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

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)µSR study of slightly pressurized organic superconductor
κ-(ET)4 Hg2.89 Br8 , II
D. P. Sari,∗1,∗2 Y. Cai,∗3,∗4 K. Kojima,∗3,∗4 I. Watanabe,∗2 Y. Ishii,∗1 and H. Taniguchi∗5The
hole-dopedorganicsuperconductor
κ-(ET)4 Hg2.89 Br8 , (κ-HgBr), where bis (ethylenedithio) tetrathiafulvalene is abbreviated as ET, is a key
to bridge the knowledge gap between half-ﬁlled organics and doped cuprate systems. Both κ-HgBr andcuprate superconductors exhibit a peculiar metallic
state at high temperature and pressure where theirresistivity exhibits a linear temperature dependence,ρ ∝ T ; this is non-Fermi-liquid (FL) behavior. In κHgBr, this non-FL region gradually changes to the FL
state by owing to pressure,1,2) similar to the change
in metallic state from optimally doped to overdoped
cuprates. A heat capacity study suggested that theenhanced antiferromagnetic ﬂuctuations toward lowtemperature cause the non-FL behavior of κ-HgBr.3)
This evidence may locate the superconducting κ-HgBrstate near the quantum critical point in between theFL and localized states; incoherent conductivity wasobserved in its non-FL state.1,2) In this region, low
superﬂuid density was proposed by the macroscopic
measurement.4)
Our zero-ﬁeld µ+ SR experiment indicated that therelaxation rate from a temperature of approximately
10 K down to 0.3 K is temperature-independent.5) Thisis consistent with a superconducting state that preserves time-reversal symmetry. Furthermore, we measured the transverse-ﬁeld (TF) µ+ SR on ARGUS at
the RIKEN-RAL/ISIS muon facility. There was almost no change in the 10 mT of transverse-ﬁeld-µ+ SR
time spectra at 0.3 K and above superconducting (SC)temperature Tc = 4.6(3) K, indicating that the London penetration depth λbc , is longer than the order ofmicrometer, that is, low superﬂuid density. To conﬁrm this result, measurements using ano-ther geometric setup are necessary to minimize the sample misalignment and determine the absolute value of λbc .
Further, we performed the µ+ SR experiment at theM15 beamline at TRIUMF. The µ+ spin rotator wasused so that the sample setup illustrated in Fig. 1 could
be arranged. In all, 100 mg of the sample was used.
The plate-like crystals we aligned so that the magnetic
ﬁeld is perpendicular to the conducting bc-plane. Using this setup on the M15 beamline, the crystal misalignment can be minimized, and larger TF 10 times
higher than Hc1 = 25(5) Oe, which lies in the vortex
solid regime6) can be applied.
∗1
∗2
∗3
∗4
∗5Fig. 1. Sample setup in the perpendicular mode µ+ [sr]stand for the muon beam with a spin rotator. In theperpendicular mode, the TF is applied perpendicular
to the plate-like conducting plane. The right panel displays a photograph of the sample setup on top of Ag
plate of the cryostat sample holder.

Furthermore, the bulk SC state must be conﬁrmed.
Therefore, we ﬁrst measure the TF-µ+ SR upon zeroﬁeld cooling and compare the spectra above and below Tc = 4.6(3) K. Figure 2 depicts the TF-µ+ SR
time spectra measured at 0.3 and 10 K under 50 mT,respectively. The spectrum measured at 0.3 K wasclearly damped compared to that at 10 K owing to thebulk SC state. The spectra were analyzed by applying the Gaussian-type-damped cosine equation (represented by lines in Fig. 2) as,Fig. 2. TF-µ+ SR time spectra upon zero-ﬁeld cooling in
κ-HgBr. The pink and cyan circles represent TF-µ+ SR
time spectra measured at 0.3 and 10 K, respectively.
The red and blue solid lines represent the ﬁtting lines
for the 0.3 and 10 K data, respectively.

A(t) = A1 exp[−σ 2 t2 ] cos(γµ B1 t + ϕ)
+ A2 cos(γµ B2 t + ϕ),College of Engineering, Shibaura Institute of Technology
Meson Science Laboratory, RIKEN
SBQM Institute, University British Columbia
CMMS, TRIUMF
Department of Physics, Saitama University(1)

where the ﬁrst and second terms on the right-hand side

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

RIKEN Accel. Prog. Rep. 56 (2023)

describe muon components from the sample and background, respectively. The damping rate σ at 0.3 K wasdetermined as 0.53 µs−1 . The next step is to measure
the temperature dependence of the London penetration depth in the vortex state under the ﬁeld coolingcondition.
References
1) H. Taniguchi et al., J. Phys. Soc. Jpn. 76, 113709(2007).
2) H. Oike et al., Nat. Commun. 8, 756 (2017).
3) A. Naito et al., Phys. Rev. B 71, 054514 (2005).
4) K. Wakamatsu et al., arXiv.2201.10714.
5) D. P. Sari et al., J. Phys. Conf. Ser. 2462, 012061(2023).
6) D. P. Sari et al., in Materials Science Forum, Vol. 966(Trans Tech Publications Ltd., Sanur, Bali, Indonesia,2019), p. 296.

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