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

RIKEN Accel. Prog. Rep. 53

Non-magnetic ground state of In2 Ru2 O7 probed by muon spin rotation
T. Takayama,∗1,∗2 A. Krajewska,∗1,∗2 I. Watanabe,∗3 D. P. Sari,∗3,∗4 and H. Takagi∗1,∗2,∗5The interplay of charge, spin, and orbital degrees offreedom in correlated systems gives rise to a variety of
exotic electronic states. Among these materials, ruthenium oxides containing Ru4+ cations (d4 electron count),which host orbital degeneracy, display a variety of spinorbital ordering phenomena such as spin singlet dimer
formation in Li2 RuO3 1) and La2 RuO5 .2) A particularlyinteresting situation may be found when Ru4+ cations
are placed on a frustrated lattice, for example, in pyrochlore ruthenate Tl2 Ru2 O7 , which displays a metalto-insulator transition with a spin-singlet ground state.3)
The spin-singlet state is discussed to originate from theformation of 1D zig-zag Haldane chains and is potentially the first example of such a system emerging in
a 3D lattice. While there is still discussion on theplausibility of the Haldane-chain scenario, the origin of
this unique state and behavior distinct from those ofother pyrochlore ruthenates remains unclear. Pyrochoreruthenates appear to be an interesting playground fornovel electronic states.
We discovered a new pyrochlore ruthenate In2 Ru2 O7
which adopts a weakly distorted pyrochlore structure at
300 K. In2 Ru2 O7 , is an insulator and shows Curie-Weissbehavior at high temperatures. At Tm ∼ 220 K, a sharp
drop in magnetic susceptibility χ(T ), similar to those ofother spin-singlet ruthenate compounds,1–3) is observed.
To elucidate the nature of the ground state of In2 Ru2 O7
and the absence of a long-range magnetic ordered state,
the temperature dependence of zero-field muon spinrotation (ZF-µSR) is necessary. In this report, we
present the result of a ZF-µSR experiment performedon CHRONUS at ISIS, UK. We recorded the time evolution of ZF-µSR spectra of In2 Ru2 O7 from 300 K down
to 10 K (Fig. 1(a)). No oscillation down to the lowest temperature was observed, excluding the presenceof long-range antiferromagnetic order and pointing to a
non-magnetic spin singlet state. The data over the wholestudied temperature range were fitted to a “stretched”
exponential:βPz (t) = e−(λt) ,(1)where Pz (t) is the asymmetry, t is the time, λ is therelaxation rate, and β is the shape parameter. We observed a cross-over from a Gaussian-like to a Lorentzianlike damping with decreasing temperature (Figs. 1(a)and (b)). The high-temperature and low-temperature
response can be explained by the slowing behaviour ofthe fluctuations of dilute Ru spins, which originate from
∗1
∗2
∗3
∗4
∗5Max Planck Institute for Solid State ResearchInstitute for Functional Matter and Quantum Technologies,
University of Stuttgart
RIKEN Nishina Center
Graduate School of Engineering and Science, Shibaura Institute of Technology
Department of Physics, University of Tokyo

Fig. 1. (a) Time evolution of ZF-µSR spectrum of In2 Ru2 O7
powder sample at various temperatures. (b) Temperaturedependence of λ and β fitted using equation (1).

the defects present in the system, or by the appearanceof slowly fluctuating quasi-particle spins excited fromthe spin-singlet state induced by injected muons. Such aresponse was reported in other materials hosting a spinsinglet ground state.5,6) In addition, a sharp change in λand β is observed at Tm (Fig. 1(b)). The enhancementof λ below Tm would point to the opening of a gap.
In this report, the ground state of In2 Ru2 O7 was investigated with ZF-µSR. No evidence of long-range magnetic order was found; hence, the non-magnetic groundstate was confirmed. The temperature dependence ofthe relaxation rate implies the opening of a gap. Othertechniques such as resonant inelastic x-ray scattering
could aid the determination of the gap size.
References

1) Y. Miura et al., J. Phys. Soc. Jpn. 76, 033705 (2007).
2) H. Wu et al., Phys. Rev. Lett. 96, 256402 (2006).
3) S. Lee et al., Nat. Mat. 5 (2006).
4) K. Kojima et al., Phys. Rev. Lett. 74, 3471 (1995).
5) S. J. Blundell et al., Phys. Rev. B 77, 094424 (2008).
6) M. Miyazaki et al., Phys. Rev. B 75, 09440 (2007).

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