Femtosecond core-level photoemision spectroscopy on $1T$-TaS${}_2$ using a 60-eV laser source

Time-resolved photoelectron spectroscopy (trPES) can directly detect transient electronic structure, thus bringing out its promising potential to clarify nonequilibrium processes arising in condensed matters. Here we report the result of core-level trPES on 1$T$-TaS${}_2$, realized by developing a high-intensity 60-eV laser obtained by high-order harmonic generation. Ta$4f$ core-level trPES offers the transient amplitude of the charge-density wave (CDW), via the site-selective and real-time observation of Ta electrons. The present result indicates an ultrafast photoinduced melting and recovery of CDW amplitude, followed by a peculiar long-life oscillation (i.e., collective amplitudon excitation) accompanying the transfer of electrons among adjacent Ta atoms. core-level trPES offers a broad range of opportunities for investigating the ultrafast atom-specific electron dynamics in photorelated phenomena of interest.

Figure 1

Schematic diagram of the trPES apparatus. (Inset) Photoemission spectrum from Au obtained at 300 K using this HH-laser PES system. The inset shows the Fermi cutoff fitted by Gaussian-convolved Fermi-Dirac function. By fitting, the total energy resolution is estimated to be 230 meV.

Figure 2

(a) Schematic picture of Ta atom network in the high-$T$ phase (top) and low-$T$ commensurate CDW state (bottom). (b) $T$-dependent Ta$4f_{7/2}$ PES spectra obtained by synchrotron x-ray light source ($h\nu =1200$ eV, SPring-8 BL17SU). (c) Ta$4f_{7/2}$ PES spectra recorded at 300 K and 190 K, using HH 60-eV laser PES system.

Figure 3

(a) Transient PES spectra for temporal region of $-0.5⩽t⩽1.6$. Red curves are the fitting to respective PES spectra, blue and green markers show the peak positions obtained by the fitting. (b) PES intensity image as a function of binding energy and time delay clearly showing the shift and smearing of peak $c$ at around $t=0$. (c) Split energy between the doublet peaks plotted as a function of delay time. Gray and red curves indicate a guide for eyes and the calculated function, respectively. (d) Linewidth of the doublet peaks as a function of delay time.

Figure 4

Color (a) trPES spectra recorded before ($t=-0.5$ ps) and after ($t=0$) pumping, obtained by two sets of measurements with $I_{\mathrm{pump}}=1$ and 1.5 mJ cm${}^{-2}$, respectively. (b) $T$-dependent PES spectra obtained by $h\nu =1200$ eV, broadened by Gaussian convolution to match the energy resolution of trPES data. (c) Schematic of the Ta$5d$ and $\mathrm{S}3p$ valence bands and the optical transition by pumping (orange arrow). The top subband forms a Mott gap at $E_F$ below the Mott transition temperature. (d) Schematic of a Star of David cluster.