RIKEN Accel. Prog. Rep. 50 (2017)

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

Study of muon spin rotation of the superconducting state of organic
superconductor λ-(BETS)2GaCl4D. P. Sari,*1,*2 R. Asih,*1,*2 K. Hiraki,*3 Y. Ishii,*4 T. Nakano,*2 Y. Nozue,*2 and I. Watanabe*1
The Cooper pairing symmetry of the third-generation
organic superconductor λ-(BETS)2GaCl4 has attracted
interest owing to the strongly correlated nature of this systemlying near the Mott insulating phase.1) A recent highresolution thermodynamic measurement reported d-wave
pairing symmetry 2) whereas other experiments such as
microwave conductivity measurement reported s-wave.3) We
report our experimental result of zero field (ZF) andtransverse field (TF) μSR in the fields of 30 G down to 0.3
K at the RIKEN-RAL Muon Facility in the UK. From ZFμSR, a slight increase in the muon-spin relaxation rate wasobserved below TC = 5 K indicating a signature of theappearance of an unconventional SC state. The TF-μSR time
spectrum at the base temperature of 0.3 K shows a dampingbehavior in comparison with that of the one at the normal
state at 10 K owing to the appearance of the flux state, which
produces a distribution of penetrated magnetic fields in thesample, as shown in Fig. 1.

Fig. 1. TF-μSR time spectra of λ-(BETS)2GaCl4 in TF = 30
G at 0.3 K and 10 K. The red and blue dashed line are theresults of fitting the data with Eq. (1).
We analyse the time spectra by using the following function
 .

8 8 < %  (=5 1 >  !3 % : # : &  =!3 & : #> (1)Here, σ is the Gaussian damping rate representing thesymmetric field distribution in the vortices felt by muons.
The % is the averaged field at the muon site in the sample
and & is the one in the Ag foil. % and & are initialasymmetry parameters of the Gaussian-type damping and thebackground components, respectively. A2 was fixed to be that
achieved at 0.3 K, and # is the phase of the muon-spin
precession. In the normal state, σ was estimated to be 0.1172± 0.0023 μs-1.
The Gaussian damping rate in the SC state at 0.3 K, σSC,is estimated to be 0.1708 ± 0.0022 μs-1 by the relation σ2 =σSC2 + σNM2 where σNM is the Gaussian damping rate by the1RIKEN Nishina Center
Department of Physics, Osaka University
3
Department of Physics, Gakushuin University
4
Department of Physics, Shibaura Institute of Technology
2Fig. 2. Extracted temperature dependence of the normalized
inverse magnetic penetration depth squared. The red, blue,and green solid lines show the result of fit using Eq. (2).
nuclear moment in the normal state. For a pollycrystalline
sample and large Ginzburg-Landau parameter κ70, σSC isrelated to the superconducting penetration depth by therelation σSC =  $ "& , where $ = 2.07 x 10-15 is
the flux quantum.
We could find a d–wave pairing contribution in thepower-law fitting of "(T) as a deviation from that expected
for a typically full-gap s–wave pairing, power-value of whichis 4. Furthermore, "(T) curves can be fit in the clean limit
using the following expression 4):
298 =.>

 982<:% &4 )60
=.> 6,4 $,/,/7, 8 (8 =7 .>(2)where  < A :   *  B(% is the Fermi function andthe temperature dependence of the gap is approximated by  <   ? A  +  ;  B$ '% @ . The angularfunction  # < $ in the s-wave model and  # <$ cos=#> in the d-wave model, where $ is constant. Thecurve can be well fit by the s-wave model. Interestingly, thedata just below the Tc down to about 3 K well follow thefitting of the d-wave model and show a close agreement withthe extracted critical temperature TC = 5.1(1) K. Furthermore,
we attempted to fit the data by using a simple superpositionof a single-gap s-wave and single-gap d-wave, as shown inFig. 2, in order to study if there is any hint of coexistence ofs- and d-wave. Further experiments on TF-μSR under
different fields and with higher statistics may be needed tomake the current experimental results convincing.
References1) H. Kobayashi et al., Phys. Rev. B. 56, 8526(R) (1997).
2) S. Imajo et al., J. Phys. Soc. Jpn., 85, 043705 (2016).
3) T. Suzuki et al., Physica C. 440, 17 (2006).
4)

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S. Chandrasekhar, D. Einzel, Ann. Phys. 505, 535 (1993).