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

RIKEN Accel. Prog. Rep. 48 (2015)

Partial magnetic order in the quantum spin system NH4CuCl3T. Goto,＊1 K. Matsui,＊1 T. Suzuki,＊2 H. Guo,＊4 A. Oosawa,＊1 M. Fujisawa,＊3 H. Tanaka,＊3 I. Watanabe＊4
The ground states of boson systems with randomness
have been a puzzle for a long time. In a uniform system
consisting of mobile bosons, the ground state is theBose-Einstein condensed fluid. When the mobility ofparticles is reduced by introducing randomness orfrustration, the system becomes solid-like, that is, the Bose
glass or the Wigner crystal, details of which, however, arestill unknown1). For studying the property of the bosonsystem, NH4CuCl3 is suitable model compound, where both
the density and mobility of the field-induced magnons canbe tuned by the magnitude of applied field and the state of
non-centrosymmetric NH4 molecules, respectively.
The quantum spin magnet NH4CuCl3 is an S =1/2
three-dimensional dimer system with the crystal structure
isomorphic to TlCuCl3, which is a spin-gapped system andis known for the Bose-Einstein condensation offield-induced magnons. The ground state of NH4CuCl3,
however, is different from that of TlCuCl3, that is, gaplessin zero field, showing a magnetic order at TN = 1.25 K.
When a high magnetic field is applied, two-stepmagnetization plateaus are observed2). The mechanism of
the two-step plateaus attracts much interest, for it is related
to the fundamental property of the field-induced magnons.
One of the models3) proposed so far is the localized
three sublattice dimer model that claims an existence of thethree inequivalent dimers A, B and C with different criticalfields Hc, one of which i.e. HcA is zero. This model
elegantly explains the two-step plateaus: the two plateaus
appear in the field regions where the dimer A and B
saturates, and the net magnetization saturates when all threesaturate.
According to this model, only dimer A
magnetically orders at zero field, indicating the partial
magnetic order.
The elastic neutron scattering experiment4) confirms thedoubling of the unit cell along b, supporting the existence ofthree inequivalent dimers in an elongated unit cell.
However, recent high-field NMR observation5) disproves
this model. That is, in the high field of the second plateau
region, the two of the three inequivalent dimers are
saturated and only C is paramagnetic, and hence the NMR
signal corresponding to C is expected to be observed atnearly zero-shift position. However, in the high-field NMR
spectra, a signal corresponding to either A or B is also
observed with that for C. This contradiction gives us agood motivation to propose carrying out the SRexperiment on this compound. If the model is correct, oneexpects that only one site possesses a large hyperfine field
while the other three quarter sites are expected to remain in
the paramagnetic state, or that when the interaction among
Department of Physics, Sophia University
Department of Physics, Shibaura institute of Technology＊3
Department of Physics, Tokyo Institute of Technology＊4
RIKEN Nishina Center＊1＊2inequivalent three dimers is appreciable, three different
hyperfine fields will appear at each dimer, similar to thecase of SrCu2(BO3)2.
So far, zero field (ZF) - SR measurements on single
crystals of NH4CuCl3 were carried out at the Riken-RAL
Muon Facility using a pulsed surface-muon beam with amomentum of 27 MeV/c. In the ordered state below TN, aclear muon spin rotation is observed at zero-field. At thelowest temperature 0.31 K, one can see in the
depolarization curve a clear beat with two different
frequencies and also an extremely fast term thatmonotonically decreases with time. This leads us tochoose reasonably the three-component functionGKT(;)·(A1e-1+A2e-2cos2+A3e-3cos3) to fit thedata below TN. Above TN, the depolarization curves
simply consist of one component, that is, the first term in
the above function.
Figure 1 shows the typicaldepolarization curves with the fitted function at varioustemperatures. The two hyperfine fields 1/ and 2/
increased monotonically below TN and reached 150 and 90Oe. The volume fraction of each component for A1, A2
and A3 were 25, 70 and 5 % at 0.3 K.
Although the observed data clearly demonstrate that theordered state consists of three different parts, the fraction ofeach part is not in accord with the model, which declaresthat A1, A2 and A3 should be in the ratio 1:2:1. For further
quantitative comparison of this result with thethree-sublattice model, the detailed knowledge of the muon
stopping site in the unit cell is indispensable. 6)

Fig. 1. Muon spin depolarization curves at various
temperatures below and above TN=1.25 K. Solid curves
arefittedfunctionwiththreecomponents
GKT(;)·(A1e-1+A2e-2cos2+A3e-3cos3).
References1) K. Kodama, Science 298, 395 (2002).
2) W. Shiramura et al., J. Phys. Soc. Jpn. 67, 1548 (1998).
3) M. Matsumoto, Phys. Rev. B 68, 180403 (2003).
4) Ch. Rüegg et al., Phys. Rev. Lett. 93, 037207 (2004).
5) H. Inoue et al., Phys. Rev. B 79, 174418 (2009).
6) K. Matsui et al. J. Phys.: Conf. Ser. 551 012007 (2014).

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