Fig. 4: Ultrafast bulk and surface dynamics.
a–d Energy-momentum dispersions along the \(\overline{\,\text{M}\,}\)-\(\overline{{{\Gamma }}}\)-\(\overline{\,\text{K}\,}\) direction measured by time- and angle-resolved photoemission at selected time delays following optical excitation. To increase the visibility of higher-energy states, the spectra have been normalized by a Fermi function with kBT ≈ 175 meV. Photoemission intensity is given in arbitrary units (arb. u.). e, f Time evolution of the momentum-resolved intensities at various energies above EF, as indicated by horizontal dashed lines in a–d. Surface-state bands, surface resonances and pure bulk states are denoted as SS, SR, and BS, respectively. g Momentum-integrated dispersion as a function of time delay, Δt. Note the logarithmic color scale. h Electronic temperature (blue circles) extracted from fits of the image in g to Fermi–Dirac distributions. Changes in the transient chemical potential are negligible. The error bars (±one standard deviation) are a metric for the deviation from a new equilibrium state of the excited electron population within the experimental resolutions. The blue solid line is a fit to an exponential decay convolved with a Gaussian profile to account for the time resolution, δt, as measured from the pump-probe cross correlation (inset). i Fits (red lines) of energy-distribution curves (black lines) to a Fermi–Dirac (F–D) distribution at selected time delays.
