Nobel quantum phase transition in multispin systems - from multipole view point of view -

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We proposed a theory for complicated quantum spin systems in which plural spins interact with each other strongly. The strongly interacting part can be regarded as a multispin unit and we introduced a form factor that depends on magnetic structure of the multispin unit. We applied the theory to a particular materials and investigated intensity of inelastic neutron scattering. When the magnetic ion is located at a site without inversion symmetry, quadrupole operator becomes equivalent to the electric polarization. This results in maltiferroic properties. In a particular material, there are intersite quadrupole interactions. We reported that they can determine the easy axis.