Evidence from Mechanical Measurements for Flux-Lattice Melting in Single-Crystal YBa2Cu3O7andBi2.2Sr2Ca0.8Cu2O8

Abstract

We have studied flux-lattice melting in single crystals of Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{Bi}}_{2.2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{0.8}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ using a high-$Q$ mechanical oscillator. In Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ with an applied magnetic field $\mathbf{H}\ensuremath{\perp}\stackrel{^}{\mathbf{c}}$, flux-lattice melting occurs at ${H}_{c2}$ as in a conventional three-dimensional superconductor. However, for $\mathbf{H}\ensuremath{\parallel}\stackrel{^}{\mathbf{c}}$ flux-lattice melting occurs 3.2 K below ${H}_{c2}$. We believe that this is evidence for a transition into a vortex-liquid state similar to that seen in two-dimensional superconducting films. For ${\mathrm{Bi}}_{2.2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{0.8}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ the effect is even more pronounced; with a bulk superconducting transition of 75 K, the flux-lattice melting in this material occurs in both orientations near 30 K.