Photoinduced complete melting of spin-Peierls phase in Na-tetracyanoquinodimethane revealed by frequency doubling of coherent molecular oscillations

Photoinduced melting of the spin-Peierls phase was investigated in Na-tetracyanoquinodimethane by pump-probe reflection spectroscopy with a time resolution of 49 fs. Photoirradiation generates coherent molecular oscillation, which indicates that the spin-Peierls dimerization decreased. Increasing the excitation photon density to $>0.02$ photon/molecule doubles the oscillation frequency attributable to recovery of the original symmetry, that is, complete melting of the spin-Peierls phase. We observed a complete photoinduced transition with symmetry recovery revealed by frequency doubling of coherent oscillations.

Figure 1

(a) Molecular structure of TCNQ. (b) Structures of TCNQ stacks in undimerized Mott insulator phase and dimerized spin-Peierls phase of Na-TCNQ. Spheres are $\mathrm{N}{\mathrm{a}}^{+}\mathrm{ions}$. (c) Couplings between TCNQ stacks through Na ions. (d) Polarized reflectivity $\left(R\right)$ spectrum (broken line) and photoinduced reflectivity change $\left(\Delta R\right)$ spectra (open symbols) measured with 180 fs time resolution [33]. Arrow indicates 0.83 eV. Solid line at bottom is $dR/dE$ spectrum. (e) Temporal evolution of $\Delta R/R$ at 0.83 eV (1.97-eV pump) measured with 49 fs time resolution. Electric fields of pump and probe lights are parallel to the $a$ axis. The broken line shows the background time profile for $x_{\mathrm{ph}}=0.059$ ph/TCNQ.

Figure 2

(a) Oscillatory components $\Delta R_{\mathrm{OSC}}/R$ of Na-TCNQ at 0.83 eV. Thin solid lines at top and bottom show $47\mathrm{c}{\mathrm{m}}^{-1}$ and $93\mathrm{c}{\mathrm{m}}^{-1}$ oscillations, respectively. (b, c) $\Delta R_{\mathrm{OSC}}/R$ (open circles) at 0.83 eV for (b) $x_{\mathrm{ph}}=0.011\mathrm{ph}/\mathrm{TCNQ}$ and (c) $x_{\mathrm{ph}}=0.019\mathrm{ph}/\mathrm{TCNQ}$. Orange lines show fitting curves. Three oscillatory components are shown in the lower part. (d) $\Delta R_{\mathrm{OSC}}/R$ (open circles) at 0.83 eV for $x_{\mathrm{ph}}=0.059\mathrm{ph}/\mathrm{TCNQ}$. Solid lines are proportional to $-|\cos ({\omega }_{1}t+{\varphi }_1)|$ and $-\cos ({\omega }_{1}t+{\varphi }_1^{\prime })(\hslash {\omega }_1=47\mathrm{c}{\mathrm{m}}^{-1})$.

Figure 3

(a) Wavelet analyses of oscillatory components $\Delta R_{\mathrm{OSC}}/R$ at 0.83 eV in Na-TCNQ. (b) Fourier power spectra at 0.5 ps extracted from panel (a). (c) Intensities of $47\mathrm{c}{\mathrm{m}}^{-1}$ and $93\mathrm{c}{\mathrm{m}}^{-1}$ modes obtained by integrating the Fourier power spectra from 36 to $60\mathrm{c}{\mathrm{m}}^{-1}$ and from 66 to $126\mathrm{c}{\mathrm{m}}^{-1}$, respectively. Integrated areas are indicated by colored shading in (b).

Figure 4

Schematics of transient changes in dimeric molecular displacements: (a) weak and (b) strong excitation cases. Figures (i)–(iii) show molecular configurations at typical times. $\psi$ and ${\psi }_{\mathrm{OSC}}$ are the phases of the molecular oscillation and coherent oscillation $\Delta R_{\mathrm{OSC}}/R$, respectively. (iv) Left: Time dependence of molecular configurations. Right: Time dependence of magnitudes of molecular displacements (blue solid lines) and oscillations of reflectivity changes (red solid lines). Green arrows indicate changes in molecular displacements in hypothetical equilibrium states just after photoexcitation.