Seminars Archive

Ultrafast evolution of out-of-equilibrium Mott-Hubbard materials

Abstract
The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behavior. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic thermal pathways. The prototype Mott-Hubbard material V₂O₃ presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. Recent results on this model system will be presented, combining different ultrafast techniques (time-resolved photoemission, reflectivity, and FEL-based X-ray diffraction), which revealed the formation of a transient non thermal phase triggered by selective electron-lattice interplay. An example of Mott-Hubbard material in a quadratic lattice structure is instead represented by BaCo_1-xNi_xS₂, a quasi-2D compound presenting a metal to insulator transition (MIT) for x_cr~0.22, and some similarities with the phase diagram of cuprates. With a series of ARPES experiments, we confirmed the multi-orbital band structure of BaNiS₂, identified several Dirac-like band dispersions near the Fermi level and a large Rashba splitting of the bands with hidden spin-chiral polarization at the X point resulting from the spin-orbit interaction coupled to a strong crystal field. By means of tr-ARPES we have revealed the unoccupied electronic states of the Dirac-like bands, and studied the out-of-equilibrium dynamics of the photoexcited carriers close to the Fermi level in a k-resolved fashion. The excess electrons relax to the valence band at about 600 fs thanks to the strong interband and intraband scattering with the electron-phonon coupling playing the key role.