Upper Critical Field of Pressure-Induced Superconductor EuFe${}_2$As${}_2$

Abstract

We have carried out high-field resistivity measurements up to 27\,T in EuFe${}_2$As${}_2$ at $P$\,=\,2.5\,GPa, a virtually optimal pressure for the $P$-induced superconductivity, where $T_{\mathrm{c}}$\,=\,30\,K. The $B_{c2}-T_{\mathrm{c}}$ phase diagram has been constructed in a wide temperature range with a minimum temperature of 1.6 K ($\approx 0.05\times T_{\mathrm{c}}$), for both $B\parallel ab$ ($B_{c2}^{\mathrm{ab}}$) and $B\parallel c$ ($B_{c2}^{\mathrm{c}}$). The upper critical fields $B_{c2}^{\mathrm{ab}}$(0) and $B_{c2}^{\mathrm{c}}$(0), determined by the onset of resistive transitions, are 25 T and 22 T, respectively, which are significantly smaller than those of other Fe-based superconductors with similar values of $T_{\mathrm{c}}$. The small $B_{c2}(0)$ values and the $B_{c2}(T)$ curves with positive curvature around 20 K can be explained by a multiple pair-breaking model that includes the exchange field due to the magnetic Eu${}^{2+}$ moments. The anisotropy parameter, $\Gamma =B_{c2}^{ab}/B_{c2}^c$, in EuFe${}_2$As${}_2$ at low temperatures is comparable to that of other "122" Fe-based systems.

5 pages, 4 figures, revised version to be published in Phys. Rev. B (Rapid Communications)