Mechanical Measurements of Flux Lattice Melting in Single Crystal YBa sub 2 Cu sub 3 O sub 7 and Bi sub(2.2) Sr sub 2 Ca sub (0.8) Cu sub 2 O sub 8 .

We have studied flux lattice melting in single crystals of YBa sub 2 Cu sub 3 O sub 7 and Bi sub(2.2) Sr sub 2 Ca sub (0.8) Cu sub 2 O sub 8 using a high-Q mechanical oscillator. In YBa sub 2 Cu sub 3 O sub 7 with an applied magnetic field H vec c hat, flux lattice melting occurs at H sub (c2) as in as in a conventional three dimensional superconductor. However, for H vec || c hat flux lattice melting occurs 3.2K below H sub 2. We believe this is evidence for a transition into a vortex liquid state similar to that seen in two-dimensional superconducting films. For Bi sub (2.2) Sr sub 2 Ca sub (0. 8) Cu sub 2 O sub 8 with a bulk superconducting transition of 75K the flux lattice melting occurs in both orientations near 30K.