The non-equilibrium quantum dynamics of many body systems is an unexplored frontier where new physics is waiting to be discovered. Scientifically one of the most fascinating emergent phenomenon is high temperature superconductivity. Some of the most intriguing questions concern the interplay of different energy scales and competing interactions in cuprate superconductors. One region of their phase diagram which has attracted much attention is the so-called pseudogap phase, found below a characteristic temperature T* in moderately-doped materials. The origin of this gapped, but non-superconducting state has been attributed to a variety of nanoscopic phenomena, ranging from a precursor superconducting state with pre-formed Cooper pairs without phase coherence, to a scenario where electronic correlations compete with superconductivity. Transport and tunneling measurements, optical spectroscopy and scattering experimentshave been used so far to investigate the superconducting and normal-state phase in order to identify the interactions and excitations to which these phases are coupled. Using time resolved optical measurements, excitations with visible and near-infrared light heat the electronic system into a highly non-equilibrium state exciting charge carriers across the superconducting gap. Ultrashort laser pulses allow probing the dynamics of hot electron carriers in the normal phase as they relax back to the superconducting ground state[i],[ii].
Going beyond such photo-doping dynamics, mid-infrared pulses were shown to dynamically induce non-thermal phase transitions directly by triggering low frequency excitations in complex oxides photo-inducing superconductivity in the insulating phase of La1.8-xEu0.2SrxCuO4 by melting the so-called ‘stripe’-phase, a competing charge- and spin order that suppresses superconductivity[iii],[iv]. Similar effects appear for photo-doping the closely related striped La2−xBaxCuO4where light triggers the competition between charge order and superconductivity[v]. However most striking effects appear in experiments where we set out to establish phase coherence in YBa2Cu3O6+x for temperatures far above Tc. Therefore high intensity optical pulses at mid-infrared frequencies tuned resonant to specific lattice phonons are used to dynamically modulate the crystallographic structure, created a phonon-dressed state. This creates a transient superconducting state for base temperatures far above Tc. For low doping transient superconductivity is observed even as high as room temperature [vi],[vii] (Fig. 1).
THz time-domain spectroscopy fully characterizes not only the optical properties of the dynamically riven superconducting state but also its time evolution whilst the driving phonon-modulation and its decay. The results point to a scenario in which two key observations are important: First, only a fraction of the crystal is susceptible to the excitation and the transient state and forms an inhomogeneous phase of photo-induced superconducting islands in a normal conducting backgroundvi. Second, within the superconducting islands phase coherence is established by redistributing coherent coupling strength between the layers of the superconductor and is supported as long as the vibrational modulation of the crystal structure is presentvii. The picture that emerges from these results points to a dynamical stabilization of superconducting coherence within spatial localized areas.
To prove this picture and finding ways to improve the stabilization one needs to access the complex transient response with spatial resolution that allows mapping the localized islands and measure locally both the full complex optical response but also the interaction strength of the superconducting Cooper-pairs as well as possible competing interactions like charge density wave orders. To achieve the required spatial subwavelength resolution for these measurements we want to combine the successful ultrafast optical control and THz-probe spectroscopy with scattering-type scanning near-field optical microscopy (s-SNOM). Time resolution down to the fs regime will allow measuring the dynamic within correlated materials on the relevant interaction time-scales while probing the full complex response of the participating collective excitations over the whole relevant energy scale. The optical probe via the s-SNOM basically allows investigating the “birth of a quasi-particles” due to the dressing interactions under tailored optical excitation. That makes it the ideal tool investigating both, the transient properties of the photo-induced and exclusively the properties of dynamically stabilized states under the influence of dynamic correlations on the nanoscale. In order to measure the interaction strength of such states and also possible competing order parameters the optical control setup will be extended with time resolved Raman probes [viii]. Competing order parameter can be measured directly via a collective amplitude mode of a charge density wave with Impulsive Stimulated Raman Scattering (ISRS) [ix],[x]. The realization of such a setup does not only allow solving the most pressing questions in optically induced transient superconductivity but also paves the way to manipulate material on the nanoscale opening novel ways of functionalizing complex quantum materials.
Aim of the project will be bringing the successful MIR pump-THz probe setups, as well as our Raman setups that are under development, to the nanoscale by establishing them on a s-SNOM near field microscope. We already have successfully shown the measurement of static tunable VIS-MIR near field spectra on such a setup as well as time-domain THz data at equilibrium (Fig. 2). These experiments have to be extended into a transient pump-probe setup allowing for photo-doping the sample or implementing optical sources that allow resonant phonon pumping. The latter requires applying novel MIR sources operating at high repetition rate that are being developed together with colleagues at the university (in Stuttgart). The existing and newly developed setups will be applied to the scenario of light induced superconductivity in underdoped cuprates that we investigate together with the department Keimer. Further we plan to extend it also to light-driven effects in novel complex materials, e.g. collective Higgs-phonon coupled dynamics that we could induce in excitonic insulators, a topic that we work on together with the department Takagi. These planned experiments will complement unique tr-ARPES measurement capabilities at UBC with Jones and Damascelli where the effect on phonon dressing on the (transient) band structure can be directed probed.
Stefan Kaiser (MPI-FKF) firstname.lastname@example.org
Dirk Manske (MPI-FKF) email@example.com
Andrea Damascelli (UBC) firstname.lastname@example.org
David Jones (UBC) email@example.com