Elastic anomalies in a La_1.85Sr_0.15CuO₄ single crystal under high magnetic fields

Temperature dependence of the sound velocity V_s in a La_1.85Sr_0.15CuO₄ single crystal has been measured in the (c₁₁ - c₁₂)/2 mode and in the c₃₃ mode under high magnetic fields up to 23T. In the field above 6T, V_s in the (c₁₁ - c₁₂)/2 mode anomalously decreases with decreasing temperature, followed by the increase below about 10K. In the c₃₃ mode, similar behavior of V_s can be observed above 20T. Such a temperature dependence of V_s may suggest the existence of a strain sensitive narrow band near the Fermi level or a phase transition related to the flux line lattice.