Upper critical magnetic field and multiband superconductivity in artificial high-Tc superlattices of nano quantum wells

Artificial high-T-c superlattices (AHTS) composed of quantum building blocks with tunable superconducting critical temperature have been synthesized by engineering their nanoscale geometry using the Bianconi-PeraliValletta (BPV) two-gap superconductivity theory. These quantum heterostructures consist of quantum wells made of superconducting, modulation-doped Mott insulators (S), confined by a metallic (N) potential barrier. The lattice geometry has been carefully engineered to induce the predicted Fano-Feshbach shape resonance between the gaps, near a topological Lifshitz transition. Here, we validate the BPV theory by providing compelling experimental evidence that AHTS samples, at the peak of the superconducting dome, exhibit resonant two-band, two-gap superconductivity. This is demonstrated by measuring the temperature dependence of the upper critical magnetic field, mu H-0(c2), in samples with superlattice periods 3.3 < d < 5.28 nm and Lid ratios close to the magic value 2/3 (where L is the thickness of the superconducting La2CuO4 layer and d is the superlattice period). The data reveal the predicted upward concavity in Hc(2)(T ) and a characteristic kink in the coherence length as a function of temperature, confirming the predicted two-band superconductivity with Fermi velocity ratio 0.25 and significant pair-exchange term among the two condensates.