Interacting fermions are at the core of the most studied, and yet poorly understood, properties of solids such as high temperature superconductivity, Mott-insulator, heavy fermions, and giant magnetoresistance. In addition to electron-electron interactions in many correlated electron systems the interactions with other degrees of freedom, such as Phonon, Magnons and Orbitons, is crucial to determine the electronic ground stated and therefore their physical properties. A possible way for distinguishing the roles of the different degrees of freedom and their interactions is to study the dynamical evolution of materials following a controlled perturbation. This philosophy is beyond all the studies tackling the physics of complex systems by means of ultrafast time domain studies.
The approach we are following are twofold:
1) On one hand we have developed a broadband spectroscopic probe capable of measuring ultrafast dynamics of the optical response on a large energy range. Our time domain spectroscopic techniques offers a direct view on the pump-induced changes of the reflectivity over a broad frequency range. This has significant advantages compared to a single-color pump-probe experiment. It allows, for instance, to determine the time-dependent evolution of the spectral weight of the different features characterizing the optical response. This has been used recently to address, for the first time directly, the spectral weight transferred between a condensate and high energy oscillators during a photo-induced quench of the superconductivity or, in other studies, to address the loss of kinetic energy of Hubbard exciton in light controlled spin disorder.
2) The second approach we follow is devoted to the development of experimental schemes allowing pump and probe measurements based on low photon energy excitation (E<30 THz). We are pursuing this directions inspired by the following considerations. Temperature driven phase transitions are determined by the thermal population of low energy excitations (E~KBT, which for room temperature is about 6 THz). The experimental setups we are developing are capable of performing pump and probe measurements based on pumps within this energy range. In this way it will be possible to excite the modes responsible for thermal phase transitions to highly non-thermal population allowing to address with unprecedented details the phase transitions thermodynamics and at the same time to drive phase changes on timescales not limited by thermodynamic constraints.
The direction we are currently pursuing is the joining of the two approaches: low energy excitations and broadband measurements of the dielectric functions.
Controlling quantum coherent phases of matter by THz light pulses
This SIR Project addresses the formation and annihilation of quantum coherent matter phases by means of pump and probe measurements based on THz light pulses.
ERC – Proof of Concept
In COBRAS we will establish Femtosecond Covariance Spectroscopy, a new spectroscopic technique to measure the optical response of material which is based on stochastic light pulses characterized by frequency uncorrelated intensity fluctuation.
The central idea of COBRAS is to set the way for commercial exploitation of covariance-based approaches to Raman spectroscopy. To this purpose we will develop a prototype spectrometer, study the general applicability of the covariance based methods and identify viable strategies for the commercialization of the spectrometer developed.
Excitonic insulator in two-dimensional long-range interacting systems (EXC-INS)
This project addresses the possibility to realise an `excitonic insulator’ (EI), the long-sought state of matter resulting from spontaneous formation of bound electron-hole pairs—excitons—expected when their binding energy exceeds the band gap.
EXC-INS aims at demonstrating that the EI phase is realized in two-dimensional systems where long-range Coulomb interaction is found to be virtually unscreened, and exciton binding is strongly enhanced.