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

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 47 (2014)

Muon LCR measurements for organic magnets based on [Pd(dmit)2 ]metal-complex molecules†
M. Abdel Jawad,∗1 Y. Ishii,∗2 S. Masubuchi,∗2 R. Kato,∗1 I. Watanabe,∗3 and F. L. Pratt∗4
Magnetic ground states of quasi two dimensional(Q2D) triangular Heisenberg antiferromagnetic (AF)systems are of great interest. Magnetic frustration
arising from the triangular exchange network suppresses the AF order. This kind of quantum-spinstates without either long-range magnetic order or lattice symmetry breaking is named quantum spin liquid (QSL) state. Theoretically, this QSL state has
been studied extensively and many classes of theoretical models such as Z2 spin liquid, spinon fermi surface(SFS) and spin-bose metal are proposed. Althoughexperimentalist have sought real model materials withQSL state for quite some time, only a few candidate
materials are known to this date.
A series of organic salts, (Cation)[Pd(dmit)2 ]2(dmit=1,3-dithiole-2- thione-4,5-dithiolate) has triangular exchange network of S = 1/2 unit of moleculardimers1) . The strength of the spin frustration can becontrolled by the choice of cation and most of materials belonging to this family undergo AF states.
In such triangular magnets, geometrical frustration isthough to play an important role on the magnetic
state as the AF transition temperatures is found toincrease proportionally to the deviation from the regular triangular exchange networks. It is thought that
EtMe3 Sb[Pd(dmit)2 ]2 2) do not show any AF order dueto strong spin frustrations. These materials have almost regular triangular exchange networks with exchange interaction J of the order of 200 to 300 K.
Recently, we have performed longitudinal field (LF)
µSR measurements on EtMe3 Sb[Pd(dmit)2 ]2 a QSLcandidate. Preliminary analysis suggest that field dependenceof muon relaxation rate, λ is proportional to√
1/ B behaviour in a field range of 1 ≤ Bext ≤ 1000
Gauss at low-temperatures. Such a behaviour is expected from spins diffusing along a one-dimensional
direction. In an ideal 1D spin system model, the fielddependence of λ is approximately described as follows:λ(ω) =1A2
42D� ω(1)where D� is the diffusion rate of spinon, ω = γe B isthe Larmor frequency and A is the scalar hyperfine
coupling constant3) . In order to determine D� andits temperature dependence quantitatively, a Muon
level-crossing resonance measurements (LCR) of thePd(dmit)2 molecule was made. For this purpose, we
∗1
∗2
∗3
∗4

Condensed Molecular Materials Laboratory, RIKENTokyo Medical University
RIKEN Nishina CenterISIS Facility- 257 -used (EtMe3 Sb)2 [Pd(dmit)2 ], a non-magnetic material, instead of the neutral Pd(dmit)2 molecule due tochemical stability issues.

Fig. 1. Longitudinal polarization for the muon radical in(EtMe3 Sb)2 [Pd(dmit)2 ] at 10K and 300K together withfitted repolarization curves..

Results of the longitudinal field dependence in(EtMe3 Sb)2 [Pd(dmit)2 ] at port-2 of the RIKEN-RAL
Muon facility are shown in Fig. 1. The Muon repolarization curve show no resonance in fields up to 0.4 Teslaand show an incredibly broad distribution of hyperfine
couplings at 10K. Preliminary DFT calculation withmuon radicals at the sulphur ends of the [Pd(dmit)2 ]−2molecule predicts anisotropic hyperfine parameters (A,D1, D2)=(360, 11, 5) MHz and a resonance around 1T.
Fit of the curves require at least a four-term equation
with hyperfine values shown within the graph. The
deduced values appear to be at least twice those foundfrom the DFT calculation which support the hypothesis of the Muonium formation at the sulphur ends ofthe [Pd(dmit)2 ]−2 molecule. However another large
hyperfine term with GHz values must be added to fit
the repolarization curve indicating at least one more
predominant Muonium site with a higher field resonance.
Further LCR measurements, in magnetic fields up
to 5 Tesla in the Hi-Fi magnet at the ISIS muon facility have recently been awarded and will complete thisstudy to understand the Muonium sites on a Pd(dmit)2
molecule and the hyperfine coupling constant associated with such sites.
References
1) K. Kanoda and R. Kato: Annu. Rev. Condens. Matter
Phys. 2, 167-188 (2011).
2) M. Yamashita et al.: Science, 328, 1246-1248 (2010).
3) F. L. Pratt et al.: Phys. Rev. Lett. 96, 247203 (2006).

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