Nonlinear elastic properties of La 1.8Sr 0.2CuO 4 under pressure

Abstract

The effect of pressure on the elastic constants of high temperature superconductor La 1.8Sr 0.2CuO 4 (LSCO) has been studied theoretically using finite strain elasticity theory. The pressure derivatives of the second-order elastic constants of La 1.8Sr 0.2CuO 4 are calculated from the knowledge of its second- and third-order elastic constants. The strain energy density ϕ is estimated by taking into account the interactions of nine nearest neighbors of each atom in the unit cell of La 1.8Sr 0.2CuO 4. The energy density ϕ thus obtained is compared with the strain-dependent lattice energy derived from continuum model approximation. The expressions for the effective second-order elastic constants of LSCO 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 (d C 33/d p) along c-axis direction than that along ab-plane, i.e. (d C 11/d p), corroborates the observation that the layers close-up substantially under hydrostatic pressure while change in interatomic distance in a layer is smaller in La 1.8Sr 0.2CuO 4.