Ultrafast photoinduced melting of spin-Peierls phase in the organic charge-transfer compounds alkali-tetracyanoquinodimethane

Abstract

Photoinduced phase transitions in spin-Peierls (SP) systems of alkali $(M=\mathrm{K},\mathrm{Na})$-tetracyanoquinodimethane (TCNQ) have been studied by a reflection-type femtosecond (fs) pump-probe spectroscopy. The SP phase is destabilized by the generation of photocarriers through the breaking of the spin-singlet states in dimers. It results in the decrease of the dimeric molecular displacements within a few hundred of femtoseconds over several tens of TCNQ molecules. It is accompanied by the displacive-type coherent oscillations, which consist mainly of three modes with the frequencies of 20, 49, and $90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ and of two modes with the frequencies of 49 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in Na-TCNQ. By taking into account the temperature dependence of the Raman scattering spectra, the mode with $20\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ and the modes with 49 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in Na-TCNQ are assigned to the phonon modes in the SP ground state, while the modes with 49 and $90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in K-TCNQ are assigned to the local modes originating from the photoexcited states. Polarization dependence of the Raman scattering signals shows that the $20\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of K-TCNQ and the $49\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of Na-TCNQ are longitudinal optical (LO) modes, whereas the $99\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode of Na-TCNQ is a transverse optical (TO) mode. Namely, the LO mode plays an important role on the stabilization of the SP phase in K-TCNQ, while both the LO and TO modes in Na-TCNQ. Such a difference is discussed by scrutinizing the difference of the crystal structures and the nature of the SP transitions in the two compounds.