Absence of superconductivity in Nd_0.8Sr_0.2NiO_x thin films without chemical reduction

The recently reported 9–15 K superconductivity in Nd_0.8Sr_0.2NiO₂/SrTiO₃ heterostructures that were fabricated by a soft-chemical topotactic reduction approach based on precursor Nd_0.8Sr_0.2NiO₃ thin films deposited on SrTiO₃ substrates, has excited an immediate surge of research interest. To explore an alternative physical path instead of chemical reduction to realizing superconductivity in this compound, using pulsed laser deposition, we systematically fabricated 63 Nd_0.8Sr_0.2NiO_x (NSNO) thin films at a wide range of oxygen partial pressures on various oxide substrates. Transport measurements did not find any signature of superconductivity in all the 63 thin-film samples. With the oxygen content reducing in the NSNO films by lowering the deposition oxygen pressure, the NSNO films are getting more resistive and finally become insulating. Furthermore, we tried to cap a 20-nm-thick amorphous LaAlO₃ layer on a Nd_0.8Sr_0.2NiO₃ thin film deposited at a high oxygen pressure of 20 Pa to create oxygen vacancies on its surface and did not succeed in obtaining higher conductivity either. Our experimental results together with the recent report on the absence of superconductivity in synthesized bulk Nd_0.8Sr_0.2NiO₂ crystals suggest that the chemical reduction approach could be unique for yielding superconductivity in NSNO/SrTiO₃ heterostructures. However, SrTiO₃ substrates could be reduced to generate oxygen vacancies during the chemical reduction process as well, which may thus partially contribute to conductivity.