HIGHER-ORDER ELASTIC-CONSTANTS OF HIGH-TEMPERATURE SUPERCONDUCTOR ND1.85CE0.15CUO4

The complete set of third order elastic constants of the high temperat ure superconductor Nd1.85Ce0.15CuO4 is calculated, by taking into acco unt the interactions of nine nearest neighbour atoms in the lattice. H ere we have used the finite deformation theory to obtain the strain en ergy density phi of a tetragonal 2-1-4-type single crystal of high tem perature superconductor. This is compared with the strain dependent la ttice energy in continuum model approximation. The second order deriva tive of the potential function is obtained from the measured C-11. The third order derivative is evaluated from the Mie-Gruneisen's interato mic potential. The second order elastic constants of other members of the cuprate family of superconductors as well as Nd1.85Ce0.15CuO4 have been evaluated and compared with measured values. The agreement of se cond order elastic constants with the experimental values is good. The third order elastic constants of Nd1.85Ce0.15CuO4 are negative and th e absolute magnitude of the third order elastic constants are one orde r higher than the second order elastic constants. Also the values of t he combination C-111+ 6C(112)+2C(123) and C(144+)2C(166) are in reason able agreement with the experimental values of Fanggao et al. [Phys Re v B, 43 (1991) 5526].