Mid-gap state of the quasi-one-dimensional mott-Hubbard system: charge trasnsfer complex composed of 1, 6-pyrenediamine and tetracyanoquinodimethane(DAP-TCNQ)

Optical and magnetic properties of the charge-transfer(CT) complex of 1,6-pyrenediamine(DAP) and tetracyanoquinodimethane(TCNQ) were investigated. DAP-TCNQ is a quasi-one-dimensional Mott-Hubbard system with the spin-Peierls instability. The degree of CT is about 0.9, indicating that the band-filling deviates slightly from the half-filled regime. In the optical absorption spectrum, a mid-gap band at 0.5 eV appears at 40 K below the Hubbard gap at 0.8 eV. The mid-gap band may be assigned to a spinless charged soliton, which is formed as a spinless domain wall between the two-fold ground states of the dimerized chains.