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. 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 induced superconductivity and (ii) transient Higgs dynamics of the superconducting condensate.
Within project (i) we want to realize phonon-pumped time resolved ARPES probes on cuprate high temperature superconductors bringing together the advanced high harmonics tr-ARPES setup of D. Jones (UBC), the phonon-pump MIR-THz expertise of light induced superconductivity and OPA-lasers of S. Kaiser (MPI-FKF) and the ARPES expertise on cuprates both in equilibrium and time resolved by A. Damascelli (UBC). Addressing the energy and momentum dependent dynamics of driven superconducting states will allow an unprecedented view on the formation of a transient superconducting gap itself. 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) and M. Imada (U Tokyo), that will allow testing present models for light induced superconductivity and identifying possible driving mechanisms.
Experimentally the novel technique also will be extended to other interesting classes of complex correlated materials that are in the combined interest of the participating groups ranging from density-wave systems to excitonic condensates in condensed matter systems. Here transient optical probes in the far– and spatially resolved near-field regime in Stuttgart will complement momentum dependent probes in Vancouver.
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, which was formulated in 1962. 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. 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, in a first step, to advance the pioneering experiments of the Shimano group on the s-wave SC NbN to multiband systems and unconventional gap symmetries. 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 (together with colleagues from theory A. Schnyder, D. Manske, both MPI-FKF) for a beam-time at the TeraFERMI beam-line at the FERMI project in Trieste (Italy) to perform first Higgs experiments on MgB2 and cuprate (YBCO) superconductors. That will allow us to experimentally test theoretical predictions of our colleagues for the Higgs dynamics in coupled bands as well as for momentum dependent Higgs dynamics in d-wave systems. We also applied to advance these experiments at the more elaborate THz-FEL TELBE in Dresden. 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 possibly also in tr-ARPES. The latter would naturally then also link back to the project (i) described above.
The s–wave two–band superconductors exhibit beside the Higgs and the phase (gauge) mode also a new collective excitation, which occurs due to the Josephson coupling between the bands and corresponds to the fluctuations in the relative phase of the two coupled order parameters. In other words, this mode represents out–of–phase oscillations of the superconducting condensates. In a recent publication, we have theoretically studied the coupling between Higgs and Leggett modes in a two-band superconductor. We predict that the dispersion of the Leggett mode is changed in a characteristic way. An illustration for both modes in two–band superconductors is shown in Fig. (iii).
Dirk Manske (MPI-FKF), firstname.lastname@example.org
Andreas P. Schnyder (MPI-FKF), email@example.com
Stefan Kaiser (MPI-FKF), firstname.lastname@example.org
Andrea Damascelli (UBC), email@example.com
David Jones (UBC), firstname.lastname@example.org
Masatoshi Imada (U Tokyo), email@example.com
Ryo Shimano (U Tokyo), shimano_at_phys.s.u-tokyo.ac.jp