Complete elastic constants and giant softening ofc66in superconductingLa1.86Sr0.14CuO4

A novel resonant ultrasound technique is used to measure all elastic constants and their temperature dependence in single-crystal La{sub 1.86}Sr{sub 0.14}CuO{sub 4}. The in-plane shear modulus {ital c}{sub 66} drops by nearly half upon cooling to 227 K, an effect attributed to order-parameter fluctuations (of 2D Gaussian character) near the tetragonal-orthorhombic phase boundary. Implications for current models of the structural phase transition are discussed, as well as the effect of domains in the low-temperature orthorhombic state.