A unified model for underdoped and overdoped cuprate superconductors based on a spinodal transition
Many years of intense research on cuprate superconductors have led to several discoveries, such as the pseudogap and charge density waves (CDW), yet a complete theory is still lacking. By analyzing some experiments and performing calculations, we provide a full interpretation of their properties; from the undoped insulator to the overdoped metallic compounds. The variation of the anomalous Hall coefficient ($R_{\rm H}(T)$) with temperature at half-filling ($n = 1$) and, combinations of undoped ($p = 0$) insulators and metallic films, which, among other things, are indicative of a thermodynamic transition. On the overdoped side, recent experiments near the superconducting-to-metal transition detecting superconducting puddles and a considerable degree of charge disorder, suggest that a similar thermodynamic transition operates at all doping levels. We propose a spinodal or charge-separation transition starting near the pseudogap temperature $T^*(p)$, which among other things generates the CDW domains with a typical double-well Landau free-energy functional. Thus, from the half-filled to the overdoped region, the free energy forms an array of wells with $n = 1$ {\it static} holes. With doping, {\it mobile} holes tend to occupy these wells with alternating high and low densities, generating the CDW pattern. The confined holes in small regions develop local superconducting amplitudes, giving rise to a mesoscopic granular superconductor. Similar to the XY model, the grains develop correlation effects mediated by Josephson coupling, which is proportional to the local superfluid density. This approach yields a unified theory of cuprate superconductors.