Seminar "Ultrafast Dynamics of Charge-Density Waves" by Prof. K. Rossnagel
Post date: Jul 7, 2017 9:59:47 AM
2017. 07. 06 @ NAIST
The talk was about "Ultrafast Dynamics of Charge-Density Waves" investigated by time resolved photoemission spectroscopy.
The talk started with an impressive animation of valence bands and core levels during K adsorption. We saw a vivid change of electronic states by K adsorption. The time scale of the animation was around 10 s.
Then, the talk moved to physics with the time resolution of femtosecond.
The power of trARPES became clear through the talk. Data was so nice and clear. Entire talk was so energetic and inspiring. I think students understand what Prof. Rossnagel is interested in, what he wants to make clear, and his future plan.
The lecture was planned from 14:00- 15:00, plus discussion for 30min. But it was too short for such fascinate talk. We also had so many questions. So we had another 30 min. or more for questions.
Some key points:
1. two mechanisms were introduced for the gap opening.
a. the conventional peierls transition-type
b. excitonic insulator transition
2. Very very impressive data on "amplitudon" of CDW phase observed by pump and probe.
After the shot of pumping light (around 1.5eV), the valence band edge starts to oscillate up and down with the
time scale of 125fs! "amplitudon" and "phason" are the terms I have to cover.
Every snap shot was taken within 2min -5min and there are around 200 snaps in total.
3. In the pump and probe experiment, electron get heated by the pump light. This process instantly occurs (less than 100fs.).
At this stage, the electron temperature is huge (10000 K)
Then the relaxation of hot electron takes place. This process requires energy transfer from hot electron to something else.
Phonon can receive the energy from the hot electron. But the time scale of this energy transfer process is around picosecond.
Then, the electron and phonon reaches to "quasi equilibrium" state. This lasts for 200 -300ps. Then, energy damping occurs.
4. 22eV laser made by Ar gas excitation was used. The repetition frequency is not high (10kHz?).
6eV laser ARPES and 22eV laser ARPES covers different physics. You can access to the BZB with 22eV.
5. At Hamburg, "XFEL+Momentum microscope + spin" project is now going on.
trPED is one of their target. There seems to be some room to discuss on the acceptance angle of momentum microscope at soft X-ray region.
6. Since the Prof. Skibowski time, they have made materials like TiSe2 and TiS2 by themselves.
References I need to check:
PRM Aoki 86 779 (2014)
papers from Prof. Rossnagel group.
.