Spin-0 soliton formation in spin-Peierls systems: Charge-transfer complexes DAP-TCNQ and DAP-DMTCNQ

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Abstract
The optical and magnetic properties of the spin-Peierls system, segregated-stack charge-transfer complexes, 1,6-pyrenediamine (DAP)-tetracyanoquinodimethane (TCNQ) and DAP-2,5-dimethyl-TCNQ (DMTCNQ), are investigated to gain insight into the nonlinear excitations in the spin-Peierls system. The band filling of the system slightly deviates from the half-filled regime and the lattice is dimerized along the chain. In the optical reflectance spectra, a midgap band below the Hubbard band is found. By referring to the magnetic properties, the midgap band is ascribed to a spin-0 soliton formed by a spinless domain wall between the twofold degenerate ground states of the dimerized chain. The amplitude of dimerization is modified by the interchain hydrogen bond.