Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2

Danilo Kühn,Raphael M Jay,Daniel Sánchez-Portal,Florian Sorgenfrei,Erika Giangrisostomi,Moritz Müller,Nils Mårtensson,Ruslan Ovsyannikov,Alexander Föhlisch

doi:10.1038/s41598-018-36637-0

Abstract

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.

Highlights

: First of all, an exponential background of the form IB(Ekin) = ahν ⋅ [1 − b ⋅ exp(−α ⋅ Ekin)], which describes the low energy secondary electron cascade and the high energy XPS loss tail, was subtracted from all electron spectra
All experimental data were measured at the BESSY II UE56/1 PGM soft X-Ray beamline in the single bunch mode with 13 mA ring current
The estimated kinetic energy resolution is better than 20 meV in the whole energy range under consideration

Summary

1T-TaS2 exhibits, despite of being a prototypical layered dichalcogenide, surprisingly strong electronic interlayer coupling between the commensurate layers of the low temperature CCDW phase, leading to isotropic three-dimensional charge transfer on the sub-femtosecond timescale. Two times slower out-of-plane than in-plane polarized charge transfer occurs for the nearly commensurate NCDW phase. This reflects the apparently reduced interlayer CDW coupling in the NCDW phase as the loss of commensurability sets in, resulting in anisotropic two-dimensional charge transfer. Selective preparation of the propagating excited state wave packet and sub-femtosecond temporal information from the soft X-ray spectroscopic core-hole-clock method allows to establish, in direct comparison to density functional theory, this presumably general mechanism for phase transitions and tailored properties of layered dichalcogenide materials involving interlayer coupling and charge density wave physics

Methods

Author Contributions

Additional Information