Enhanced superconductivity in co-doped infinite-layer samarium nickelate thin films

Abstract: Rare-earth infinite-layer nickelates represent an emerging class of unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report the synthesis and characterization of phase-pure superconducting samarium-based infinite-layer nickelate thin films, including the first demonstration of Sm$_{1-x}$Sr$_x$NiO$_2$, along with co-doped variants incorporating europium and calcium. These films, grown on LSAT (001) substrates, exhibit coherent lattice structures up to $\sim$ 9 nm thickness with minimal stacking faults. The co-doped compounds achieve a record-small c-axis parameter of 3.26 {\AA} and display remarkable superconducting transition temperatures up to 32.5 K. We observe distinct magnetotransport behaviors in europium-containing samples, including negative magnetoresistance and field-enhanced superconductivity. By establishing a clear correlation between decreasing c-axis parameter and increasing critical temperature across different rare-earth systems, these findings establish clear materials design principles for achieving higher transition temperatures in infinite-layer nickelate superconductors through structural engineering of the rare-earth site.