Complete elastic constants and giant softening of c66 in superconducting La1.86Sr0.14CuO4

A novel resonant ultrasound technique is used to measure all elastic constants and their temperature dependence in single-crystal La_1.86Sr_0.14CuO₄. The in-plane shear modulus c₆₆ 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.