TY  - JOUR
A1  - Kühn, Danilo
A1  - Müller, Moritz
A1  - Sorgenfrei, Florian
A1  - Giangrisostomi, Erika
A1  - Jay, Raphael Martin
A1  - Ovsyannikov, Ruslan
A1  - Martensson, Nils
A1  - Sanchez-Portal, Daniel
A1  - Föhlisch, Alexander
T1  - Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2
JF  - Scientific reports
N2  - For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.
Y1  - 2019
U6  - https://doi.org/10.1038/s41598-018-36637-0
SN  - 2045-2322
VL  - 9
IS  - 488
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Kühn, Danilo
A1  - Giangrisostomi, Erika
A1  - Jay, Raphael Martin
A1  - Sorgenfrei, Florian
A1  - Föhlisch, Alexander
T1  - The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission
JF  - New journal of physics : the open-access journal for physics
N2  - Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources.
KW  - space-charge effects
KW  - mean-field model
KW  - x-ray photoemission
KW  - electron spectroscopy
KW  - pump-probe
KW  - ARTOF
Y1  - 2019
U6  - https://doi.org/10.1088/1367-2630/ab2f5c
SN  - 1367-2630
VL  - 21
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  -