Magnetic and dielectric properties have been studied for quasi one-dimensional spin 1/2 systems which are formed of edge-sharing CuO 4 square planes called CuO 2 ribbon chains. Due to the geometrical characteristic of the crystal structure of these systems, the nearest-neighbor exchange interaction J 1 between spins is relatively weak and often ferromagnetic, and the second neighbor interaction J 2 is antiferromagnetic. Under these situations, the helical ordering is often realized. Actually, LiVCuO 4 with the CuO 2 ribbon chains has the helical magnetic structure. For LiVCuO 4, we found that ferroelectric transition takes place simultaneously with a magnetic transition at T N =2.4 K (multiferroic) [1] as theoretically discussed in ref. 2. We have also found that PbCuSO 4 (OH) 2 with the CuO 2 ribbon chains has the helical magnetic structure and multiferroic behavior [3,4]. The quantum spin system can be considered as a new kind of multiferroics which may provide different aspects of physics from classical spin systems consisting of Fe 3+ and Mn 3+ with S > 1. Here, magnetic and dielectric properties of Rb 2 Cu 2 Mo 3 O 12 with CuO 2 ribbon chains are presented. The system does not exhibit a magnetic transition owing to quantum fluctuation and low dimensionality of crystal structure. We have observed anomalous increase of dielectric constant ε with decreasing T below ~50 K, which is originated from growing a short range ordering of a helical magnetic structure. For an external magnetic field H > 0.1 T, a peak structure is observed in the ε-T curve of Rb 2 Cu 2 Mo 3 O 12 at T c~8 K and the ferroelectric polarization has been observed below T c. However, the temperature dependence of magnetic susceptibility and specific heat do not have anomaly at T c in the magnetic field. These results indicate that the ferroelectric transition is found to be induced by applying field without magnetic transition. Then, these results strongly suggest that the magnetic field induced ferroelectric transition is a new type ferroelectric transition triggered by the magnetism of frustrated quantum spin systems.

Dielectric and magnetic properties have been studied for poly-crystalline samples of quasi-one-dimensional frustrated spin-1/2 system Rb 2 (Cu 1-x M x ) 2 Mo 3 O 12 (M=Ni and Zn) which does not exhibit a three-dimensional magnetic transition due to quantum spin fluctuation and low dimensionality. A broad peak in the magnetic susceptibility -temperature curves originated from a short range helical ordering at low temperature is suppressed by the Ni and Zn substitution for Cu sites. The capacitance is found to anomalously increase with decreasing T below ~50 K, which is also suppressed by the impurity doping. The behavior of the anomalous capacitance component is found to be strongly connected with that of the

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