Impact of Cation Stoichiometry on the Crystalline Structure and Superconductivity in Nickelates

Abstract

The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high-quality nickelate films using molecular beam epitaxy. We found that stoichiometry is crucial in optimizing the crystalline structure and realizing superconductivity in nickelate films. In precursor NdNiO3 films, optimal stoichiometry of cations yields the most compact lattice while off-stoichiometry of cations causes obvious lattice expansion, influencing the subsequent topotactic reduction and the emergence of superconductivity in infinite-layer nickelates. Surprisingly, in-situ reflection high energy electron diffraction indicates that some impurity phases always appear once Sr ions are doped into NdNiO3 although the X-ray diffraction data are of high quality. While these impurity phases do not seem to suppress the superconductivity, their impacts on the electronic and magnetic structure deserve further studies. Our work demonstrates and highlights the significance of cation stoichiometry in the superconducting nickelate family.

Investigation of Hydrogen Incorporations in Bulk Infinite-Layer Nickelates

“…For IL nickelates, a close relation and possible analogy to cuprate superconductors was suggested already in 1999 [11], and since the first discovery of superconductivity in the IL nickelate (Nd,Sr)NiO 2 [12], the observation of superconductivity has been confirmed [13][14][15] and extended to (Pr,Sr)NiO 2 [16], (La,Sr)NiO 2 [17], (La,Ca)NiO 2 [18], and Nd 6 Ni 5 O 12 [19]. Furthermore, a recent work reported superconductivity not only for films grown on SrTiO 3 substrates but also on LSAT [20], which provides enough evidence to consider thin-film nickelates as a novel class of superconductors.…”

Thin-Film Aspects of Superconducting Nickelates

“…Two of them may be just stoichiometry and sample quality. The R:Ni flux ratio used during the growth of the perovskite films, for example, has been reported to be an important factor for their subsequent reduction into the infinite-layer phase and hence for the observation superconductivity [40]. Beyond that, samples synthesized in nominally similar conditions can easily show substantial variations in the superconducting T c and even stay nonsuperconducting [5].…”

Ab Initio Downfolding Based on the GW Approximation for Infinite-Layer Nickelates

“…The discovery of nickel superconductors [1] has attracted renewed attention to superconductivity in strongly correlated electron systems [2][3][4][5][6][7]. So far, superconductivity has been found in film samples of doped infinite-layer nickelates RNiO 2 (R = Nd, Pr, and La) [1,[8][9][10][11][12][13][14][15][16] and a quintuple-layer nickelate Nd 6 Ni 5 O 12 [17]. Although the nature of the superconductivity is largely unknown, the pairing mechanism is likely to be unconventional: Theoretically, a phonon calculation for NdNiO 2 has shown that the electron-phonon coupling is too weak to explain the superconductivity with a transition temperature on the order of 10 K [18].…”