Gapped Collective Charge Excitations and Interlayer Hopping in Cuprate Superconductors.

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr_{0.9}La_{0.1}CuO_{2}. We detect a plasmon gap of ∼120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in Sr_{0.9}La_{0.1}CuO_{2} are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered t-J-V model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping t_{z}. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t_{z}.

Interplay of spin-orbit interactions, dimensionality, and octahedral rotations in semimetallic SrIrO(3).

We employ reactive molecular-beam epitaxy to synthesize the metastable perovskite SrIrO(3) and utilize in situ angle-resolved photoemission to reveal its electronic structure as an exotic narrow-band semimetal. We discover remarkably narrow bands which originate from a confluence of strong spin-orbit interactions, dimensionality, and both in- and out-of-plane IrO(6) octahedral rotations. The partial occupation of numerous bands with strongly mixed orbital characters signals the breakdown of the single-band Mott picture that characterizes its insulating two-dimensional counterpart, Sr(2)IrO(4), illustrating the power of structure-property relations for manipulating the subtle balance between spin-orbit interactions and electron-electron interactions.

Low temperature hidden Fermi-liquid charge transport in under doped La x Sr1-x CuO2 infinite layer electron-doped thin films.

We have studied the low temperature electrical transport properties of La x Sr1-x CuO2 thin films grown by oxide molecular beam epitaxy on (1 1 0) GdScO3 and TbScO3 substrates. The transmission electron microscopy measurements and the x-ray diffraction analysis confirmed the epitaxy of the obtained films and the study of their normal state transport properties, removing the ambiguity regarding the truly conducting layer, allowed to highlight the presence of a robust hidden Fermi liquid charge transport in the low temperature properties of infinite layer electron doped cuprate superconductors. These results are in agreement with recent observations performed in other p  and n doped cuprate materials and point toward a general description of the superconducting and normal state properties in these compounds.

Strain-engineering Mott-insulating La2CuO4.

The transition temperature Tc of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La2-xSrxCuO4 thin films, such substrates are sub-optimal and the highest Tc is instead obtained using LaSrAlO4. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in Tc and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La2CuO4 thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest Tc under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates.

Atomic-scale control of competing electronic phases in ultrathin LaNiO3.

In an effort to scale down electronic devices to atomic dimensions, the use of transition-metal oxides may provide advantages over conventional semiconductors. Their high carrier densities and short electronic length scales are desirable for miniaturization, while strong interactions that mediate exotic phase diagrams open new avenues for engineering emergent properties. Nevertheless, understanding how their correlated electronic states can be manipulated at the nanoscale remains challenging. Here, we use angle-resolved photoemission spectroscopy to uncover an abrupt destruction of Fermi liquid-like quasiparticles in the correlated metal LaNiO3 when confined to a critical film thickness of two unit cells. This is accompanied by the onset of an insulating phase as measured by electrical transport. We show how this is driven by an instability to an incipient order of the underlying quantum many-body system, demonstrating the power of artificial confinement to harness control over competing phases in complex oxides with atomic-scale precision.