Inter-site pair superconductivity: origins and recent validation experiments

The challenge of understanding high-temperature superconductivity has led to a plethora of ideas, but 30 years after its discovery in cuprates, very few have achieved convincing experimental validation. While Hubbard and t-J models were given a lot of attention, a number of recent experiments appear to give decisive support to the model of real-space inter-site pairing and percolative superconductivity in cuprates. Systematic measurements of the doping dependence of the superfluid density show a linear dependence on superfluid density - rather than doping - over the entire phase diagram, in accordance with the model's predictions. The doping-dependence of the anomalous lattice dynamics of in-plane Cu-O mode vibrations observed by inelastic neutron scattering, gives remarkable reciprocal space signature of the inter-site pairing interaction whose doping dependence closely follows the predicted pair density. Symmetry-specific time-domain spectroscopy shows carrier localization, polaron formation, pairing and superconductivity to be distinct processes occurring on distinct timescales throughout the entire superconducting phase diagram. The three diverse experimental results confirm non-trivial predictions made more than a decade ago by the inter-site pairing model in the cuprates, remarkably also confirming some of the fundamental notions mentioned in the seminal paper on the discovery of high-temperature superconductivity in cuprates.Comment: Dedicated to Prof. K. A. Mueller on the Occasion of his 90th Birthda

SrTiO3: Thoroughly Investigated but Still Good for Surprises

Peer reviewed: TruePublication status: PublishedFor decades, SrTiO3 has been in the focus of research with seemingly never-ending new insights regarding its ground state properties, application potentials, its surface and interface properties, the superconducting state, the twin boundaries, domain functionalities, etc. Here, we focus on the already well-investigated lattice dynamics of STO and show that four different temperature regimes can be identified which dominate the elastic properties, the thermal conductivity, and the birefringence. These regimes are a low-temperature quantum fluctuation-dominated one, followed by an intermediate regime, a region of structural phase transition at ~105 K and its vicinity, and at high temperatures, a regime characterized by precursor and saturation effects. They can all be elucidated by lattice dynamical aspects. The relevant temperature dependences of the soft modes are discussed and their relationship to lattice polarizability is emphasized.</jats:p