RIKEN54(2021)(2021)RIKENAccel.
Accel.Prog.
Prog. Rep.
Rep. 54

I. HIGHLIGHTS OF THE YEARµSR study of slightly pressurized organic superconductor κ-(ET)4 Hg2.89 Br8

∗1
∗2
∗3

College of Engineering, Shibaura Institute of Technology
RIKEN Nishina Center
Department of Physics, Saitama University

Normalized Asymmetry

0.0
‒ 0.2
‒ 0.4
‒ 0.6
‒ 0.8
‒ 1.0 0(a)135234Temperature (K)

56(c)

0.8
0.6
0.4
0.2
0.002468Time (μs)

1210120.35

λ (μs ‒1)

252015100.300.25

5001.00.4030

H c1 (Oe)Organic superconductors are single-band system, similar to high-critical-temperature (high-Tc ) cuprates superconductors. A distinct diﬀerence can be observed intheir lattice, i.e., squared cuprates and triangular organics. In the case of squared cuprates, the Mott insulating state has an antiferromagnetic (AF) ground statein the 1/2-ﬁlled band case, while the anisotropic triangular (t = t′ ) organics also exhibit AF state. Here, tand t′ are the nearest and next nearest transfer integral between sites, respectively. For a triangular lattice
Mott insulator, owing to geometrical frustration (t ∼ t′ ),the system cannot be magnetically ordered down to themili-Kelvin order, i.e., a spin liquid state. Owing to hole
doping, it should also become metallic and superconducting (SC). However, the realistic candidate material
was limited until the discovery of hole-doped organic superconductor κ-(ET)4 Hg2.89 Br8 (κ-HgBr). In metallicstate, resistivity exhibits the linear-temperature dependence, ρ ∝ T , which is not a Fermi-liquid behavior. The
susceptibility from 300 to 2 K is nearly perfectly scaled
to that of a non-doped spin liquid organic insulator κ(ET)2 Cu(CN)3 .1) By pressure, this non-Fermi-liquid behavior turns into conventional Fermi-liquid behavior at
the Pc = 0.5 GPa, where Tc is also the highest (∼7 K)
in the pressure-temperature phase diagram.2) The nonFermi-liquid behavior at low-pressure region is similar
to the metallic state of high-Tc cuprates or low-pressure
range of multi-band heavy fermion CeCoIn5 , and it isreferred to as a strange metallic region.1,2) Because κHgBr exhibits similarities with other strongly correlated
electron systems, it is interesting to determine the typeof Cooper pairing that occurs in κ-HgBr.
We aim to determine the pairing symmetry in κHgBr by µSR. We performed µSR measurement down
to 0.3 K on the ARGUS spectrometer at the RIKENRAL muon facility with HELIOX cryostat and ﬂy-path
setup. We developed a technique for applying a decent pressure on κ-HgBr crystals because at ambientpressure, the SC state of κ-HgBr is not bulky due tothe inhomogeneous state.3) Approximately 130 mg crystals were carefully aligned and stuck together using diluted polymer glue. This strategy was eﬀective for applying enough pressure on the sample as we conductedmagnetization measurement using SQUID with a similar sample setup. Figures 1(a) and (b) show the resultof SQUID measurement. The demagnetization eﬀect dueto sample shape was treated for all analyses. The temperature dependence of susceptibility determined Tc =4.6(2) K whereas the ﬁeld dependence of magnetization(MH-curve) was measured at several temperatures; theSC volume fractions were estimated at each temperature. Consequently, the estimated SC volume fraction
at 2 K was found to be approximately 90%, as shown inDiamagnetic Volume Fraction

D. P. Sari,∗1,∗2 Y. Ishii,∗1 I. Watanabe,∗2 and H. Taniguchi∗3(b)1234
5Temperature (K)60.20
0.1(d)
24 64 610Field (G)24 6100Fig. 1. (a) Temperature dependence of SC volume fraction
measured by SQUID. (b) Temperature dependence of Hc1measured by SQUID down to 2 K compared with µSRmeasurement. (c) ZF-µSR time spectra before and after
applying TF of 40 G. The solid lines are the exponential
decay fitting lines. (d) Relaxation rate of ZF-µSR time
spectra after applying several TF, from 2 to 40 G.

Fig. 1(a). Comparing this with the referred result obtained from ac-susceptibility measurement,3) a pressureof at least 0.3 GPa was applied to the sample. This expectation was conﬁrmed by the estimation of the lowercritical ﬁeld, Hc1 , by SQUID, resulting in the absolutevalue of Hc1 at 0 K, i.e., 27 Oe. The result is consistent
with the measurement performed using zero-ﬁled (ZF)
µSR at 0.3 K. Hc1 was estimated as follows. The sample was ZF-cooled to 0.3 K. Transverse-ﬁeld (TF) wasapplied for approximately 10 min to destroy the shielding ﬁeld. After cutting oﬀ the TF, the ZF-relaxation
rate was measured. At TF = 25 Oe, the relaxation
rate started to increase. Therefore, we concluded thatthe sample was in bulk condition and a slight pressure,P ≳ 0.3 GPa, was applied. Using these results, we measured the ZF-µSR relaxation rate in κ-HgBr down to0.3 K. The temperature independence of ZF-relaxation
rate was observed, indicating that spontaneous internalﬁelds do not appear in the superconducting state. This
result is consistent with that in Ref. 4), in which the measurement was performed down to 1.5 K and at ambientpressure. Thus, the possibility of spin triplet and d + idwave symmetry in κ-HgBr due to the expected triangularlattice in κ-HgBr becomes very low. The temperaturedependence of London penetration depth measurement
using TF-µSR is necessary for determining the SC gap
symmetry, which will be discussed in the future work.
References

1) H. Oike et al., Nat. Commun. 8, 756 (2017).
2) H. Taniguchi et al., J. Phys. Soc. Jpn. 76, 113709 (2007).
3) H. Oike et al., Phys. Rev. Lett. 114, 067002 (2015).
4) K. Satoh et al., Physica B 404, 597 (2009).

- S30 -1