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

RIKEN Accel. Prog. Rep. 49 (2016)

Investigation of magnetic ground states in mixed kagome systems(Rb1−x Csx )2 Cu3 SnF12 II
T. Suzuki,∗1,∗3 K. Katayama,∗2 I. Watanabe,∗3 and H. Tanaka∗2∗1∗2∗3Shibauta Institute of Technology
Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center- 214 -measurements in the sample of xc = 0.53 down to 0.3
K, and we reported that internal magnetic ﬁelds ﬂuctuate at 0.3 K, and that no tendency towards a magnetic
phase transition is observed. The purpose of this study
is to microscopically investigate the spin dynamics including ﬂuctuating frequencies and values of internal
magnetic ﬁelds down to lower temperatures.
In zero ﬁeld (ZF), a low frequency rotational behavior due to dipole-dipole interactions between F − andµ+3) is observed, and to extract the Cu spins dynamics from the entire spectra includung rotational signals,
longitudinal ﬁelds above 100 gauss are applied. Figure
1 (a) shows the LF-µSR time spectra at 26 mK in thecase of xc = 0.53 above 100 gauss. The time spectra are analyzed using the two components function as
follows:
A(t) = A1 exp (−λ1 t) + A2 exp (−λ2 t)(1)Here, λ1 and λ2 are the muon spin relaxation rates. A
ratio of A1 /A2 is ﬁxed to be 0.97. Fig. 1 (b) shows
the obtained LF dependence of muon spin relaxations
λ1 and λ2 . No step-like change in the LF-dependence
appears, and λ1 and λ2 monotonously decrease withincreasing the LF. These results suggest that internal
magnetic ﬁelds at the muon sites ﬂuctuate with widely
spread frequencies down to 26 mK.

13(a)

(b)

26 mK

λ1, λ2 (µs-1)

Corrected asymmetry

The Heisenberg kagome antiferromagnet (HKAF)
has attracted much attention in magnetism because
lots of frustration and quantum eﬀects have been indicated by theories. For example, in√the classicalspin√
model case for HKAF, the q = 0 or 3 × 3 magnetic
structure is stabilized when the next-nearest-neighborinteraction is considered. In the case of S = 1/2
kagome lattice, exotic magnetic ground states havebeen theoretically predicted. For example, numerical calculations revealed that the ground state is amagnetically disordered spin liquid. In the groundstate, triplet excitations are gapped, and there exists the continuum of low-lying singlet states below
the triplet gap. Valence-bond crystals having a periodic arrangement of singlet dimers have also been
proposed as the magnetic ground state of S = 1/2
HKAF. Experimentally, many types of HKAFs havebeen investigated as the candidate for the ideal kagome spin lattice material. For example, ”jarosite”
materials AM3 (OH)6 (SO4 )2 (A = NH4 , Na, or K, M= Fe, Cr), m−MPYNN·BF4 , Cu3 V2 O7 (OH)2 ·2H2 O,[Cu3 (titmb)2 (OCOCH3 )6 ·H2 O, β−Cu3 V2 O8 , etc.
A new candidate for the ideal kagome lattice which
has an exotic magnetic ground state was reported1) .
The cupric compound A2 Cu3 SnF12 (A = Cs, Rb),
which is the subject in this study, is a newly synthesized family of S = 1/2 HKAF. For Cs2 Cu3 SnF12 , theq = 0 magnetic structure is observed below TN = 20K. On the other hand, for Rb2 Cu3 SnF12 , the ﬁrst realization of the ”pinwheel” valence bond solid groundstate in the S = 1/2 HKAF is conﬁrmed by inelastic
neutron measurements.
Recently, the ground state of mixed kagome(Rb1−x Csx )2 Cu3 SnF12 was investing by Katayama etal2) . By magnetic susceptibility and speciﬁc heat measurements on single crystals, they reported a groundstate phase diagram, which shows the existence of aquantum phase transition from the valence-bond-glass(VBG) phase to the AF phase at xc = 0.53. In this critical concentration, the spin gap and the ordered state
disappear at least down to 1.8 K from the magnetic
susceptibility measurement results. The characteristic properties at the quantum phase transition point
is of great interest, and the information about it willcontribute to a further understanding of the properties of the ground states in the kagome lattice compound. In the previous beam time, we carried out
the longitudinal-ﬁeld muon-spin-relaxation ( LF-µSR)

0.50036t (µs)

912λ1λ2210 1
10102
103
HLF (gauss)

104

Fig. 1. (a) LF-µSR time spectra at 26 mK in 100, 700,1000, and 3000 gauss shown from bottom to top.
The solid lines represent the fitted results. (b) LFdependence of the muon spin relaxation rates at 26 mK.

References
1) T. Ono et al., Phys. Rev. B 79, 174407 (2009).
2) K. Katayama et al., Phys. Rev. B 91, 214429 (2015).
3) J. H. Brewer et al., Phys. Rev. B 33, 7813 (1986).