  Magnetic properties of a one-dimensional frustrated quantum spin system
Rb2Cu2Mo3O12 is investigated by the muon spin relaxation (muSR) technique.
Although this system shows an incommensurate spin correlation in neutron
scattering, it has not yet been determined whether the ground state reaches to
a nonmagnetic spin-singlet state or not. In this study, our zero-field (ZF)
muSR result undoubtedly reveals a nonmagnetic ground state, which establishes
formation of the nonmagnetic incommensurate spin-singlet state in combination
of the previous neutron scattering study. Furthermore, we found that the
internal field is slightly enhanced below ~7 K. This temperature dependence
resembles that of electric polarization under the magnetic field reported
recently, indicating a possibility that a small change occurs even under ZF. In
longitudinal-field measurements, a fast fluctuation around a narrowing region
is observed. This relaxation is expected to reflect characteristics of
formation of the spin-singlet state.
