Fundamental interactions governing the non equilibrium electronic structure in low dimensions

Abstract

This thesis explores the electronic structure and ultrafast dynamics of two lowdimensional materials with focus on the role of intrinsic interactions and couplings to the environment. Charge carriers in the matter are never completely independent: they interact among each other and couple to lattice vibrations (phonons) and other excitations. Their behavior is also influenced by the environment in which the material is embedded. Moreover, confining charges to low dimensions promotes interactions and enhances the impact of the environment. All these factors lead to a variety of static and dynamic properties, and potentially to the emergence of new phases of matter. Investigating a system out of its equilibrium helps the assignment of each fundamental interaction to the related physical property. Remarkably, addressing ultrafast dynamics can also uncover novel properties which otherwise would not be accessible at equilibrium. The first study of this thesis explores the modification of the unoccupied electronic structure of ultrathin films of SiO2 by electron quantum confinement and investigates the electronic coupling at the interface with the Ru(0001) substrate. By means of timeresolved two-photon photoelectron spectroscopy, the formation of quantized states is resolved, whose energies are altered by the image potential of the metal. The second and major study deals with the quasi-one-dimensional material Ta2NiSe5 which shows a combined electronic and structural phase transition upon heating and likely exhibits an excitonic insulator ground state. Here, the ultrafast charge carrier, exciton and lattice dynamics are disclosed by complementary time-resolved photoelectron and optical spectroscopies. The electron relaxation rate follows an anomalous dependence on the excess energy and is reduced by the transient increase of screening of the Coulomb interaction. The coherent phonon dynamics are generated by the photoinduced displacement of the charges. Optical absorption saturation restrains the number of photoexcited charges thereby hindering a photoinduced structural change. Also, the electronic band gap is transiently modulated by means of light. Nontrivially, it widens upon photoinduced strengthening of the excitonic insulator order parameter in remarkable agreement with Hartree-Fock calculations. These findings show that intrinsic interactions highly impact on the properties of Ta2NiSe5. Moreover, they demonstrate that it is possible to optically control the out-of-equilibrium electronic structure of a strongly interacting system on an ultrafast timescale. These studies show that unraveling the role of fundamental interactions in low dimensions provides profound understanding and potential control of the equilibrium electronic structure and the photoinduced ultrafast dynamics of very diverse materials