  The recently reported superconductivity 9-15 K in Nd0.8Sr0.2NiO2/SrTiO3
heterostructures that were fabricated by a soft-chemical topotactic reduction
approach based on precursor Nd0.8Sr0.2NiO3 thin films deposited on SrTiO3
substrates, has excited an immediate surge of research interest. To explore an
alternative physical path instead of chemical reduction for realizing
superconductivity in this compound, using pulsed laser deposition, we
systematically fabricated 63 Nd0.8Sr0.2NiOx (NSNO) thin films at a wide range
of oxygen partial pressures on various different oxide substrates. Transport
measurements did not find any signature of superconductivity in all the 63
thin-film samples. With reducing the oxygen content 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 Nd0.8Sr0.2NiO3 thin film deposited at a
high oxygen pressure of 150 mTorr to create oxygen vacancies on its surface and
did not succeed in higher conductivity either. Our experimental results
together with the recent report on the absence of superconductivity in
synthesized bulk Nd0.8Sr0.2NiO2 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.
