Coherent many body dynamics
The interplay of correlations that leads to electronic order in the Mott transition or Charge Density Wave state is one of the key challenges in correlated electron systems. It gives rise to a large variety of different ground states ranging from highly unconventional (“bad”) metals to charge density wave states or superconductivity.
The case of the Mott insulator is due to high electronic correlations competing with the kinetic energy of the conducting electrons. Free electrons in a metal tend to delocalize over the lattice to minimize their energy. However the Coulomb repulsion between them works against that and localizes the electrons on the lattice sites. That reduces the density of states (DOS) at the Fermi energy and the Mott gap opens. The DOS splits into the so-called lower and upper Hubbard band.
Doping this Mott insulating state tunes the hierarchy of the competing interactions, where besides the pure electronic interactions also coupling to spin or lattice degrees of freedom are involved. This is especially true for the doped 2D Mott insulator which shows some of the most intriguing phenomena of correlated electron systems: High temperature superconductivity.
In our research we control the Mott state in various ways: Ultrafast photo-doping directly excites charges above the gap and allows investigating the peculiar properties of the photo-induced states. These do not show simple metallic properties but are still dominated by the strong electron-electron (or also electron-phonon) interactions. The photo-excited electron feels the Coulomb attraction by the hole that it leaves behind. Ultrafast optical probes on timescales of the electron hopping allow us to directly measure such drag-back dynamics for example in 1D organic charge transfer salts (ET-F2TCNQ) . Applying external pressure we tune the relative strength of the electronic interactions and therefore control the recombination dynamics of the excited electrons back to the ground state .
On the other hand in strongly electron-phonon coupled Mott-insulator 1T-TaS2 we observe a response that is dominated by the polaronic properties in the photo-induced state .
In the presence of strong electron-phonon coupling a ground state of matter often realized is a so-called Charge Density Wave state. These are solids that exhibit spatial modulations of the electron density, accompanied by a collective lattice distortion. The dynamics of the electronic and lattice system is coupled. One of the most interesting features is that such charge density waves are often in competition with superconductivity, and frustrate its emergence.
With light, we wish to understand and control the collective CDW dynamics (amplitudon and phason modes), but also to re-direct the material toward alternative ground states. Since CDWs involve band gap openings at the edges of the Brioullin zone time-resolved ARPES probes are the ideal tools for clocking the decoupling between electronic and lattice properties in the photoexcited state as we could demonstrate in the photo-excited state of 1T-TaS2  or revealing possible couplings between the collective amplitude and phason modes of a CDW in the blue bronze K0.3MoO3 .
Another way controlling strongly correlated materials is to excite directly vibrational, spin, or orbital properties that are coupled to the ground state or the electronic properties of the system.
In the organic charge transfer salt ET-F2TCNQ we use resonant excitations of local vibrational modes to alter the local molecular orbitals. That modulates the electronic on-site properties and makes the effective interactions in the Hubbard model time dependent. This method of selective modulation of a single degree of freedom in the solid state allows to experimentally deconstruct the Hubbard Hamiltonian by exposing couplings that otherwise would have vanishingly small contributions in equilibrium .
| Quantum interference between charge excitation paths in a solid state Mott insulator|
|S.Wall, D. Brida, S. R. Clark, H. P. Ehrke, D. Jaksch, A. Ardavan, S. Bonora, H. Uemura, Y. Takahashi, T. Hasegawa, H. Okamoto, G. Cerullo and A. Cavalleri|
|Nature Physics, 7, 114 (2011)|
| Pressure-Dependent Relaxation in the Photoexcited Mott Insulator ET-F2 TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates|
|M. Mitrano, G. Cotugno, S.R. Clark, R. Singla, S. Kaiser, J. Stähler, R. Beyer, M. Dressel, L. Baldassarre, D. Nicoletti, A. Perucchi, T. Hasegawa, H. Okamoto, D. Jaksch and A. Cavalleri|
|Physical Review Letters, 112, 117801 (2014)|
| Polaronic Conductivity in the Photoinduced Phase of 1T-TaS2|
|N. Dean, J. C. Petersen, D. Fausti, R. I. Tobey, S. Kaiser, L. V. Gasparov, H. Berger, and A. Cavalleri|
|Physical Review Letters, 106, 016401 (2011)|
| Clocking the Melting Transition of Charge and Lattice Order in 1T-TaS2 with Ultrafast Extreme-Ultraviolet Angle-Resolved Photoemission Spectroscopy|
|J. C. Petersen, S. Kaiser, N. Dean, A. Simoncig, H. Y. Liu, A. L. Cavalieri, C. Cacho, I. C. E. Turcu, E. Springate, F. Frassetto, L. Poletto, S. S. Dhesi, H. Berger, and A. Cavalleri|
|Physical Review Letters 107, 177402 (2011)|
|→ press release|
| Possible observation of parametrically amplified coherent phasons in K0.3MoO3 using time-resolved extreme-ultraviolet angle-resolved photoemission spectroscopy|
|H. Y. Liu, I. Gierz, J. C. Petersen, S. Kaiser, A. Simoncig, A. L. Cavalieri, C. Cacho, I. C. E. Turcu, E. Springate, F. Frassetto, L. Poletto, S. S. Dhesi, Z.-A. Xu, T. Cuk, R. Merlin, and A. Cavalleri|
|Physical Review B, 88, 045104 (2013)|
| Optical Properties of a Vibrationally Modulated Solid State Mott Insulator|
|S. Kaiser, S. R. Clark, D. Nicoletti, G. Cotugno, R. I. Tobey, N. Dean, S. Lupi, H. Okamoto, T. Hasegawa, D. Jaksch & A. Cavalleri|
|Scientific Reports, 4, Article number: 3823 (2014)|