Absence of superconductivity in Nd0.8Sr0.2NiOx thin films without chemical reduction

Abstract

The recently reported superconductivity 9-15 K in Nd0.8Sr0.2NiO2/SrTiO3heterostructures [Li et al. Nature 572, 624 (2019)] 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 [Liu et al. Phys. Rev. X 3, 021010 (2013)] 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 [Li et al. arXiv: 1911.02420 (2019] 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 [Liu et al. Phys. Rev. Lett. 107, 146802 (2011)].

Nickelate superconductors—a renaissance of the one-band Hubbard model

“…Most noteworthy Sr 0.2 Nd 0.8 NiO 2 which was found to be superconducting in experiment 11 is close to optimal doping n d x 2 Ày 2 ¼ 0:85 or Sr 0.16 Nd 0.84 NiO 2 . Our results call for a more thorough investigation of superconductivity in nickelates around this doping, which is quite challenging experimentally [12][13][14] .…”

Antiferromagnetism in Ni‐Based Superconductors

“…Based on our previous experimental study, [ 13 ] we have further utilized CaH 2 powder to chemically reduce 20 nm‐thick Nd 0.8 Sr 0.2 NiO 3 films, which were prefabricated to epitaxially grow on STO substrates at an oxygen pressure of 150 mTorr and 700 °C by pulsed laser deposition, in the pulsed laser deposition vacuum chamber at 350 °C for 2 h. Figure a shows a typical X‐ray diffraction pattern of an NSNO/STO heterostructure after the chemical reduction. The (001) peak located at 26.5° and the (002) peak at 54.6° correspond to an out‐of‐plane lattice constant c ≈ 3.36 Å, which is in good agreement with previous experimental reports on the superconducting NSNO thin films.…”

Superconductivity in infinite-layer nickelates

“…It is worth noting that several other groups have now reproduced superconductivity in doped infinitelayer nickelate film samples [29,[31][32][33][34], although, in the early stage, there was a report of failure in reproducing the superconductivity in thin film samples [35]. The phase diagram of Nd 1−x Sr x NiO 2 thin film with different strontium concentration x (Fig.…”