Evidence from Mechanical Measurements for Flux-Lattice Melting in Single-Crystal YBa2Cu3O7 and Bi2.2Sr2Ca0.8Cu2O8

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 $\mathbf{H}\perp \hat{\mathbf{c}}$, flux-lattice melting occurs at $H_{c2\mathrm{}}$ as in a conventional three-dimensional superconductor. However, for $\mathbf{H}\parallel \hat{\mathbf{c}}$ flux-lattice melting occurs 3.2 K below $H_{c2\mathrm{}}$. 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.