Scientific Interests
Finite-momentum nonlinearities of driven quantum materials
Recent developments in ultrafast science allowed observing hidden and perturbation-enhanced phases and enabled manipulating band structures at fundamental timescales. Central to these advances is the ability to unfold higher orders of the electronic correlation function \(\tilde{\chi}^{(n)}({\bf k_1}\omega_1,{\bf k_2}\omega_2,\ldots,{\bf k_n}\omega_n)\) or to modulate the electronic states with fields oscillating in time at frequencies comparable with (or higher than) the intrinsic interaction energy scales. Measuring \(\tilde{\chi}^{(n)}\) is crucial, as this quantity encodes information about the propagation of electronic excitations inside a material and determines the material’s response to external electromagnetic fields. Our group is interested in studying THz-driven quantum materials (in particular Mott insulators, strongly correlated metals, and superconductors) by directly probing higher-order correlation functions with time-resolved electron energy loss spectroscopy (trEELS). This technique, which simultaneously provides energy and momentum resolution, will enable probing the propagation of collective excitations in driven quantum solids and will allow us extending nonlinear optics to finite momenta.
Manipulation of magnetic phases
On-demand control of effective exchange interactions Jex remains one of the ultimate goals of the field of ultrafast magnetism. Promising control protocols are either quenching the hopping amplitudes or modulating the electronic states with fields oscillating at frequencies comparable to (or higher than) the interaction energies (Floquet engineering) Although fascinating, ultrafast magnetism still lacks a finite-momentum, spin-sensitive probe that can directly measure Jex using the dispersion of spin waves. We are interested in using strong laser perturbations and probing the collective electronic response with time-resolved resonant inelastic X-ray scattering (trRIXS). This technique, in a suitable scattering geometry, is spin-sensitive and allows a direct measurement of the dispersion of spin waves. Since the dispersion of these excitations is determined by the exchange interactions at play in a magnetic phase, trRIXS represents effectively a finite-momentum, spin-sensitive probe that is currently lacking in the portfolio of modern ultrafast research.
Hydrodynamics of electronic crystals
One of the most fascinating problems in the physics of quantum materials is the existence of electronic liquid crystals (ELC) that self-organize by spontaneously breaking one or more symmetries of the underlying lattice. Although purely electronic and quantum in nature, these states closely mimic the symmetries occurring in classical liquid crystals and likely play an important role in the development of high-Tc superconductivity and anisotropic transport in correlated materials. From hydrodynamic considerations, it is known that classical liquid crystals must possess a rich fluctuation spectrum of propagating or diffusive collective modes. However, how these fluctuations manifest in the quantum case is currently unknown. We are interested in measuring ELC collective modes in the time-domain and as a function of momentum by means of ultrafast x-ray scattering. Ultrafast laser pulses will perturb the characteristic order parameter —typically corresponding to peaks in the finite-momentum density-density correlation function \(\chi''({\bf q},\omega)\)—and impulsively generate a non-equilibrium population of collective modes in a quasi-linear perturbation regime. Ultrafast x-rays are then used to provide a stroboscopic measurement on the hydrodynamic ELC fluctuations with sub-meV energy resolution. With this research we aim to establish a direct connection between classical and quantum liquid crystals and elucidate how ELC contribute to the appearance of superconductivity and other emergent phases.