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Zusammenfassung:
Detection of the energy conversion pathways between photons, charge carriers, and the lattice is of fundamental importance to understand fundamental physics and to advance materials and devices. Yet, such insight remains incomplete due to experimental challenges in disentangling the various signatures on overlapping timescales. Here, we show that attosecond core-level x-ray absorption fine-structure spectroscopy (XANES) meets this challenge by providing an unambiguous and simultaneous view on the temporal evolution of the photon-carrier-phonon system. We provide surprising new results by applying the method to graphite, a seemingly well-studied system whose investigation is complicated by a variety of mechanisms occurring across a wide range of temporal scales. The simultaneous real-time measurement of electrons and holes reveals disparate scattering mechanisms for infrared excitation close to the Fermi energy. We find that ultrafast dephasing of the coherent carrier dynamics is governed by impact excitation (IE) for electrons, while holes exhibit a switchover from impact excitation to Auger heating (AH) already during the 11-fs duration of the infrared light field. We attribute this switchover to the limited scattering phase space in the n-doped material. We further elucidate the excitation mechanisms of strongly coupled optical phonons (SCOPs). The coherent excitation of both SCOPs is nondisplacive and is explained by the strong electron-phonon scattering, i.e., via a seemingly incoherent process. We identify the A′1 phonon as the dominating channel for dissipation of electronic coherence. Moreover, unobserved in graphite, we find high-frequency oscillations up to 90 THz, which arise from the modulation of the electronic density of states by the atomic displacements along the E2g and A′1 modes. These measurements establish the utility of core-level XANES with attosecond temporal resolution to achieve an unambiguous and simultaneous view on the temporal evolution of the photon-carrier-phonon system with surprising new results even for a seemingly well-studied system like graphite. While the graphite measurement was conducted around the K edge of carbon, adapting the methodology to other materials only requires spectra coverage of the respective elemental edge of the material’s constituent. This flexibility makes our methodology widely applicable to detect and distinguish the various dynamic contributions to the flow of energy inside materials on their native timescales.
