Flux lattice melting in YBCO

We studied the melting transition of the flux lattice by transport measurements on very high quality single crystals of YBCO of typical dimensions 50×100×100gmm3. Using the configuration H//ab, I//c we identified the melting transition from an extremely sharp drop in the R vs T curves (ΔT=20 mK). The very sharp resistive drop is accompanied by strong non-ohmic effects and universal scaling below Tm. Careful thermal cycling of the sample around its melting temperature reveals hysteresis, as expected with first order melting. The effect appears only in the heating cycle, promoting superheating as the principal mechanism of hysteresis in this material. This suggests an important role of the surface on the mechanism of melting. The above results establish first order melting as the most plausible effect in the clean case. © 1994.