Effect of metamaterial engineering on the superconductive properties of ultrathin layers of NbTiN

The electronic transport and optical properties of high quality multilayers of NbTiN/AlN with ultrathin NbTiN layers were characterized.   The anisotropy of the dielectric function of the multilayers confirmed their hyperbolic metamaterial properties. The superconductive transition temperature, T_c, of these engineered superconductors was enhanced up to 32% compared to the T_c of a single ultrathin NbTiN layer while the resistivity per NbTiN layer remained unchanged.  We have demonstrated that this T_c increase can be attributed to enhanced electron-electron interaction in superconducting hyperbolic metamaterials.  The behaviour of the critical fields will be discussed.