Inverse Iron Isotope Effect on the Transition Temperature of the $(\mathrm{Ba},\mathbf{K}){\mathrm{Fe}}_2{\mathrm{As}}_2$ Superconductor

We report that the $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_2{\mathrm{As}}_2$ superconductor (transition temperature, $T_c\sim 38\mathrm{K}$) has an inverse iron isotope coefficient ${\alpha }_{\mathrm{Fe}}=-0.18(3)$ (where $T_c\sim M^{-\alpha \mathrm{Fe}}$ and $M$ is the iron isotope mass); i.e., the sample containing the large iron isotope mass depicts a higher $T_c$. Systematic inverse shifts in $T_c$ were clearly observed between the samples using three types of Fe isotopes (${}^{54}\mathrm{Fe}$, natural Fe, and ${}^{57}\mathrm{Fe}$). This indicates the first evidence of the inverse isotope effect in high-$T_c$ superconductors. This anomalous mass dependence on $T_c$ implies an exotic coupling mechanism in Fe-based superconductors.

Figure 1

X-ray diffraction patterns of the samples $S2({}^{57}\mathrm{Fe})$ and $S2({}^{54}\mathrm{Fe})$ synthesized simultaneously in a BN crucible by high-pressure synthesis technique. The inset shows the sample cell assembly for the high-pressure synthesis method.

Figure 2

(a)–(c) Temperature ($T$) dependence of susceptibility ($\chi$) of the three sample sets using three different combinations of the isotopic Fe: (a) $S1({}^n\mathrm{Fe})$ and $S1({}^{57}\mathrm{Fe})$, (b) $S2({}^{57}\mathrm{Fe})$ and $S2({}^{54}\mathrm{Fe})$, and (c) $S3({}^{54}\mathrm{Fe})$ and $S3({}^n\mathrm{Fe})$. The $\chi$ is normalized at the values of the ZFC curve at 5 K.

Figure 3

Temperature ($T$) dependence of resistivity ($\rho$) of the sample $S2({}^{57}\mathrm{Fe})$ and $S2({}^{54}\mathrm{Fe})$. The Fe isotope exponent is estimated to be ${\alpha }_{\mathrm{Fe}}=-0.22(2)$.

Figure 4

The iron isotope mass dependence of $T_{c(\chi )}$ for the seven sets of the samples $S1–S7$. Fe isotope exponent ${\alpha }_{\mathrm{Fe}}$ is $-0.18(3)$.