Coincidence of the metal-insulator transition and epsilon near zero condition: implications for superconductive T_cenhancement

A study of the magnetic, electronic transport, and dielectric properties of a series of sputter grown amorphous FeCuB samples with varying metal/B content was performed to explore the relationship between the room temperature values of the dielectric function e(w) and distance to the disorder-driven metal-insulator transition (MIT). This relationship is relevant to observations that the superconductive transition temperatures (T_c) of many systems is enhanced near the MIT as well as other studies showing that T_c can be enhanced using metamaterial engineering where e(w) is negative and near zero (ENZ). The results demonstrate that, as expected, e(w) ~ 0 at metal concentrations similar to those near the MIT so that the ENZ condition is satisfied near the MIT on the metallic side. Furthermore, a composition near the MIT (and thus, satisfying the ENZ condition) was found that displayed signatures of superconductivity. These results imply that the enhancement of T_c in disordered superconductors is due to the presence of the ENZ condition which causes an enhancement of the attractive electron-electron interaction.