Electronic structure and collective excitations in correlated materials [electronic resource] : frequency and time domain perspectives
Keywords: Applied Physics

In condensed matter, electrons are not independent but are coupled to their neighbors via interactions; they are said to be correlated. This fact that electronic states are aware of their neighbors' states and changes thereof gives rise to a rich variety of properties and physics that defines the nature around us. Correlations in condensed matter can range in their effect from slight changes in the band structure to the emergence of order and symmetry breaking, which lead to novel properties and phases. These correlations exhibit themselves on different time scales. Mott-insulators, in which strong Coulomb repulsion between electrons splits a half-filled conduction band to result in an insulator, can have a band gap of several eV. On the other hand, transitions into the superconducting or charge density wave state, for example, happen on energy scales within several tens to hundreds of meV of EF. Despite the plethora of phenomena arising from these correlations, this many-body problem is hard to tackle and there is still much to be learned. This work investigates different aspects of correlated systems in and out of equilibrium with angle resolved photoemission (ARPES), and time resolved ARPES, two powerful spectroscopic techniques that are able to elucidate the dynamics and mechanics of correlations. Starting out, the experimental ARPES setup is introduced, and the ARPES system located in the Geballe Laboratory for Advanced Materials at Stanford University is described in more detail, since its maintenance and enhancement were an integral part of this work. In the following, the high energy properties of Nd2-xCexCuO4+d (NCCO) are investigated with ARPES. NCCO belongs to the electron doped side of a class of materials called high temperature superconducting Cuprates (HTSCs), so called because of their unusually high transition temperatures. The HTSCs exhibit a plethora of rich physics; including the anti-ferromagnetic insulating phase of the undoped parent compound dominated by Mott-Hubbard physics to the superconducting dome which features superconductivity with a d-wave symmetry whose origin is still a mystery. The hole doped HTSCs show a vertical band dispersion in ARPES measurements around 0.3 eV which was termed the high energy anomaly (HEA). Here, a systematic study of high energy features on NCCO revealed a similar HEA, albeit around 0.6 eV binding energy. We were able to successfully explain the HEA within the Hubbard model as being a cross-over from the quasi particle band resulting from doping and the lower or upper Hubbard band, depending on doping. The simulations also captured the difference in energy scale between hole and electron doping. Next, focusing on energies within several tens of meV of EF, a different energy scale of NCCO is explored. In the hole doped HTSCs, a discontinuity of the electronic dispersion around 50-70 meV was observed both in the region of the Brillouin zone were the d-wave superconducting gap had a node ("nodal") and where it had a maximum ("antinodal"). Conversely, in the electron doped materials this kink could only be observed in the antinodal region. If the discontinuity or "kink", which is hypothesized to originate from electron phonon coupling to certain Oxygen modes, is related to superconductivity, one would imagine it to have the same universality as the observed superconductivity. Our work demonstrates that new and improved ARPES data show a kink in the nodal region of NCCO as well, giving this discontinuity universality among the HTSCs. Superconductivity (SC) (at least conventional, electron phonon mediated superconductivity according to Bardeen-Cooper-Schrieffer) is closely related to its brethren, the spin- (SDW) or charge-density waves (CDW): all are mediated by different channels of the same type of interaction. The non-equilibrium dynamics and excitation modes of CDWs and SCs are closely related. Time resolved ARPES (tr-ARPES) is able to probe both non-equilibrium dynamics and collective excitation modes in real time. Continuing, this work explores the non-equilibrium physics of TbTe3, a model system for studying charge density waves. Our systematic study of the transient dynamics in dependence of excitation density revealed a complex picture ranging all the way from a weakly perturbed regime, in which collective modes were evident to a strongly perturbed regime, where we could observe the transient melting of the CDW state. This unprecedented insight into the dynamics of interactions will enhance our future understanding of correlated materials. Through the strengths of tr-ARPES, we were able to assign one of the collective modes we observed to the amplitude mode of the CDW. This provides a major stepping stone towards seeing a similar amplitude mode in superconductors. Especially in novel superconductors like the HTSCs or the Pnictides, where the mechanism of superconductivity is still debated, the observation of such collective modes could greatly aid towards their understanding which is needed in order to eventually exploit their huge potential for real-life applications.