Excitation-Photon-Energy Selectivity of Photoconversions in Halogen-Bridged Pd-Chain Compounds: Mott Insulator to Metal or Charge-Density-Wave State

Ultrafast photoinduced transitions of a one-dimensional Mott insulator into two distinct electronic phases, metal and charge-density-wave (CDW) state, were achieved in a bromine-bridged Pd-chain compound [$\mathrm{Pd}{(\mathrm{en})}_2\mathrm{Br}]{({\mathrm{C}}_5\text{−}\mathrm{Y})}_2{\mathrm{H}}_2\mathrm{O}$ ($\mathrm{en}=\text{ethylenediamine}$ and ${\mathrm{C}}_5\text{−}\mathrm{Y}=\text{dialkylsulfosuccinate}$), by selecting the photon energy of a femtosecond excitation pulse. For the resonant excitation of the Mott-gap transition, excitonic states are generated and converted to one-dimensional CDW domains. For the higher-energy excitation, free electron and hole carriers are produced, giving rise to a transition of the Mott insulator to a metal. Such selectivity in photoconversions by the choice of initial photoexcited states opens a new possibility for the developments of advanced optical switching and memory functions.

Figure 1

(a) Crystal structure of [$\mathrm{Pd}{(\mathrm{en})}_2\mathrm{Br}]{({\mathrm{C}}_5\text{−}\mathrm{Y})}_2{\mathrm{H}}_2\mathrm{O}$. H atoms of ${\mathrm{C}}_5\text{−}\mathrm{Y}$ are omitted for clarity. (b) Temperature dependence of the optical-gap energy $E_g$ and the Pd-Pd distance $L$ (open circles: cooling run, open squares: heating run) [24]. Schematics of (c) photoinduced Mott-insulator to metal transition and (d) photoinduced Mott-insulator to CDW transition.

Figure 2

(a),(b) Reflectivity ($R$) spectrum (thick solid lines) and its photoinduced changes ($\mathrm{\Delta }R$) of $[\mathrm{Pd}{(\mathrm{en})}_2\mathrm{Br}]{({\mathrm{C}}_5\text{−}\mathrm{Y})}_2{\mathrm{H}}_2\mathrm{O}$ at 12 K; (a) 1.55 eV pump and (b) 0.56 eV pump. Arrows indicate pump energies. (c),(d) Spectrum of the imaginary part of the dielectric constant (${ϵ}_2$) (thick solid lines) and its photoinduced changes ($\mathrm{\Delta }{ϵ}_2$) at 12 K; (c) 1.55 eV pump and (d) 0.56 eV pump. Pump and probe lights are $\parallel$ $\mathbf{b}$.

Figure 3

(a),(b) ${ϵ}_2$ spectra along $\mathbf{b}$ at 12 K and at $t_d=1\mathrm{ps}$ in $[\mathrm{Pd}{(\mathrm{en})}_2\mathrm{Br}]{({\mathrm{C}}_5\text{−}\mathrm{Y})}_2{\mathrm{H}}_2\mathrm{O}$; (a) 1.55 eV pump and (b) 0.56 eV pump (thick solid lines). Blue shaded areas show the responses of the Mott-insulator state. Green shaded area in (a) shows the response of the photoinduced metallic state. Red shaded area in (b) shows the response of the photoinduced CDW state. Thin solid line is the fitting curve. (c) Calculated $E_g$ as a function of the transfer energy $t_0$. (d) Optical-gap energy $E_g$ and (e) frequency $\hslash \Omega$ of the symmetric Pd-Br stretching mode as a function of the Pd-Pd distance $L$.

Figure 4

(a) Time profiles of $\mathrm{\Delta }R/R$ at 12 K in $[\mathrm{Pd}{(\mathrm{en})}_2\mathrm{Br}]{({\mathrm{C}}_5\text{−}\mathrm{Y})}_2{\mathrm{H}}_2\mathrm{O}$ for $x_{\mathrm{ph}}=0.0066\mathrm{ph}/\mathrm{Pd}$. (b) Oscillatory component $\mathrm{\Delta }{R}_{\mathrm{OSC}}/R$ (open circles) extracted from $\mathrm{\Delta }R/R$ by the 0.56 eV pump and 0.32 eV probe shown in (a). The upper solid line is a fitting curve, which is the sum of the three damped oscillations shown in the lower part. The inset exhibits the symmetric Pd-Br stretching mode responsible for the $90{\mathrm{cm}}^{-1}$ oscillation. (c) Fourier power spectrum of the oscillatory component shown in (b) (open circles) and the fitting curve (solid line). (d) Polarized Raman spectrum for the 1.96 eV excitation at 210 K (CDW). $x$ corresponds to the $\mathbf{b}$ axis and $\mathrm{z}$ $z$ is normal to the $bc$ plane.