Guoqing Wu, W. G. Clark, R. L. Greene, A. P. Reyes, P. L. Kuhns, W. G. Moulton, Bing Wu, Feng Wu
We report measurements of the irreversibility field $(H_{\rm{m}})$ in single crystals of the electron-doped high-$\it{T}_{c}$ superconductor (HTSC) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO, $x$ = 0.15 and 0.17) for alignments ($\parallel$ and $\perp$ to the CuO$_{2}$-plane) of the applied magnetic field ($B_{0}$) up to 28 T. The measurements use the resonant frequency ($f_{R}$) of a lab-built nuclear magnetic resonance (NMR) probe to track the changes in the radio frequency susceptibility of the sample that are caused by the changes in the superconducting vortex phases as a function of $B_{0}$ and temperature ($T$). One result is that $H_{\rm{m}}(T)$ is highly anisotropic. Our analysis also indicates that as $T$ $\rightarrow$ 0 the obtained upper critical field [$H_{c2,\parallel c}$(0)] at $B_{0}$ $\parallel$ $c$, is far less than the Pauli limit and is very different from that at $B_{0}$ $\perp$ $c$. A phase diagram that involves vortex solid and/or vortex liquid states depending on the alignment of $B_{0}$ relative to the lattice c-axis, is proposed for PCCO. The obtained anisotropic upper critical field $H_{c2}(0)$ character along with the evaluated zero $T$ coherence length $[\xi_{ab(c)}(0)]$ and penetration depth $[\lambda_{ab(c)}(0)]$ is compared with those of hole-doped cuprate HTSCs and typical Fe-based superconductors. We find that the low temperature anisotropic character of PCCO is rather similar to that of hole-doped cuprate HTSCs, but apparently larger than that of typical Fe-based superconductors. These measurements also prove a new sensitive probe of detecting rich properties of unconventional superconductors with the use of the resonant frequency of a NMR probe circuit.
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http://arxiv.org/abs/1305.5467
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