Absence of superconductivity in bulk Nd$_{1-x}$Sr$_x$NiO$_2$

TL;DR

This study synthesizes bulk Nd$_{1-x}$Sr$_x$NiO$_2$ samples and finds no superconductivity, suggesting that superconductivity in films may originate from interface effects or stress rather than the bulk material.

Contribution

The paper provides the first bulk synthesis of Nd$_{1-x}$Sr$_x$NiO$_2$ and demonstrates the absence of superconductivity in these samples, challenging the assumption that bulk and film behaviors are identical.

Findings

Bulk samples are insulating without superconductivity.

High pressure suppresses insulating behavior but does not induce superconductivity.

Superconductivity in films may be due to interface or stress effects.

Abstract

Recently superconductivity at 9 - 15 K was discovered in an infinite-layer nickelate (Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ films), which has received enormous attention. Since the $Ni^{1+}$ ionic state in NdNiO$_2$ may have the $3d^9$ outer-shell electronic orbit which resembles that of the cuprates, it is very curious to know whether superconductivity discovered here has similar mechanism as that in the cuprates. By using a three-step method, we successfully synthesize the bulk samples of Nd$_{1-x}$Sr$_x$NiO$_2$ (x=0, 0.2, 0.4). The X-ray diffractions reveal that all the samples contain mainly the infinite layer phase of 112 with some amount of segregated Ni. This has also been well proved by the SEM image and the EDS composition analysis. The resistive measurements on the Sr doped samples show insulating behavior without the presence of superconductivity. Temperature dependence of the magnetic moment under high magnetic fields exhibits a Curie-Weiss law feature with the paramagnetic moment of about 2$\mu_B$/f.u.. By applying pressure on Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ up to about 50.2 GPa, we find that the strong insulating behavior at ambient pressure is significantly suppressed, but superconductivity has not been observed either. Since the lattice constants derived from our XRD data are very close to those of the reported superconducting films, we argue that the superconductivity in the reported film may not originate from the expected Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, but arise from the interface or the stress effect.