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

Abstract

Photoinduced phase transitions in spin-Peierls ͑SP͒ systems of alkali ͑M =K,Na͒-tetracyanoquinodimethane ͑TCNQ͒ have been studied by a reflection-type femtosecond ͑fs͒ 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 cm −1 in K-TCNQ and of two modes with the frequencies of 49 and 99 cm −1 in Na-TCNQ. By taking into account the temperature dependence of the Raman scattering spectra, the mode with 20 cm −1 in K-TCNQ and the modes with 49 and 99 cm −1 in Na-TCNQ are assigned to the phonon modes in the SP ground state, while the modes with 49 and 90 cm −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-cm −1 mode of K-TCNQ and the 49-cm −1 mode of Na-TCNQ are longitudinal optical ͑LO͒ modes, whereas the 99-cm −1 mode of Na-TCNQ is a transverse optical ͑TO͒ 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.

Detecting charge and lattice dynamics in photoinduced charge-order melting in perovskite-type manganites using a 30-femtosecond time resolution

“…Such coherent oscillations have previously been reported in other examples of photoinduced phase transitions. [29][30][31][32][33][34][35] Here, let us briefly discuss semiquantitatively the amplitude of the coherent oscillation due to the JT mode, which is the largest among the four oscillations. As seen in the lower panel of Fig.…”

Ultrafast photoinduced transitions in charge density wave, Mott insulator, and metallic phases of an iodine-bridged platinum compound

“…1 It is because a photoinduced phase transition in an ultrafast time scale may be used as a useful mechanism for future optical switching devices. Several transient photoinduced transitions between nearly degenerate two phases have been reported so far, e.g., insulator-metal transitions in VO 2 , 2 the neutral-ionic transition in tetrathiafulvalene-p-chloranil, 3,4 the charge-/ orbital-order insulator to ferromagnetic metal transition in the manganite, 5-7 the diamagnetic spin-Peierls phase to paramagnetic phase transition in K-tetracyanoquinodimethane, 8,9 and so on. In all of these transient photoinduced transitions, the conversion from one phase to another is not perfect, resulting to coexistence of original phase and the photoinduced phase.…”

Sub-10 fs spectroscopy of K-TCNQ crystal for observation of intramolecular vibration modulation in melting of the Peierls dimer

“…In addition, they are the organic charge-transfer compounds [4][5][6][7][8][9][10][11]. Ikegami et al reported the pump-probe spectroscopy of K-and Na-TCNQ crystals in the IR region with 130 fs laser pulses generated by OPA [12,13]. They claimed that electron-hole pairs generated by photoexcitation melt the generalized Peierls phase to induce intermolecular coherent oscillations.…”