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We report pilot time-resolved X-ray photoemission spectroscopy (tr-XPS) experiments performed at the SXP (Soft X-ray Photoemission) instrument [1] of the European XFEL. The samples were pumped with 1.5 eV laser pulses at fluences up to 2 mJ/cm⁻² and probed with XFEL pulses of 1 keV with a fluence of ~5 μJ/cm⁻², a temporal resolution of ~60 fs and an energy resolution of ~100 meV. We explored non-equilibrium physics of two systems:
Non-equilibrium charge carriers in InAs. In equilibrium, the downward band bending at the InAs(110) surface creates a built-in electric field. Following pump excitation, this field drives photoexcited electrons toward the surface and holes into the bulk, resulting in the surface photovoltaic (SPV) effect. However, because the electron mobility exceeds that of holes by about two orders of magnitude, electrons diffuse much more rapidly into the bulk than holes, lagging near the surface. The resulting transient charge separation (the photo-Dember effect) forms a dipole layer that counteracts the SPV-related drift of the charge carriers [3]. We follow the transient interplay between carrier drift and diffusion through the time-dependent energy shifts of the In 4d core levels measured with the probe X-ray pulse.
Non-equilibrium charge-density waves (CDWs) in TaSe2. The non-equivalent Ta sites in the commensurate-CDW phase of TaSe2 show up in the XPS spectra as energy splitting of the Ta 4f core levels [2]. The pump excitation transiently suppresses the CDW order, which is reflected in the reduction of the Ta 4f splitting magnitude. The remnant splitting may reflect either persistence of the equilibrium-CDW domains, a reduction in the amplitude and/or long-range phase coherence of this phase, or the transient formation of high-temperature CDW phases. The equilibrium-CDW state recovers on a time scale of ~20 ps. This transient process overlaps with periodic oscillations of the Ta 4f splitting, which reflect coherent excitation of CDW modes coupled to dipole-active lattice vibrations.
These two cases illustrate the unfolding power of tr-XPS for exploring dynamics of non-equilibrium electronic processes in various solid-state systems.
[1] P. Grychtol et al. The diagnostics and laser infrastructure at the Soft X-ray Port of the European XFEL. J. Synchrotron Radiat. 33 (2026) in print
[2] A. Reklaitis. Terahertz emission from InAs induced by photo-Dember effect: Hydrodynamic analysis and Monte Carlo simulations. J. Appl. Phys. 108 (2010) 053102
[3] S. Hellmann et al. Ultrafast Melting of a Charge-Density Wave in the Mott Insulator 1T-TaS₂. Phys. Rev. Lett. 105 (2010) 187401
| Scientific Topics | Solid State Physics |
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