Upper critical field for optimally-doped YBa₂Cu₃O_7−δ

Abstract

We have observed clear indication of the transition from super- to normal-state conductivity in optimally doped $\text{YBa}{}_2\text{Cu}{}_3\text{O}{}_{\mathrm{7-\delta }}\text{(T}{}_{\mathrm{<mtext>c</mtext>}}\text{∼90}\text{K}\text{)}$ in high magnetic fields up to $\text{400}\text{T}$ applied parallel to the CuO₂ plane $\text{(B}\text{||}\text{CuO}{}_2\text{)}$, using the electromagnetic flux compression system. In order to perform high-sensitivity magnetoresistance measurements in short-pulsed ultra-high magnetic fields, we developed a contactless radio frequency transmission technique. Therefore, a complete phase diagram of the upper critical field (H_c2) for optimally-doped YBCO was constructed as a function of temperature on both cases of B⊥CuO₂ and $\text{B}\text{||}\text{CuO}{}_2$. In the previous experiment, we found that the H_c2 for B⊥CuO₂ would attain to $\text{120}\text{T}$ at $\text{T=0}\text{K}$, and follow the WHH phase boundary where quenching of the superconductivity was mainly governed by the orbital effect. On the other hand, it turned out that the observed H_c2 for B||CuO₂ would reach around $\text{250}\text{T}$ at $\text{T=0}\text{K}$, which is consistent with the WHH theory that takes into account the spin Zeeman and spin–orbit effect.