Spin-Peierls transition to a Haldane phase in molecular-based materials*

 Our material design using organic radicals with diverse molecular structures realizes unconventional spin systems, such as the ferromagnetic-leg ladder, quantum pentagon, and random honeycomb, which have not been realized in conventional inorganic materials. The flexibility of the molecular orbital (MO) in our radicals enabled us to design spin arrangements composed of intermolecular exchange interactions through molecular design.

 Recently, we synthesized single crystals of radical salts through our spin arrangement design. They exhibit a spin-Peierls (SP) transition to an effective spin-1 antiferromagnetic uniform chain, that is, the Haldane chain. The clear disappearance of magnetization, accompanied by a structural phase transition, is well explained by the deformation to an effective spin-1 Haldane chain. The flexibility of the molecular orbitals in the organic radical compound allows the transformation of the exchange interactions into the Haldane state with different topologies. The SP transition in the present compound demonstrates a mechanism different from that of the conventional systems, paving another path for research in quantum phenomena originating from spin-lattice couplings.