Spin-Peierls Transition in the Charge-Ordered Organic Conductor θ-BEDT-TTF_2RbZn(SCN)_4(Condensed matter: structure and mechanical and thermal properties)

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X-ray investigation of θ-BEDT-TTF_2RbZn(SCN)_4 was carried out around the temperature of the phase transition to the nonmagnetic state. θ-BEDT-TTF_2RbZn(SCN)_4 exhibits a metal-insulator transition accompanied by a charge ordering at T_<CO> = 200K. In the one-dimensional spin chain formed by this charge ordering, BEDT-TTF donor molecules are aligned uniformly along the a-axis obeying the 2_1 screw symmetry of the space group P2_12_12_1 below T_<CO>. At the ground state, the system becomes nonmagnetic. The transition temperature T_c to the nonmagnetic phase seems to scatter around 20K and may depend on the sample. We found that the extinction rule of [h00: h = 2n] no longer applied below approximately 20K. This indicated a symmetry breaking of the 2_1 screw symmetry along the a-axis in the nonmagnetic state. The observed peak intensities violating the extinction rule were about 10^<-4> of other general Bragg reflection peak intensities. This structural change should be interpreted as a spin-Peierls transition in the charge-ordered system. In addition, a corresponding anomaly was observed in the lattice parameter a.
社団法人日本物理学会の論文
2007-12-15