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

RIKEN Accel. Prog. Rep. 51 (2018)µSR study of FeSe1−x Sx around nematic critical point
Y. Tanabe,∗1 R. Asih,∗2,∗3,∗4 I. Watanabe,∗2,∗3 A. Takimoto,∗1 T. Urata,∗1 and K. Tanigaki∗1,∗5Iron-based superconductors (FeSCs) provide an intriguing platform where the spin and orbital degreeof freedom can contribute to the emergence of exotic
phenomena including high temperature superconductivity. Superconductivity was usually observed in mostFeSCs when antiferromagnetism in the parent materials was suppressed by chemical doping, indicating thecompetition between superconductivity and antiferromagnetism.1) On the other hand, another electronic
state called electronic nematicity was clarified where
the lattice C4 symmetry is spontaneously broken bysome electronic origin.2) The relation between superconductivity and electronic nematicity is one of key
piece to understand the complex phase diagram. The
recent nonmagnetic nematic states in FeSe may provide
an opportunity to clarify this key piece. By sulfur doping on the selenium site, electronic nematicity was suppressed and superconductivity tended to be enhanced
around the nematic critical point.3) This may indicate
a competitive relationship between superconductivity
and electronic nematicity, whereas the magnetic phase
diagram is still controversial in the FeSe1−x Sx system.
In this report, we studied the magnetic phase diagramof FeSe1−x Sx (0.09 ≤ x ≤ 0.15) from the view of electrical transport and muon spin relaxation.
From electrical transports, the electronic phase diagram of FeSe1−x Sx was still unclear below the superconducting transition temperatures. Figure 1(b)–(d) shows the temperature dependence of the electricalresistivity in FeSe1−x Sx single crystals under various
magnetic fields (Bs). At zero magnetic fields, the resistivity curves showed a kink due to the tetragonal tothe orthorhombic structural transition at temperature
ranges from 30–60 K in the present sulfur doping range.
At low temperatures, superconducting transitions wereobserved at around 10 K. Both the superconducting
transition temperature (Tc ) and the structural transition temperature (Ts ) were summarized in Fig. 1(a).
By sulfur doping, Ts continuously decreased and disappeared around x = 0.16 together with the broad maximum of Tc around 0.125. Since the electronic nematic
phase were previously clarified below x = 0.15 in theorthorhombic phase, the normal state resistivity curves
unveiled by high magnetic fields would provide further
information about the relationship between superconductivity and electronic nematicity. Under the magnetic fields, the resistivity curves showed a suppression
∗1
∗2
∗3
∗4
∗5

Department of Physics, Tohoku University
RIKEN Nishina Center
Department of Physics,Graduate School of Science, OsakaUniversity
Department of Physics, Faculty of Mathematics and NaturalScience, Institut Teknologi Sepuluh Nopember
WPI-AIMR, Tohoku University

Fig. 1. (a) Electrical phase diagram of FeSe1−x Sx . (b)–(d)Electrical resistivity curves for x = 0.09 − 0.15 undervarious magnetic fields. (e) µSR time spectra for x =0.15 from 1.7 K to 20 K.

of Tc with an increase in B. For x = 0.125 − 0.15,
other kink structures tended to be developed around
5 K, which resemble resistivity curves for the pressure
induced antiferromagnetic transition in FeSe.4) To clarify the hidden magnetic phase below Tc , we carried out
the zero field (ZF) µSR measurements in FeSe1−x Sx
as shown in Fig. 1(e). ZF-µSR showed the relatively
exponential-like time spectra from 1.7 K to 20 K, being
similar with those of the FeSe single crystal. All spectra
considerably overlapped. These demonstrated no development of magnetism where anomalies were detected in
the resistivity curves under high magnetic fields.
For x = 0.125–0.15, resistivity anomalies under highmagnetic fields suggested a development of another newphase at low temperatures whereas no development ofmagnetism was detected by the ZF-µSR measurements.
An origin of resistivity anomaly is still unclear in thepresent states. Since the nonmagnetic nematic quantum critical point (QCP) would be a key to understandthe mechanism of the superconductivity from the comparison to the magnetic QCP, further studies clarifying
an origin of low temperature anomalies in resistivity
curves may be important to understand the relationship between superconductivity, magnetism and electronic nematicity.
References
1) H. Hosono, K. Kuroki, Physica (Utrecht) 514, 399(2015).
2) R. M. Fernandes et al., Nat. Phys. 10, 97 (2014).
3) S. Hosoi et al., Proc. Nat. Acad. Sci. USA 113, 8139
(2016).
4) J. P. Sun et al., Nat. Commun. 7, 12146 (2016).

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