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

RIKEN Accel. Prog. Rep. 52 (2019)

Superconductivity in single crystals of λ-(BETS)2 GaCl4 studied bytransverse-field µSR
D. P. Sari,∗1,∗2 K. Hiraki,∗3 R. Asih,∗4,∗2 I. Watanabe,∗2 and Y. Ishii∗1The superconducting gap structure in λ(BETS)2 GaCl4 is intriguing and has been consistently
studied using µSR. We performed transverse field (TF)
µSR at fields of 150 Oe down to 0.3 K using a 120 mg
randomly oriented sample, at the ISIS Muon Facility inthe UK. The temperature dependence of the superfluid
density deduced from that experiment is best describedby the s + d-wave with a dominant s-wave component.1)
In order to get clearer and more direct evidence in termsof the superconducting gap structure determination, we
performed similar measurements using a 75 mg wellaligned sample by applying the external field perpendicular to the conducting ac-plane. Here, we report theTF dependence of the London penetration depth, whichleads to the upper critical field H c2 estimation, in orderto check the quality of the experimental setup in aligned
single crystals of λ-(BETS)2 GaCl4 .
Figure 1 shows a typical TF-µSR spectra with a fieldof 150 Oe in the aligned single crystals at T = 0.3 K
and 10 K after being cooled through critical temperature
Tc ∼ 5.3 K.
The time spectra were fitted by using Eq. (1),()
2
A(t) = 0.3438 × e−(σt) cos γµ Hint1(sample) t + ϕ)((1)+ 0.652 cos γµ Hint2(Ag foil) t + ϕ

The two oscillation components represent about 35%
signal from the sample and 65% signal from the silverpackage. The damping rate σ becomes prominent oncethe system enters the superconducting state. The superconducting component of the damping rate σSC is222= σ 2 − σNM, where σNMis the signal
then given by σSCσT =10 K in the normal state due to the nuclear moments.
Furthermore, the TF dependence of σSC can be used todetermine the London penetration depth λ and to givean estimate for the Hc2 , following Eq. (2),Fig. 1. The normalized TF time spectra in the applied field of150 Oe measured at T = 0.3 K (black) and 10 K (green).
The red and black solid lines are the best-fit results obtained by using Eq. (1). The trace of the envelope of thecosine curve represents the damping rate σ of Eq. (1).

(2)

Fig. 2. Transverse-field dependence of damping rate of themuon-spin precession in the superconducting state, σSC ,of λ-(BETS)2 GaCl4 taken at 0.3 K. The red and green
circles indicate the data for randomly oriented and welloriented crystals, respectively. The inset figure shows thealigned crystal mounted in a silver foil. The black arrow
indicates the direction of the applied field.

where Happ = 150 Oe and λis in nm.2)
Figure 2 shows the estimation of H c2⊥ from the measurements of the aligned crystals. The resulting Hc2⊥ ,which is parallel to the b-axis, is 41 ± 19 kOe, and theabsolute value of the London penetration depth at 0 K,
λ(0) = 710 ± 12 nm. The estimation of H c2 from theprevious TF-µSR measurement on a randomly oriented
sample was 43±25 kOe.1) From magnetoresistance measurements, the H c2 of λ-(BETS)2 GaCl4 was reported tohave a high anisotropy. The H c2 parallel to a, b, andc-axes were 150 kOe, 30 kOe, and ∼ 130 kOe, respectively.3) In comparison with the H c2 in the randomly oriented crystals, the estimation of H c2⊥ is closer to the reference of the magnetoresistance measurement, although
the signal we obtained from the sample was smaller thanthat of the background from silver. It implies that thesample alignment was good enough. Taking into accountthe low sample mass, we will increase it up to ∼ 120 mgin order to obtain a stronger signal and solid results.

√[)3 ](√λ−22σSC (H) = 4.83 × 104 × (1 − Happ /Hc2 ) × 1 + 1.21 1 − Happ /Hc2

∗1∗2
∗3
∗4Graduate School of Engineering and Science, Shibaura Inst.
of Tech.
RIKEN Nishina Center
Department of Physics, Fukushima Medical University
Institut Teknologi Sepuluh Nopember

References

1) D. P. Sari, et al., to be submitted.
2) E. H. Brandt, Phys. Rev. B, 68, 054506 (2003).
3) M. A. Tanatar, et al., Supercond. 12, 511 (1999).

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