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

RIKEN Accel. Prog. Rep. 53

Investigations of the quantum critical behavior of the S = 1/2 J1 -J2square lattice antiferromagnets Sr2 Cu(Te1−x Wx )O6S. Yoon,∗1 W. -J. Lee,∗1 S. Lee,∗1,∗2 C. H. Lee,∗1 A. Berlie,∗2 I. Watanabe,∗3 and K. -Y. Choi∗1The J1 -J2 square-lattice Heisenberg model oﬀers a richreservoir of exotic quantum phases. Depending on theratio J2 /J1 (the ratio of exchange interactions betweena diagonal and a side of the square), the classical groundstates encompass Néel, columnar antiferromagnetic, anddisordered states. In particular, the B -site ordered double perovskites Sr2 Cu(Te1−x Wx )O6 (B = Cu2+ , B ′′ =Te6+ or W6+ ) with the tetragonal I4/m structure are
regarded as near-optimal realizations of the S = 1/2
Heisenberg model on a square lattice.1–4) Notably, themagnetic ground state of Sr2 Cu(Te1−x Wx )O6 can betuned from a Néel order (x = 0; J2 /J1 = 0.03) througha quantum spin liquid to a columnar antiferromagneticorder (x = 1; J2 /J1 = 7.92) by varying the B ′′ cation
between Te6+ and W6+ ions.3) Owing to the similar sizesof the Te6+ and W6+ ions, Sr2 Cu(Te1−x Wx )O6 retains
the isostructural crystal lattice.
Unlike the crystal structure, the magnetism is susceptible to a composition of the B ′′ cations that are involvedin the magnetic exchange paths mediating the Cu2+ ions.
W6+ (5d0 ) hybridizes strongly with O 2p, thereby enabling the magnetic exchange path Cu-O-W-O-Cu (J2 ).
This is in sharp contrast to the Te6+ (4d10 ) ion, whichis in favor of J1 . The distinct role of the Te6+ /W6+
ions in relation to the exchange interaction poses thequestion as to whether the theoretically predicted spin
liquid phase can be achieved by controlling the composition of the B ′′ cations. Strikingly, there is tantalizing
experimental evidence for a quantum disordered state ina wide composition range of x ≈ 0.1 − 0.7.3) However,
the observed wide range of the spin-liquid phase is notcompatible with the theoretically predicted narrow composition range of x ≈ 0.23 ∼ 0.33. Recent theoretical
calculations have proposed quenched disorders as a key
ingredient to this discrepancy.3,4) To clarify the precise
nature of the ground state, we focus on the compositionsin the range of x = 0.05 − 0.1, which is at the boundary between the Neel antiferromagnetic order and thequantum disordered state.
Muon spin relaxation measurements were performed

Fig. 1. Muon spin polarization data observed at zero field
together with fitting curves (see the text for details) for
x = 0.05, 0.08, and 0.1.
∗1
∗2
∗3

Department of Physics, Chung-Ang university
Science and Technology Facilities Council, ISIS-RAL
RIKEN Nishina CenterFig. 2. Temperature dependences of the relaxation rate (a)and its stretch exponent (b) for x = 0.05, 0.08, and 0.1.

on ARGUS and CHRONUS spectrometers at RIKENRAL towards understanding the precise nature of theground state and associated spin dynamics of the x =0.05, 0.08, and 0.1 compounds. Representative spectra
are shown in Fig. 1. No spontaneous muon spin oscillation is observed, indicating the absence of long-range
magnetic order. Moreover, all the observed µSR spectra are described by a stretched exponential function
exp[−(λt)β ], where λ is the muon relaxation rate andβ is the exponent.
Temperature dependences of the muon spin relaxation
rate, λ(T ), and its stretch exponent, β(T ), for x = 0.05,0.08, and 0.1 are plotted in Fig. 2. λ(T ) of x = 0.05
starts to increase on cooling below 15 K and subsequently becomes constant for temperatures below 0.5 K.
In the x = 0.08 and 0.1 compounds, the crossover regime
shrinks, and onset temperatures shift to 6 K and 2 K for
x = 0.08 and 0.1, respectively; on the other hand, aplateau appears at 1 K for x = 0.1. We comment thatthe x = 0.05 and 0.1 compounds reveal the persistentspin dynamics below T = 0.5 − 1 K, which is often observed in frustrated magnets.
In addition, the T -dependence of the stretching exponent, β(T ), of x = 0.08 and 0.1 resemble λ(T ) while
increasing from 1 to 1.6 with decreasing temperature. In
sharp contrast, β(T ) of x = 0.05 decreases from 1.8 to
1.3 on cooling to 25 K. Noticeably, the low-T β(T ) of thecompounds is between those of Gaussian and Lorentzianrelaxation functions. When we consider that a Gaussianlike distribution is expected for dense disordered systems
or migrating unpaired spins in the matrix of singlet spins
and that a Lorentzian-like distribution is expected for
dilute systems, the ground state for x = 0.05, 0.08, and
0.1 is largely compatible with the random singlet picture
comprising resonating singlets and propagating orphanlike spins.
References1) O. Mustonen et al., Nat. Commun. 9, 1085 (2018).
2) M. Watanabe et al., Phys. Rev. B 98, 054422 (2018).
3) O. Mustonen et al., Phys. Rev. B 98, 064411 (2018).
4) K. Uematsu, H. Kawamura, Phys. Rev. B 98, 134427 (2018).

- 151 -