Musical metals: empirical rules and predictions for superconductivity theory and materials development

Abstract A search for empirical rules for superconductivity in materials on X=O, N, C, B… etc. suggests a common origin and requirements for covalency and pairing of radical bonds with focus on the electronegative subspecies. For the cuprates an empirical linear Tc filling and overfilling dependence, which is universally calibrated to the electronegative species, indicates the involvement of a common optimal charge order. For the cuprates, maximum Tc=167 K is extrapolated from the charge order constraints but higher values should be obtainable in B and C containing systems, based on similar considerations. The high hole concentrations and the low coherence lengths for the cuprates indicate great proximity in the pairing in real space involving the overlapping anionic species. A charge ordering along strings of alternate pairs is indicated. The optimal charge order corresponds to the densest packing of these strings (alternate charge order P2 corresponding to formal Cu2.5+). Dilution of P2 results in striped underdoped materials. Cuprate phenomena, such as selfdoping steps under thermal stress, point to local elastic stress waves as pair correlation and propagation mechanisms and to lattice pressure as a universal origin for the phenomena. Lattice fit requirements place stripes exclusively into the O sublattice along lattice contraction lines. Several Tc decreases, conventionally referred to as overdoping, indicate various crystal chemical O− placement problems. Levels and limits of selfdoping on M=Bi, Hg, Tl analogs also reflect lattice pressure, induced, e.g. through E=Ba, Sr, allowing crystal chemical Tc design. Hole creation through overoxidation or selfdoping tends to a ‘musical’ level scheme, involving charge order energetics. Similar ideas also hold generally for related materials based on electronegative species.