フラストレートした１次元量子スピン磁性体で誘起される強誘電転移と新奇量子磁気状態

抄録

Neutron scattering study and measurements of magnetic and dielectric properties have been studied for quasi one-dimensional spin 1/2 systems which are formed of edge-sharing CuO₄ square planes called CuO₂ ribbon chains. Due to the geometrical characteristic of the crystal structure of these systems, the nearest-neighbor exchange interaction between spins is ferromagnetic, and the second neighbor interaction is antiferromagnetic. The CuO₂ ribbon chain systems are typical examples for the frustrated quantum spin systems driven by the competing interactions. We found that LiVCuO₄ and PbCuSO₄(OH)₂ with CuO₂ ribbon chains have helical magnetic order and exhibit a ferroelectric transition with the magnetic transition, simultaneously (called multiferroic). For the CuO₂ ribbon chain systems, exotic quantum phases are theoretically predicted such as Haldane-dimer, spin-nematic, quadrupolar-order, and chiral-order phases. Rb₂Cu₂Mo₃O₁₂ with CuO₂ ribbon chains does not exhibit the long range ordering due to the quantum spin fluctuation and low dimensionality. For Rb₂Cu₂Mo₃O₁₂, the ferroelectric transition is found to be induced by applying field without magnetic transition, which is a new type ferroelectric transition triggered by the magnetism of frustrated quantum spin systems. The obtained results strongly suggest that Rb₂Cu₂Mo₃O₁₂ exhibits some exotic quantum states.