Experimental evidence of Tc enhancement above 50 K and diode and paramagnetic-Meissner effects, in Nickelate films on highly reduced SrTiO₃

Since the discovery of superconductivity in nickelate thin films in 2019, the quest for enhancing their $\mathrm{T}_\mathrm{c}$ has been ongoing. Here we provide experimental evidence for $\mathrm{T}_\mathrm{c}$ enhancement in oxygen deficient films on highly reduced and conducting SrTiO$_3$ substrates. $\mathrm{T}_\mathrm{c}$ onset of 50-70 K was found in Meissner and transport measurements, which indicates superconductivity in islands or domains in our films, where $\mathrm{T}_\mathrm{c}$ of zero resistance is obtained at 20-25 K. In addition, we observed a giant paramagnetic-Meissner effect peak at about 48 K, which further supports the existence of a superconductive transition just above it. Furthermore, an asymmetric or nonreciprocal and non-hysteretic superconductive diode effect was observed. The latter effect could be fully polarized, and its polarity could be reversed. Our mixed phase films comprised of many Ruddlesden-Popper $\rm (Nd_{0.8}Sr_{0.2})_{n+1}Ni_nO_{3n+1}$ phases and includes the infinite-layer (IL) phase.