Vortex melting line and anisotropy of high-pressure-synthesized $TlBa2Ca2Cu3O10−y$ high-temperature superconductor from third-harmonic susceptibility studies

The fundamental and third-harmonic susceptibility response of $YBa2Cu3O7−δ$ (YBCO) and $TlBa2Ca2Cu3O10−y$ (Tl:1223) high-critical temperature $(Tc)$ superconductors with preferentially oriented crystallites has been measured in various magnetic fields up to 14 T, and with very low ac field amplitude, to probe the melting transition of the vortex matter. This method does not require large single crystals, being therefore suitable for high-$Tc$ superconductors with small crystallites, prepared with high-pressure synthesis techniques. We determine experimentally the melting line of vortices and the anisotropy factor in high-pressure-grown Tl:1223 with a high $Tc$ of 133.5 K. Results from similar measurements on YBCO with preferentially oriented grains are consistent with both commonly accepted models of the melting line and values of the anisotropy factor, supporting the validity of our approach.