EFFECT OF MAGNETIC-FIELD ON THE SOUND-VELOCITY DISCONTINUITY NEAR TC IN HIGH-TC SUPERCONDUCTORS

We have measured the velocity of sound in polycrystalline samples of Cu-oxide superconductors YBa2Cu3O7-δ (Lax-xSrx)2CuO4 (x = 0.075) and (Nd1-xCex)2CuO4 (x = 0.075), and observed a discontinuity near Tc in the sound velocity versus temperature curves except in (Nd1-xCex)2CuO4. The observed magnitude of the discontinuity was comparable to that expected from a thermodynamic estimation. By applying a magnetic field the discontinuity became broad and its magnitude decreased, while the shift of the temperature position was much smaller than that of Tc(H) estimated from Hc2(T) versus T curves which were derived from the midpoint of the transition in the resistivity curves. This behavior suggests that the "true" coherence length ξ(0) is much shorter than that derived from - dHc2 dT. The effects of fluctuation and a large anisotropy of ξ(0) on the behaviour of the sound velocity discontinuity under magnetic fields are discussed. © 1990.