ULTRASONIC STUDY OF THE SPIN-PEIERLS SYSTEM CUGEO3 UNDER PRESSURE

We present the temperature dependence of the acoustic sound velocity f or a CuGeO3 single crystal measured under pressure up to 10 kbar. Anom alous elastic behavior along the c axis, observed around 100 K, is acc ounted for by the existence of a magnetoelastic coupling. We show that this coupling is weakly affected by pressure. Moreover, the temperatu re dependence of the sound velocity below the spin-Peierls phase trans ition is analyzed using a Landau-type model from which we obtain the t emperature and pressure dependences of the order parameter. We find th at the magnetic spin-Peierls energy gap increases with pressure at a r ate of d Delta/dP=0.11 +/- 0.01 meV/kbar. [S0163-1829(98)02326-1].