New Features in the Vortex Phase Diagram ofYBa2Cu3O7−δ

Magnetic and transport measurements have been performed in high quality ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals. We demonstrate for the first time that the magnetization peak line in the phase diagram of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$, $H_p\left(T\right)$ exhibits an impressive similarity with the equivalent line for ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$. Our results show not only a similar temperature dependence but also an almost identical response to oxygen doping and a correlation to the multicritical point. At high temperatures we observe a previously unreported splitting of the magnetization peak.