Clocking the Melting Transition of Charge and Lattice Order in $1T\mathrm{\text{−}}{\mathrm{TaS}}_2$ with Ultrafast Extreme-Ultraviolet Angle-Resolved Photoemission Spectroscopy

We use time- and angle-resolved photoemission spectroscopy with sub-30-fs extreme-ultraviolet pulses to map the time- and momentum-dependent electronic structure of photoexcited $1T\mathrm{\text{−}}{\mathrm{TaS}}_2$. This compound is a two-dimensional Mott insulator with charge-density wave ordering. Charge order, evidenced by splitting between occupied subbands at the Brillouin zone boundary, melts well before the lattice responds. This challenges the view of a charge-density wave caused by electron-phonon coupling and Fermi-surface nesting alone, and suggests that electronic correlations play a key role in driving charge order.

Figure 1

Structure and ARPES intensity maps of $1T\mathrm{\text{−}}{\mathrm{TaS}}_2$. (a) Static photoemission intensity maps from center to center of the first Brillouin zone, measured with a helium discharge lamp (21.2 eV) at 20 and 300 K. A solid line denotes the Fermi level. Dashed lines indicate the approximate band structure in the low-temperature phase, showing the positions of the LHB (upper band) and U1 (lower band). In both panels, momentum labels refer to the Brillouin zone of the low-temperature structure. (b) Low-temperature lattice distortion in the Ta plane, which accompanies the formation of the commensurate, static CDW. Dashed lines indicate high- and low-temperature unit cells. Gray stars show clusters of atoms formed by the periodic lattice distortion, illustrated by arrows for one star. Also shown is a map of the high-symmetry points of the Brillouin zone. (c) Static intensity map measured with the laser-harmonic XUV source.

Figure 2

Collapse of the Mott and CDW gaps and oscillation of the LHB edge. (a) Maps of photoelectron intensity as a function of time, during and after photoexcitation. At both the $\Gamma$ and $M$ points, a transfer of intensity across the Fermi level at $t=0$ heralds the prompt destruction of the Mott gap. Mott gap recovery, after a few hundred fs, is followed by a rigid oscillation at the amplitude-mode frequency. (b) Individual energy distribution curves before and after photoexcitation. Along with the loss of the Mott gap, which causes intensity to drop in the LHB and be transferred to energies around and above the Fermi level, there is also a rapid loss of splitting between the band features at the $M$ point. The top curve (green) is a reference measurement taken with a lamp source, showing the two bands at the zone center.

Figure 3

Snapshots of momentum-dependent photoemission intensity, obtained with the laser-harmonic XUV source at 20 K. As the insulator-metal transition proceeds, transfer of intensity across the Fermi level is evident throughout both Brillouin zones. The gap between bands at the zone boundary collapses on subvibrational time scales.

Figure 4

Momentum dependence of spectral changes on fast and slow time scales. (a) Time evolution at the $\Gamma$ point, scaled, for three key energies. Changes are prompt in the lower Hubbard band and at $E_F$, but much slower in the umklapp band (U1). Red and black curves show a step response and a transient response with a lifetime of half the amplitude-mode period, each convolved with the instrumental resolution. (b) Maps of intensity changes. The first panel shows immediate changes, between $-50$ and $+50\mathrm{fs}$. The second panel shows the subsequent change, between $+50$ and $+200\mathrm{fs}$. All changes are expressed relative to the peak intensity before photostimulation. The arrow indicates the unfolding of intensity out of the first reconstructed Brillouin zone as the structural distortion begins to relax.