Low-temperature elastic constants of polycrystalline La2CuO4 and La1.85Sr0.15CuO4

Abstract Using ultrasonic methods, we measured the 295-4-K elastic constants of superconductive La 1.85 Sr 0.15 CuO 4 and nonsuperconductive La 2 CuO 4 . These materials show two elastic-constant similarities: nearly the same ambient-temperature elastic constants and an elastic-stiffness minimum in the 20–40-K region. Their principal difference is that La 2 CuO 4 softens 3% during cooling, while La 1.85 Sr 0.15 CuO 4 softens 30%. This reversible wide-temperature-range softening resembles a magnetic phase transition. Perhaps it relates to the large drop in spin-excitation intensity found by Shirane and coworkers using inelastic neutron scattering.