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

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 $B_{\mathrm{c}2}-T_{\mathrm{c}}$ 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}$$),forboth$B \parallel ab$($B_$\mathrm{c}2$^$\mathrm{a}\mathrm{b}$$)and$B \parallel c$($B_$\mathrm{c}2$^$\mathrm{c}$$)\mathrm{.}Theuppercriticalfields$B_$\mathrm{c}2$^$\mathrm{a}\mathrm{b}$$(0)and$B_$\mathrm{c}2$^$\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 $B_{\mathrm{c}2}(0)$ values and the $B_{\mathrm{c}2}(T)$ 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, $\mathrm{\Gamma }=B_{\mathrm{c}2}^{ab}\mathrm{/}{B}_{\mathrm{c}2}^c$, in EuFe${}_2$As${}_2$ at low temperatures is comparable to that of other “122” Fe-based systems.