Measurements of the anisotropic irreversibility field in the electron-doped high-T$_{c}$ superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$

We report measurements of the irreversibility field $(H_{\rm {irf}})$ in single crystals of the electron-doped high-T$_{c}$ superconductor (HTSC) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ ($x$ = 0.15 and 0.17) with an applied magnetic field ($B_{0}$) up to 28 T, using the method of the shift in a nuclear magnetic resonance (NMR) probe circuit resonance frequency (f) caused by the susceptibility of the sample. It is observed that $H_{\rm{irf}}$ is highly anisotropic, and that as the temperature $T$ $\rightarrow$ 0 the upper critical field [$H_{c2, \parallel c}$($T$ $\rightarrow$ 0)] at $B_{0}$ $\parallel$ $c$ is far less than the Pauli limit and very different from that at $B_{0}$ $\perp$ $c$. A phase diagram that involves the vortex solid and/or vortex liquid states depending on the alignment of $B_{0}$ relative to the lattice $c$-axis is proposed, and the obtained anisotropic $H_{c2}$ character along with the evaluated zero $T$ coherence length [$\xi_{ab(c)}$($T$ $\rightarrow$ 0)] and penetration depth [$\lambda_{ab(c)}$( $T$ $\rightarrow$ 0)] at $B_{0}$ $\parallel$ $ab(c)$ is compared with that of hole-doped HTSCs. This work is supported at UCLA by NSF Grants DMR-0334869 (WGC) and 0520552 (SEB), at U. Maryland by 0352735 (RLG), and NHMFL by 0084173 and the State of Florida.