Through the looking glass: optical spectroscopy of electronic correlations

Many body systems involving strongly interacting electrons exhibit various rich and interesting physical states, such as Mott insulators, superconductors, heavy Fermions and etc. Optical properties serve as an important tool to study these correlations and their resulting collective excitations. In my talk I will briefly review several examples of our experimental observations regarding: I. Pairing symmetry, energy gap, superfluid stiffness and phase collective modes in superconducting thin films [1,2,3]. II. Hybridization of localized and conduction energy bands and their optical plasmons in heavy Fermion systems [4]. III. The interplay between the coherent and incoherent sectors of the dynamic conductivity of doped Mott insulator systems [5]. I will then dwell into the latter example and show the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. I will demonstrate the similarity of the optical signature for a pseudo-gap in several systems. Based on these similarities, and the absence of a correlation between superconductivity and pseudo-gap in the doped iridate compound I will argue that the pseudo-gap is a signature of the presence of residual correlations inherited from the insulating anti-ferromagnetic state.