Absence of Superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_x$ Thin Films without Chemical Reduction

TL;DR

This study systematically investigated Nd0.8Sr0.2NiOx thin films fabricated without chemical reduction and found no evidence of superconductivity, suggesting that chemical reduction may be essential for inducing superconductivity in these materials.

Contribution

The paper demonstrates that chemical reduction is likely necessary for superconductivity in Nd0.8Sr0.2NiO2-based systems, challenging previous reports of superconductivity in similar heterostructures.

Findings

No superconductivity observed in 63 Nd0.8Sr0.2NiOx thin films.

Reducing oxygen content increased resistivity, leading to insulating behavior.

Surface oxygen vacancies alone did not induce higher conductivity.

Abstract

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.