Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 47 (2014)µSR study of the spin correlation in iron-chalcogenide
superconductors Fe1−y My Se0.3 Te0.7 (M = Co, Ni, Zn)
T. Adachi,∗1,∗2 T. Inabe,∗2 K. M. Suzuki,∗1,∗2 T. Kawamata,∗1,∗2 T. Noji,∗2 H. Guo,∗1 I. Watanabe,∗1and Y. Koike∗1,∗2
In recent years, much attention has been paid to theso-called iron-based superconductors due to their highsuperconducting (SC) transition temperatures, Tc ’s, inthe research ﬁeld of superconductivity. Hsu et al. havediscovered superconductivity with Tc = 8 K in theiron-chalcogenide FeSe.1) It has been found that Tcof FeSe increases through the partial substitution ofTe for Se, shows a maximum of 14 K at x ∼ 0.7 inFeSe1−x Tex and the superconductivity disappears at
x = 1, namely, in FeTe.2) The compound FeTe is not
SC but develops an antiferromagnetic (AF) order at
low temperatures below ∼ 67 K.2) Therefore, one may
guess the mechanism of superconductivity relating tothe AF spin ﬂuctuation in the iron-chalcogenide superconductors, which is similar to the case of high-Tc
cuprate superconductors.
In order to investigate impurity eﬀects on the SCproperties, we have grown impurity-substituted single
crystals of Fe1−y My Se0.3 Te0.7 (M = Co, Ni, Zn) with
y = 0 − 0.05 and have measured the in-plane electrical resistivity.3) Here, Co and Ni ions are expected todope one and two electrons, respectively, while Zn is
expected to dope no electrons. As a result, it has beenfound that Tc is reduced through the Co substitution
and that the reduction of Tc is more signiﬁcant in theNi substitution than in the Co substitution. On theother hand, the decrease in Tc with the Zn substitution has been found to be negligibly small. Moreover,it has been found that both Tc and the residual resistivity depending on the impurity concentration cannot beexplained by the Abrikosov-Gor’kov theory, suggestingthat a glue to form electron pairs is not the spin ﬂuctuations4,5) but the orbital ﬂuctuations.6)
Therefore, in order to investigate the spin ﬂuctuations directly, we have performed µSR measurementsof Fe1−y My Se0.3 Te0.7 (M = Co, Ni, Zn) with y =0 − 0.05.7) Zero-ﬁeld (ZF) and longitudinal-ﬁeld µSR
measurements were carried out using a Variox cryostat
at temperatures down to 1.6 K at RIKEN-RAL.
For the impurity-free crystal of y = 0, it has beenfound that ZF µSR spectra are independent of temperature at low temperatures down to 1.6 K, indicating that the crystal is in a paramagnetic state. Figure 1 shows ZF spectra of the Zn-substituted crystal of Fe1−y Zny Se0.3 Te0.7 with y = 0.05. At 29 K,
the spectrum shows slow depolarization of muon spins
similar to that observed in the impurity-free crystal.
On the other hand, the depolarization becomes fast
∗1
∗2

with decreasing temperature and the initial asymmetry is missing at 1.6 K. These results suggest signiﬁcant development of the spin correlation through theZn substitution. For the Co and Ni substitution, it
has been found that the spin correlation is enhanced
at y = 0.02 and that further substitution of impurities
leads to weakening of the spin correlation at y = 0.05.
These µSR results depending on the amount and kindof impurities strongly suggest that not the spin ﬂuctuations but the orbital ﬂuctuations may be a glue toform electron pairs in the iron-chalcogenide superconductors.
In summary, we have found impurity-induced development of the spin correlation in Fe1−y My Se0.3 Te0.7(M = Co, Ni, Zn) from µSR measurements. The
present µSR results strongly suggest that the formation of electron pairs is mediated by the orbital ﬂuctuations in iron-chalcogenide superconductors.

Fig. 1. Zero-field µSR time spectra of Fe1−y Zny Se0.3 Te0.7
with y = 0.05.

References

RIKEN Nishina Center
Department of Applied Physics, Tohoku University- 247 -1) F.-C. Hsu et al.: Proc. Natl. Acad. Sci. USA 105,14262 (2008).
2) T. Noji et al.: J. Phys. Soc. Jpn. 79, 084711 (2010).
3) T. Inabe et al.: J. Phys. Soc. Jpn. 82, 044712 (2013).
4) I. I. Mazin et al.: Phys. Rev. Lett. 101, 057003 (2008).
5) K. Kuroki et al.: Phys. Rev. Lett. 101, 087004 (2008).
6) H. Kontani et al.: Phys. Rev. Lett. 104, 157001 (2010).
7) T. Adachi et al.: in preparation.