  Based on the recent developed real-space picture of superconductivity, we
study the stability of the superconducting vortex lattices in layered
superconductors. It is shown that the effective c-axis lattice constant play a
significant role in promoting the superconducting transition temperature in
these materials. An unified expression Tc(max)=10c*-28 can be applied to
estimate the highest possible Tc(max) for a given layered superconductor with
an effective c-axis lattice constant c*. For the newly discovered iron-based
superconductors, our results suggest that their Tc cannot exceed 60K, 50K and
40K for the 1111, 21311 and 122 series, respectively. In the case of
copper-based oxide superconductors, it seems that the highest Tc can reach
about 161K without applying of the external pressure. In our theoretical
framework, we could interpret the experimental results of the completely
different superconducting transition temperatures obtained in two very similar
cuprate superconductors (La{2-x}Ba{x}CuO{4} of 40K and
Sr{2-x}Ba{x}CuO{3+\delta} of 98K). In addition, the physical reason why the
superconductivity does not occur in noble metals (like gold, silver and copper)
is discussed. Finally, we argue that the metallic hydrogen cannot exhibit
superconductivity at room temperature, it even cannot be a superconductor at
any low temperature.
