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

RIKEN Accel. Prog. Rep. 51 (2018)

Spin dynamics in the S = 1/2 zigzag spin chain magnets K2 CuCl2 SO4
and Na2 CuCl2 SO4
M. Fujihala,∗1 S. Mitsuda,∗1 and I. Watanabe∗2∗1∗2Department of Physics, Tokyo University of Science
RIKEN Nishina Center

Fig. 1. (a) Temperature dependence of the total specificheat divided by temperature C/T . (b) Temperaturedependence of magnetic susceptibility measured at 1 T(open red circles). The grey solid line represents the theoretical curve of the linear spin chain model with a AFM
interaction J = 15.5 K.
200.55 K

powde
powderr sa
sample
ple

1.2
singl
nglee ccrys
rysttal
10.8

Asym
ymm
metry

150.6
0.4

100.3 K5λ (µsec-1)

The S = 1/2 Heisenberg chain is an outstanding andversatile model system in quantum many-body physics.
The cuprate is known to exhibit magnetic propertiesof the quantum spin chain, thus providing access tothe experimental study of spin chains with various
perturbations. Among these are frustrated intrachain
exchange interactions, staggered g tensors, staggered
Dzyaloshinskii-Moriya (DM) interactions, or disorder.
We recently succeeded in synthesizing the pure-phaseof a new compound Na2 CuCl2 SO4 . This compoundhas the same crystal structure as K2 CuCl2 SO4 andK2 CuBr2 SO4 .1) On ﬁrst inspection, the ferromagnetic
nearest-neighbor interaction J1 and the antiferromagnetic next-nearest-neighbor interaction J2 through theCu-Cl(Br)-Cu path seem to compete, however, it wasreported that the linear spin chains along the a-axis
are formed by the exchange interaction J throughthe Cu-Cl(Br)-Cl(Br)-Cu path in K2 CuCl2 SO4 andK2 CuBr2 SO4 . In addition, from the crystal structure
information, these two compounds feature substantial
DM interactions that are uniform within each chain,but are anti-parallel in adjacent chains. Therefore, weconsider that Na2 CuCl2 SO4 has the unique spin frustration induced by the DM interaction.
Further, we succeeded to grow large single crystals(∼10 mm3 ). Therefore, Na2 CuCl2 SO4 is suitable forobserving the spin dynamics along and perpendicular
to the spin chain by both muon spin rotation and relaxation (µSR) and inelastic neutron scattering. The
temperature dependence of the total speciﬁc heat divided by temperature C/T and the magnetic susceptibility are shown in Figs. 1(a) and (b). Short-range
correlation is developed around 5 K, and the longrange magnetic ordering is observed at TN = 0.5 K.
The INS data measured at 1.5 K in the (h, 1.5, 0) scattering plane reveals a spinon continuum, indicatingthat the Tomonaga-Luttinger liquid state is realized in
Na2 CuCl2 SO4 above TN . Then, in order to investigatethe spin ﬂuctuations in Na2 CuCl2 SO4 , we performedµSR measurements at the RIKEN-RAL Muon facility
at the Rutherford-Appleton Laboratory, UK.
The crystal orientations were determined by Laue Xray diﬀraction. The crystal was cut into slices along
the bc-plane with a homogeneous thickness of 3 mm,
and it was mounted on a silver sample holder. Powder
sample of Na2 CuCl2 SO4 was prepared by milling single
crystals.
A clear oscillation indicative of the long-range magnetic order was observed at 0.3 K in zero ﬁeld(Fig. 2(a)). The weak LF asymmetries for T > 0.6 K

3DLRO

0 0.5 1 1.5 2 2.5 3 3.5 4Time ((µµsec)

1TLL
100.2
0100

T (K(K))

Fig. 2. (a) Selected ZF-µSR signals (asymmetry vs time).
The gray lines are the corresponding fits to the data.
(b) Temperature dependence of the muon spin relaxation rate in Na2 CuCl2 SO4 measured in longitudinal
field 50 G.

are ﬁtted by the stretched exponential function a(t) =a1 exp[−(λt)β ] + aBG , where a1 is an intrinsic asymmetry, aBG is a constant background, λ is the muon spin
relaxation rate, and β is the stretching exponent. Weobserve a decrease in the λ with increasing temperature
above 1 K in the full time window, 0–20 µs as shown in
Fig. 2(b). This behavior is expected in the TLL system;
they have been seen in other spin-liquid candidates.2)
Further the analyses of our µSR spectrum measured inboth the single crystal and the powder sample are now
in progress.
References
1) M. Hälg et al., Phys. Rev. B 90, 174413 (2014).
2) F. L. Pratt et al., Phys. Rev. B 96, 247203 (2006).

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