Condensed Matter Physics Seminar: Itai Panas

Abstract:

The synthesis of materials that exhibit superconductivity at ambient conditions remains as a holy grail in chemistry. Superconductivity is not rare though. It is found in alloys and intermetallics, in M(I)3C60, in MgB2, in the M(II)Mo6S8 Chevrel phases, in Cuprate Perovskites and in marginally Nb-doped SrTiO3, in Iron Chalcogenides and Pnictides, in the Lithium Titanate Spinel, in polymeric Sulfur Nitride (SN)x, in Tetratiafulvalene Bechgaard salts, in "Miles of Dirty Lead Wire" and more. Every time it is found however, it is where least expected. This arguably with the exception of the hydrides that exhibit near-room temperature superconductivity at ~2 million atmospheres, and in neutron stars. Indeed, machine learning experiments prove that by learning on one class of superconductors one is still unable to predict the superconductivity in other classes. But once it is found, we know what to expect: Accidental degeneracy among strongly correlated electronic states at the Fermi energy, such that this degeneracy becomes lifted by the formation and condensation of Cooper pairs. Characteristics of some of these states may actually be interrogated by DFT calculations.  This summer, superconductivity at ambient conditions was claimed for a Lead Copper Oxy-Apatite system. The scientific community reacted in three ways. The first pragmatic approach was the safe don't-believe-it-amazing-if-true-let's-wait-and-see. The second addressed the reproducibility, actually falsifying the observation as reported. The third was (and still is) concerned with how superconductivity would come about in the Apatite class of materials, were it to exist. The speaker belongs to the third category. This informal talk will include, firstly, a personal view on the turn of events, and secondly the speaker's own work based on careful re-examination of the experimental XRD data for the nominal Pb10(PO4)6O, supporting strong correlations that guide low-energy local chemical pair-making and pair-breaking processes in superstructures of superatom-motifs of Pb 6p origin in the normal state of reduced Lead (oxy-)apatite.