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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Zero-field µSR on the out-of-plane superconductivity ofλ-(BETS)2 GaCl4
D. P. Sari,∗1,∗2 K. Hiraki,∗3 I. Watanabe,∗1 and Y. Ishii∗1λ-(BETS)2 GaCl4 is a quasi-two-dimensional organic
metal and it is reported as a high-anisotropic type-II superconductor. It exhibits rich physical properties once
we substitute Ga with Fe, Cl with Br, or once we reduce the Se content in the BETS molecule (BETS =(CH2 )2 S2 Se2 C6 S2 S2 (CH2 )2 ).1) In the P-T phase diagram, the neighboring phase adjacent to the superconducting (SC) state is the Mott insulating state, which isantiferromagnetically ordered, and separated by a suspicious paramagnetic insulating state with no magnetic
ordering down to 0.3 K.2) A theoretical study based on
anisotropic spin fluctuations-mediated superconductivity suggested an anisotropic gap structure with two nodallines,3) whereas 13 C-NMR study evidenced two types ofmagnetic fluctuations of the π-electron down to 2 K.4)
There is an urgent requirement for the detailed experimental determination of the SC gap structure. We performed a series of transverse-field (TF) µSR measurement to determine the SC gap structure. First, we foundthat the SC gap structure of λ-(BETS)2 GaCl4 was unusual as it exhibited both d-wave and s-wave symmetry
characteristics.4,5) Furthermore, considering the study
by Powell and McKenzie for entire families of organic
superconductors, we plan to confirm whether time reversal symmetry breaking is related to this unusual SCgap structure. Zero-field (ZF) µSR is a powerful tool todetect such breaking symmetry in the SC state.
We recently performed a series of zero-field µSR using aligned single crystals. We prepared ∼130 mg single crystals and oriented them in the same direction.
Measurements were performed using the ARGUS spectrometer at the RIKEN-RAL pulse muon facility with aHELIOX cryostat and fly-path setup. The single pulse
mode was used to collect 80M events. In a previous
report6) we showed the result when the polarized muon
beam direction was nearly perpendicular to the conducting plane. We prepared another sample setup (∼100 mg)
to measure with the polarized muon beam direction parallel to the conducting plane. In this report, we compare
both measurements to gain information regarding outof-plane superconductivity.
Figure 1 shows normalized time spectra at several
temperatures below and above the critical temperature(Tc ∼ 5.5 K). Open circle curves indicate normalized
time spectra with the muon beam direction perpendicular to the conducting plane.6) Blue solid lines indicate fitting lines using a stretched exponential function
A(t) = A0 exp(−(λt)β ). At 1.5 K, λ and β values were0.2281(35) µs−1 and 1.123(16), respectively. No temperature dependence was observed between the normaland SC states. Closed circle curves indicate normal∗1∗2∗3

College of Engineering, Shibaura Institute of Technology
RIKEN Nishina Center
Department of Physics, Fukushima Medical UniversityFig. 1. Normalized ZF-µSR time spectra with the setup ofµ+ injected parallel and perpendicular to the conducting plane. The solid lines represent fitting lines obtainedusing the stretched exponential function.

ized time spectra with the muon beam direction parallel
to conducting plane, which reflects the intrinsic behavior of the out-of-plane properties. Although there wasno temperature dependence between the normal and SCstates, we found that the shape of the spectra was distinct with the open circle spectra showing in-plane properties in λ-(BETS)2 GaCl4 . All spectra were best fit by astretched exponential function. At 1 K, λ and β values
were 1.881(21) µs−1 and 1.34(2). In-plane properties
have a stronger relaxation while the shape is toward asingle exponential function. λ-(BETS)2 GaCl4 has highanisotropy SC properties such as the large ratio of inplane/out-of-plane upper critical field and transfer integral, Hc2|| /Hc2⊥ > 4 7) and t|| /t⊥ ∼ 13,3) respectively.
Then, the stronger in-plane λ can be related to the antiferromagnetic fluctuations of the π-electron within theconducting plane, which mediates the superconductivity.
These results confirm that the exotic SC state behavior proposed in this material based on TF-µSR could bebeyond triplet superconductivity or time-reversal symmetry breaking properties. The next step is to relatethis ZF-µSR result with the TF-µSR results using singlecrystals.
References

1) H. Kobayashi et al., Chem. Rev. 104, 5265 (2004).
2) T. Minamidate, RIKEN Accel. Prog. Rep. 51, 205 (2018).
3) H. Aizawa et al., J. Phys. Soc. Jpn. 87, 093701 (2018).
4) T. Kobayashi, A. Kawamoto, PRB 96, 125115 (2017).
5) D. P. Sari, PhD Thesis, Osaka University (2018).
6) D. P. Sari, et al., submitted (2020).
7) D. P. Sari, RIKEN Accel. Prog. Rep. 53, 155 (2018).
8) M. Tanatar et al., J. Superconductivity 12, 511 (1999).

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