Session L41: Ultrafast Control of Correlated Materials by Terahertz Light

Probing and Controlling Low Energy Excitations in Complex Materials with Using Terahertz Pulses

Single–cycle terahertz (THz) pulses enable both the understanding and control of low-energy excitations in materials on ultrafast timescales. Combining excitation with THz pulses with a time-delayed probe based on second harmonic generation enables the observation of ultrafast structural changes with surface sensitivity. Using this technique, we investigate the dynamics of surface and bulk carriers, as well as nonlinear phonon dynamics, in the topological insulator Bi₂Se₃. We will also describe the use of THz pulses as a probe to reveal spin dynamics in insulating antiferromagnets.   

Terahertz Light-Quantum-Tuning of a Metastable Correlated Phase Hidden by Superconductivity

“Sudden” quantum quench and prethermalization have emerged as a cross-cutting theme
for discovering emergent states of matter, such as quantum criticality and generalized Gibbs
ensembles of cold atoms, quark-gluon plasmas, or phase transitions in the early universe.
Yet this remains challenging in electron matter, especially superconductors (SC), despite
recent progress. The grand open question of what is hidden underneath the “SC dome”
occurring in several quantum materials appears to be universal, yet the new, even thermodynamically
forbidden states have been unexplored by quantum quench of the SC. Here we reveal a hidden quantum phase of prethermalized, gapless electron fluid, which evolves
following single-cycle, resonant terahertz quench of the SC gap above a critical fluence.
Its “non-Fermi-liquid” conductivity is characterized by a sharp coherent peak and a vanishing
scattering rate, most pronounced around full depletion of the SC condensate, which
is absent for high frequency pump and decreases almost linearly at low frequency. Above
threshold, such quantum behaviors with memory persist as an unusual prethermalization
plateau, without relaxation to normal metallic or SC states for order of magnitude longer
than the quasi-particle energy relaxation times. Switching to such a metastable quantum
fluid signals a dynamic coexistence of SC and hidden electronic orders, and
implies novel organization principles beneath superconductivity.   

Ultrafast Optical Control of Complex Quantum Materials

It is a well known fact that phase transitions in condensed matter can by triggered by external parameters such as temperature, pressure, electric or magnetic field. Finding systems that show light induced transitions became a particular interesting field of research. Advanced nonlinear optical methods based on pump-probe spectroscopy open new ways of controlling ultrafast dynamics in complex materials on unprecedented timescales. In quantum materials, finding new ways of manipulating the interplay of electronic phases or effectively tuning electronic interactions opens new avenues in controlling physical properties and designing new functionalities.
I will discuss different scenarios like the balancing between competing phases via ultrashort light pulses or possibilities of dynamical stabilization of new states in periodically driven light fields. One finds remarkable possibilities to induce superconductivity in high-Tc cuprate superconductors [1] by melting competing “stripe”-order [2] or promoting SC to temperatures far above Tc [3,4]. Possible light-induced superconductivity in doped fullerides K₃C₆₀ [5] serves as important example that inducing such intriguing effects is a more general effect based on non-linear effective interaction control [6,7]. Going beyond superconductors, for excitonic insulators (a BEC of excitons) probing the coherent response of the order parameter reveals possible couplings to the lattice structure [8].

[1] S. Kaiser, Physica Scripta 92, 1003001 (2017).
[2] D. Fausti et al. Science 331, 189 (2011).
[3] S. Kaiser et al. Phys. Rev. B 89, 184515 (2014).
[4] W. Hu et al. Nature Materials 13, 705 (2014).
[5] M. Mitrano et al. Nature 530, 461 (2016).
[6] S. Kaiser et al., Scientific Reports 6, 3823 (2014).
[7] R. Singla et al., Phys. Rev. Lett. 115, 187401 (2015).
[8] D. Werdehausen et al. arxiv:1607.02314 (2016).   

Nonequilibrium steady states and transient dynamics of conventional superconductors under phonon driving

The resonant excitation of mid-infrared phonon modes provides a new way of controlling material properties. An interesting result of such experiments is a light-induced superconducting (SC)-like behavior [1], for which several possible mechanisms have been theoretically proposed. However, in driven systems, the competition from heating and nonthermal energy distributions must be taken into account in order to assess the ultimate effect of such nonequilibrium states on SC.

In the present work, we focus on the scenario that the parametric excitation of Raman-active phonons through nonlinear couplings to the mid-infrared phonons enhances the phonon-mediated attractive interaction and hence SC [2]. We systematically investigate the effects of the parametric phonon driving on SC in the Holstein model by analyzing the transient dynamics and the non-equilibrium steady states (NESS) using the non-equilibrium dynamical mean-field theory [3]. For NESS, the Floquet dynamical mean-field theory for electron-phonon systems is newly formulated. In the strong electron-phonon coupling regime, even though the attractive interaction can indeed be enhanced by the driving, the SC gap and the superfluid density are always suppressed in the NESS, in particular, at the parametric resonance. Even in the transient dynamics towards the NESS, the net effect is a suppression of SC. In the weakly-coupled regime, we study how the SC fluctuations evolve under the driving. Generically these correlations decay faster in the presence of driving and the decay becomes particularly fast around the parametric resonance. Our systematic analysis shows that, in a wide parameter range, the heating of the system is the dominant effect and that the parametric phonon driving has a negative effect on SC.

[1] M. Mitrano, et al., Nature 530, 461 (2016).
[2] M. Knap, et al., Phys. Rev. B 94, 214504 (2016); M. Babadi, et al., Phys. Rev. B 96, 014512 (2017).
[3] Y. Murakami, et al., Phys. Rev. B 96, 045125 (2017).