Lattice instability in the normal state of La2?x Sr x CuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.