Optimum doping of high-temperature superconductors (HTSC) defines a superconducting unit volume for each HTSC. For a single-mode HTSC, e.g., a cuprate with one CuO 2 plane, the volume is given by V sc =cx 2 , where c is the unit cell height and x the doping distance. The experimental resistivity at T c is connected to the structure by ρ(exp)≈c×h/(2e 2 ). Combining this result with the classical definition of resistivity leads to an equation similar to Einstein's diffusion law x 2 /(2τ)=h/(2M eff )=D, where τ is the relaxation time, M eff =2m e and D the diffusion constant. It has also been shown that the mean free path d=x. The Einstein–Smoluchowski diffusion relation D=μk B T c provides a connection to T c .