Magnetic Excitation in Interacting Spin Trimer Systems Investigated by Extended Spin-Wave Theory

Magnetic property of interacting spin trimer systems is studied theoretically focussing on (C 5 H 11 NO 2 ) 2 ·3CuCl 2 ·2H 2 O as a typical example, where the trimer is composed of three S =1/2 spins. It is reported that (C 5 H 11 NO 2 ) 2 ·3CuCl 2 ·2H 2 O has low-field and high-field magnetically ordered phases at low temperatures, where the two phases are separated by a 1/3 magnetization plateau region. The quantum phase transition and magnetic excitation are investigated on the basis of an extended spin-wave theory which was developed in studying interacting spin dimer systems. Formulation of dynamical spin correlation function is presented by introducing form factors for trimers with which interference effects inside the trimer can be taken into account appropriately. Introducing the form factors enables us to treat a trimer as a kind of an atom and makes the formulation simple. Magnetic excitations are classified with transverse and longitudinal modes. It is found that each mode is characterized by i...