Interplay between lattice softening and high-Tc superconductivity in La1.86Sr0.14CuO4

High-resolution ultrasonic measurements have been performed on single-crystalline La1.86Sr0.14CuO4 in magnetic fields H up to 14 T. We found that by reducing temperature the transverse elastic constant (C11-C12)/2 starts to soften from about 50 K and continues to soften down to Tc(H), even when Tc(H) is reduced to 14 K by magnetic field. The softening turns to rapid hardening by the emergence of superconductivity. This behavior of C(11-C12)/2 is a consequence of the structural instability of the orthorhombic phase and disappearance of the instability caused by emergence of the superconducting state.