Key goals within the project are characterizing and understanding the non-equilibrium dynamics of light-induced phase transitions or photo-stabilized non-equilibrium phases. A particular key aspect is optical control of superconductivity or similar coherent condensates. Therefore both experimental and theoretical groups throughout the center work closely together addressing various aspects of the transient order parameter dynamics or possible models behind light driven phase transitions.
From the experimental point of view 2 collaborations within the center combine their individual expertise to realize novel experiments to advance the understanding of (i) light control of superconductivity or other correlated electron states and (ii) transient Higgs dynamics of the superconducting condensate.
Within project (i) we want to realize optically or phonon-pumped novel states in complex quantum materials that we probe time resolved over a broad frequency range using short THz up to UV light pulses and even momentum resolved via tr-ARPES techniques. That brings together the technology expertise of advanced high harmonics laser setups of D. Jones (UBC), the MIR-THz expertise and OPA-lasers of S. Kaiser (MPI-FKF) and the ARPES expertise by A. Damascelli (UBC) and time of flight techniques by Y. Morita (U Tokyo).
Research areas interlink closely to research groups within the center like with B. Keimer (MPI-FKF), H. Takagi (MPI-FKF, U Tokyo), G. Logvenov (MPI-FKF), or D. Bonn (UBC). They range from exploring and controlling superconductivity, to more elusive states like spin-liquids or excitonic insulators. Here transient optical probes in the far– and spatially resolved near-field regime in Stuttgart will complement momentum dependent probes in Vancouver (Fig. 1,2).
In particular addressing the energy and momentum dependent dynamics of driven states will allow an unprecedented view on the formation of transient quasiparticles or the dynamics of order-parameters; for light induced superconductivity e.g. the formation of the superconducting band-gap. Further it opens an experimental probe to measure transient electron-phonon or interaction-couplings and possible band renormalizations. Together with theory, D. Manske (MPI-FKF), that will allow testing present models e.g. for light induced superconductivity and identifying possible driving mechanisms.
The power of on-equilibrium techniques was shown e.g. by probing the fill in of the superconducting gap in cuprates by quenching the condensate via ultra-short light pulses  or the possibilities of inducing superconductivity by mode selective phonon excitation . Theoretical models of quenches and pulse driven dynamics foster our understanding of underlying mechanisms . In the field of excitonic insulators phononic coupling to the excitonic condensate could be identified  and possibilities for optical band engineering were explored .
Closely related to this non-equilibrium control and dynamics is project (ii) that aims to probe in particular the collective dynamics and the eigenmodes of coherent condensates. A fundamental property of all types of superconductors is the appearance of a collective excitation, which is a result of breaking the continuous U (1) symmetry. This is the statement of the general Goldstone theorem. For instance, in neutral superfluids with a two–component order parameter one can excite a so–called Anderson–Bogoliubov or gauge mode, which corresponds to the angular excitation in the Mexican hat potential of the free energy F (Figure 3). The existence of this phase mode is necessary to restore the particle conservation law. On the other hand, in charged systems, like single band superconductors, this collective excitation is shifted to the plasma mode according to the Anderson–Higgs mechanism and appears in most cases in the quasiparticle continuum. This is the result of the long–range Coulomb interaction. Moreover, one–band superconductors exhibit, in addition, amplitude fluctuations of the order parameter. Due to the approximate particle–hole symmetry of the superconducting excitations and similarities to the Lorentz invariant theory, this radial excitation in the Mexican hat potential of the free energy F corresponds to the Higgs mode from high energy physics.
Within project (ii) we address the coherent amplitude dynamics of the superconducting condensate via their Higgs mode oscillations. That allows a full characterization of the condensates and their intrinsic couplings via “Higgs-Spectroscopy”. Experimentally S. Kaiser (MPI-FKF) and R. Shimano (U Tokyo) team up together with colleagues from theory A. Schnyder (MPI-FKF) and D. Manske (MPI-FKF), in a first step, to advance the pioneering experiments of the Shimano group on the s-wave SC NbN  to unconventional gap symmetries [7,8,9]. These experiments call for access at large scale facilities that can deliver sufficiently high field THz-pulses. For this purpose we have just recently applied for a beam-time at the TELBE beam-line at the HZDR in Dresden to perform first Higgs experiments on cuprate superconductors . From the theory side the challenge is to classify the driven non-equilibrium modes and their mutual possible couplings [7,10].
In the long run a goal would be to also identify Higgs mode dynamics in the light induced superconducting states both optically via transient THz spectroscopy but also in tr-ARPES. The latter would naturally then also link back to the project (i) described above.
 F. Boschini et al. Nature Materials 17, 416 (2018).
 S. Kaiser Physics Scripta 92, 103001 (2017).
 N. Bittner et al. arXiv:1706.09366 accepted at JPSJ (2019).
 D. Werdehausen et al. Science Advances 4, eaap8652 (2018).
 K. Okazaki et al. Nature Communications 9, 4322 (2018).
 R. Matsunaga et al. Science 345, 1145 (2014).
 B. Fauseweh et al. arXiv:1712.07989 (2017).
 K. Katsumi et al. Phys. Rev. Lett. 120, 117001 (2018).
 H. Chu et al. et al arXiv:1901.06675 (2019).
 H. Krull et al. Nature Communications 7, 11921 (2016).
Stefan Kaiser (MPI-FKF), firstname.lastname@example.org
Andrea Damascelli (UBC), email@example.com
David Jones (UBC), firstname.lastname@example.org
Ryo Shimano (U Tokyo), shimano_at_phys.s.u-tokyo.ac.jp
Dirk Manske (MPI-FKF), email@example.com
Andreas P. Schnyder (MPI-FKF), firstname.lastname@example.org