Mode Grueneisen parameters and the low temperature thermal expansion of high-Tc superconductor La1.8Sr0.2CuO4

The anisotropy of low temperature thermal expansion of La1.8Sr0.2CuO4 (LSCO) is analysed theoretically using quasiharmonic theory of thermal expansion. Vibrational anharmonicity as determined by the generalised Grueneisen parameters (GPs) γ'j and γ''j of the various normal mode frequencies of the lattice has been studied. γ'j refers to change in frequencies due to a uniform areal strain perpendicular to the c-axis and γ''j to the change in frequencies due to a uniform areal strain along the c-axis of LSCO crystal. The generalised GPs are calculated using the second- and third-order elastic constants. The second- and third-order elastic constants are calculated using deformation theory. The strain energy density phi is estimated by taking in to account the interactions of nine nearest neighbours of each atom in the unit cell of LSCO. The energy density thus obtained is compared with the strain-dependent lattice energy from continuum model approximation to obtain the complete set of non-vanishing second- and third-order elastic constants of LSCO. All the generalised GPs are positive. Generally, γ''j exhibits higher values than γ'j except for the case of γ2''. Thus the crystal is more anisotropic along the c-axis than in ab-plane. The longitudinal acoustic mode γ3'' is the most anisotropic of all the mode gammas. The low temperature limits of the GPs γ-barperp(0) and γ-barparallel(0) are found to be almost equal. This is presumably due to the absence of discrepancies in anharmonicity along the c-axis and along the ab-plane in LSCO at low temperatures. The low temperature limit of the lattice thermal expansion γL is calculated from mode Grueneisen gammas. γL has been obtained as 4.3 for LSCO. The low temperature limit γL is positive and therefore, we expect the volume lattice thermal expansion to be positive down to absolute zero in LSCO