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

RIKEN Accel. Prog. Rep. 53

Magnetic ground state of Jeﬀ = 0 Mott insulator Ag3 LiRu2 O6 underpressure
T. Takayama,∗1,∗2 M. Blankenhorn,∗1,∗2 A. Krajewska,∗1,∗2 D. P. Sari,∗3,∗4 I. Watanabe,∗4 and H. Takagi∗1,∗2,∗5
The interplay of strong spin-orbit coupling (SOC)and electron correlation in heavy transition-metal oxides has been found to give rise to unprecedented electronic phases. Among such oxides, Mott insulators witha d4 (t42g ) conﬁguration are expected to exhibit exoticmagnetism. In the d4 systems, Hund’s coupling produces an S = 1 and Leﬀ = 1 state. SOC thus yields
a spin-orbit-entangled Jeﬀ = 0 ground state. Although
the Jeﬀ = 0 state is non-magnetic, it may interact magnetically with each other through the excited Jeﬀ = 1
levels separated by SOC. It was proposed that the system undergoes magnetic ordering when the energy gainby magnetic interactions overcomes the energy cost of
the Jeﬀ = 0 to 1 excitation, which is dubbed excitonic
magnetism.1) Particularly, when the Jeﬀ = 0 state is arranged on a honeycomb lattice, exotic magnetic groundstates such as a spin-nematic phase or magnetic triplon
liquid, are expected to emerge.
The Jeﬀ = 0 honeycomb lattice may be realized in
Ru4+ (4d4 ) oxides. However, a honeycomb ruthenate
Li2 RuO3 is known to display spin-singlet dimerizationaccompanying orbital ordering.2) In the dimerized state,
the spin and orbital degrees of freedom are quenched,and spin-orbit coupling is inactive. In order to suppress the dimerization and realize the Jeﬀ = 0 state,
we employed an ion-exchange reaction and obtained thesilver-intercalated compound Ag3 LiRu2 O6 .3) The crystal structure at room temperature shows no appreciable
distortion in the Ru honeycomb lattice, indicating thedisappearance of dimerization.
The magnetic susceptibility χ(T ) of Ag3 LiRu2 O6 increases on cooling from room temperature and showssaturation below 100 K. Interestingly, no magnetic orderis observed down to 0.3 K. From inelastic neutron scattering and 7 Li-NMR measurements, Ag3 LiRu2 O6 was
found to have a gapped magnetic excitation of ∼35 meV,as expected for the Jeﬀ = 0-type singlet ground state.4)
The singlet ground state suggests that the magnetic exchange interactions are not strong enough to drive excitonic magnetism. We thus investigated the magnetic
properties under high pressures as an attempt to enhance magnetic interactions. Indeed, under a pressure
above 0.4 GPa, χ(T ) shows a broad peak below room
temperature, indicating the appearance of a pressureinduced electronic phase transition. However, the natureof the magnetic ground state under pressure remains un∗1
∗2
∗3
∗4
∗5Max Planck Institute for Solid State ResearchInstitute for Functional Matter and Quantum Technologies,
University of Stuttgart
Graduate School of Engineering and Science, Shibaura Inst.
of Tech.
RIKEN Nishina Cente
Department of Physics, University of Tokyo

Fig. 1. Zero-field time spectra of muon asymmetry for
Ag3 LiRu2 O6 under a pressure of 0.64 GPa.

clear.4)
In order to understand the magnetic ground state under pressure, we performed a muon spin rotation measurement on Ag3 LiRu2 O6 under pressure by using ARGUS, ISIS. At ambient pressure, the time dependenceof muon asymmetry at low temperatures only shows amonotonic decrease, which is consistent with the nonmagnetic singlet ground state. As shown in Fig. 1, thetime dependence of muon asymmetry did not show any
pronounced change down to 2 K even at a pressure of0.64 GPa, while χ(T ) showed a broad peak at approximately 150 K at that pressure. This suggests that thebroad peak observed in χ(T ) under pressure is not associated with a magnetic ordering but represents anotherelectronic phase transition while retaining the nonmagnetic ground state.
The nature of the pressure-induced nonmagnetic
phase, revealed by the µSR measurement, remains elusive. The x-ray diﬀraction under pressure excluded therestored dimerization in this pressure range. The candidates of possible ground states include a spin-orbit
triplon liquid, molecular orbital formation, or ﬂuctuating dimers. We are currently attempting to determine
the nature of this phase in combination with other spectroscopic probes.
References

1) G. Khaliullin, Phys. Rev. Lett. 111, 197201 (2013).
2) Y. Miura et al., J. Phys. Soc. Jpn. 76, 033705 (2007).
3) S. A. J. Kimber et al., J. Mater. Chem. 20, 8021 (2010).
4) T. Takayama et al., in preparation.

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