ORDER OF THE FUNDAMENTAL VORTEX TRANSFORMATION IN YBA2CU3O7-DELTA

In high-Tc materials with low levels of static disorder, a small magnetization jump is thought to occur at the vortex solid to vortex liquid transformation. Using a null technique, the temperature dependence of the magnetization of an untwinned single crystal of YBa2Cu3O7-δ has been investigated. No jump could be detected at the transformation, the data establishing an upper bound that is 1% of the value anticipated theoretically for a first-order melting transition. Residual disorder in the particular crystal used may have suppressed the jump to some extent. However, the result indicates that the first-order character of the fundamental transformation is likely to be weaker than anticipated. © 1995 The American Physical Society.