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

RIKEN Accel. Prog. Rep. 51 (2018)

Muon spin relaxation study on the new organic spin liquid materialλ-(STF)2 GaCl4T. Minamidate,∗1 D. P. Sari,∗2,∗3 N. Matsunaga,∗1 and I. Watanabe∗2
Research on quantum spin liquid (QSL) states infrustrated quantum magnetism is a critical issue inthe ﬁeld of condensed matter physics. In recent
research, some candidate QSL materials have beenfound in molecular-based organic systems, such as
κ-(ET)2 Cu2 (CN)3 1) and EtMe3 Sb[Pd(dmit)2 ]2 .2) The
spin systems in these salts have been well described as
an antiferromagnetic (AF) spin system with a nearly
equilateral regular triangular lattice. Recently, wefound a new candidate QSL material, λ-(STF)2 GaCl4 ,which is related to the organic superconductor λ(BETS)2 GaCl4 . As shown in Fig. 1, down to 2 K,
the static susceptibility of λ-(STF)2 GaCl4 is very welldescribed by the AF Heisenberg model with a regulartriangular lattice, which is the typical behavior of theQSL system.3) In contrast to the good agreement of thesusceptibility behavior, the calculated geometry of thespin interaction in λ-(STF)2 GaCl4 is quite distorted
from the regular triangle. Therefore, it can be the key
material to investigate the stabilization mechanism of
the QSL state.

Fig. 1. Temperature dependence of the static susceptibilityof λ-(STF)2 GaCl4 . The solid line is the susceptibility
curve calculated for the S = 1/2 AF Heisenberg modelin a triangular lattice with an exchange coupling constant J/kB = −165 K.3)

We succeeded in synthesizing a high-quality sampleof λ-(STF)2 GaCl4 with the total mass ∼ 75 mg andconducted ZF-µSR measurement down to 0.3 K. Figure 2 shows the µSR time spectra measured at 0.3 Kin ZF and under an LF of 100 G. The ZF-µSR time
∗1
∗2
∗3

Department of Physics, Hokkaido University
RIKEN Nishina Center
Department of Physics, Osaka University- 205 -spectrum is well described by the simple exponentialfunction. Since the µSR time spectrum is well decoupled by applying LF of 100 G, there is no static internal ﬁeld due to AF long-range ordering. On the otherhand, we can see a slow relaxation behavior even at0.3 K under an LF of 100 G. This implies that there
are some eﬀects of ﬂuctuating internal ﬁelds originating from surrounding electronic spins, and the sample
does not show the long-range AF ordering although it
has a strong AF interaction J/kB ∼ −165 K, at least
down to 0.3 K.
This behavior is similar to that observed in theµSR measurement for the ﬁrst organic QSL salt, κ(ET)2 Cu2 (CN)3 .4) Additionally, the authors of Ref. 4)
found that the LF-µSR time spectrum is described bya two-component exponential function and pointed outthat the system undergoes phase separation into paramagnetic islands and a singlet phase at a low temperature. Therefore, we are now planning an additional
experiment for λ-(STF)2 GaCl4 to observe the spin behavior in the range between ZF and an LF of 100 G.
With the progress of this research, the microscopic spin
dynamics in the QSL state and the stabilization mechanism of the QSL state in a distorted lattice system
can probably be clariﬁed.

Fig. 2. Muon spin depolarization curves of λ-(STF)2 GaCl4
at 0.3 K under zero field and a longitudinal field of
100 G.

References
1) Y. Shimizu et al., Phys. Rev. B 91, 107001 (2003).
2) T. Itou et al., Phys. Rev. B 77, 104413 (2008).
3) T. Minamidate et al., J. Phys. Soc. Jpn. 84, 063704(2015).
4) S. Nakajima et al., J. Phys. Soc. Jpn. 81, 063706 (2012).