|Near Gap Excitation of the Amplitude Mode in CeTe3|
|Full Author List||J.A. Sobota, S.-L. Yang, A.F. Kemper, P. Giraldo-Gallo, R.G. Moore, I.R. Fisher, P.S. Kirchmann, T.P. Devereaux and Z.-X. Shen|
SIMES (Stanford Institute for Material and Energy Science)
We present time- and angle-resolved photoemission spectroscopy measurements on the prototypical CeTe3 charge density wave (CDW) system. Optical transitions across the CDW gap and transient population of the unoccupied band structure, reveals a CDW gap size of 0.59 eV. Coherent excitation of three distinct lattice modes leads to coherent modifications in the Te5p band structure.
In a first step, a transient tight-binding model dispersion was applied to disentangle coherent changes in the electronic structure, in both energy- and momentum-space, depending on the distinct spatial polarisation of the coherently excited modes. DFT frozen phonon calculations identify an out-of-plane optical A1g phonon mode at 2.98 THz, which rigidly renormalises the energy scaling of the measured 5px(z) band. In contrast to that, the collective in-plane modes at 2.19 THz and 2.68 THz are found to affect both, the CDW gap size as well as the rigid energy scaling. Periodic modulation of the CDW gap size is attributed to a change in hybridisation between the in-plane electronic 5px(z) bands and their corresponding CDW shadow bands, solely driven by the in-plane lattice modes.
In a second step, the pump-photon energy has gradually been lowered towards the CDW gap size. We find a decreasing oscillation strength of the amplitude mode (2.19 THz) for decreasing pump-photon energies. The observed quantitative trend is in line with the calculated momentum integrated joint density of states for optical transitions between the hybridised upper and lower 5px(z) bands. This can be interpreted such that quasi-elastic scattering processes determine the CDW response and not direct optical transitions. Furthermore the results confirm, that the frequency of the driving field has to overcome the CDW band gap in order to coherently excite the order parameter.