Ultrafast Electron Dynamics
For this project, we are collaborating closely with the work groups of Prof. Martin Wolf, Department of physics at the Freie Universität Berlin and Department of Physical Chemistry of the Fritz-Haber-Institut, Germany, and of Prof. Ian Fisher, Department of physics at Stanford university, USA.
For this project, we are collaborating closely with the work groups of Prof. Martin Wolf, Department of physics at the Freie Universität Berlin and Department of Physical Chemistry of the Fritz-Haber-Institut, Germany, and of Prof. Ian Fisher, Department of physics at Stanford university, USA.
These microscopic interactions and rearrangements happen in very short times (femtosecond to picosecond scale). In order to make them visible, we use time-resolved ARPES to take a look at the changes in the electronic band structure in real time. An ultra-short (50 fs) infrared (1.5 eV) pump pulse excites the system, and after a variable time delay, an ultra-short (90 fs) ultraviolet (6 eV) probe pulse takes a snapshot of the excited system, having propagated for the set pump-probe delay.
This time-resolved angle-resolved photoemission spectroscopy (trARPES) allowed us to observe the effects of collective excitations and collective vibrations via their interaction with the electronic band structure. Namely, we were able to resolve a k-dependent, anisotropic correlation and excitation: the pumping leads to transient melting of the CDW and thus to the time-dependent closing of the electronic CDW gap in the electronic band structure. Furthermore, we revealed two collective modes that coherently modulate the electronic band structure due to electron-phonon coupling. Since trARPES allows us to obtain the single particle spectral function and the influence of collective modes on it at the same time, thus revealing the connections between those, we were able to conclude that the observed collective vibrations are intimately connected to the CDW physics.
We believe we have observed the transient effects of the amplitude mode on the band structure, and furthermore, we documented the melting of the electronic CDW gap in real time. To our knowledge, this is the first time that momentum-dependent dynamics were recorded with this technique. Observing the amplitude mode, which is at the core of correlated CDW physics, is a major breakthrough in uncovering the mechanics of collective phenomena in solid state physics. Future experiments will include and investigate the influence and effect of the amplitude mode in the superconducting phase of the high temperature superconducting cuprates and the newly discovered Fe-based superconductors, which are both of relevance and are both currently investigated by our group. In fact, we think that discovering a collective mode in these materials similar to the one we demonstrated observable in the CDW system investigated by us could mean a major breakthrough in understanding the underlying physics in these systems.
Pictures taken from Schmitt et al., Science 321, 1649-1652 (2008).
