In high-${\mathit{T}}_{\mathit{c}}$ 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 ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ 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.

• Received 8 November 1994

DOI:https://doi.org/10.1103/PhysRevB.51.9148