are employed to obtain the pressure derivatives of the effective second-order elastic constants. The results that the larger pressure derivative (dC33/dp) along c-axis direction than that along ab-plane, i.e. (dC11/dp), corroborates the observation that the layers close-up substantially under hydrostatic pressure while change in interatomic distance in a layer is much smaller in NCCO