Observation of perfect diamagnetism and interfacial effect on the electronic structures in infinite layer Nd0.8Sr0.2NiO2 superconductors

Abstract

Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

Infinite-Layer Nickelate Superconductors: A Current Experimental Perspective of the Crystal and Electronic Structures

“…On the other hand, the T c,0 of Nd 0.8 Sr 0.2 NiO 2 thin films monotonically increases with film thickness (4.6-10.1 nm) observed from both resistivity and susceptibility measurements [24]. In addition, a systematic evaluation of the thickness-dependent electronic structure has been shown with the change in Hall coefficients and XAS spectra of Nd 0.8 Sr 0.2 NiO 2 thin film from 4.6 to 10.1 nm [24]. The results imply strain modulation and interface effect in the infinite-layer nickelate.…”

“…This implies the stronger correlation between T c,0 and normal state resistivity but not the film thickness [25]. On the other hand, the T c,0 of Nd 0.8 Sr 0.2 NiO 2 thin films monotonically increases with film thickness (4.6-10.1 nm) observed from both resistivity and susceptibility measurements [24]. In addition, a systematic evaluation of the thickness-dependent electronic structure has been shown with the change in Hall coefficients and XAS spectra of Nd 0.8 Sr 0.2 NiO 2 thin film from 4.6 to 10.1 nm [24].…”

“…Unlike cuprate, which was first synthesized in the bulk form, superconducting nickelate was only realized recently in the thin-film form [14][15][16][17][18][19][20][21], with Ni 1+ in the infinite-layer phase that can be achieved through topotactic reduction from the perovskite compound. Missing superconductivity in the bulk nickelate [22,23] and limitation in stabilizing the infinite-layer phase above ~10 nm from the substrate [24][25][26] demands answers on the thickness-dependent crystallinity and electronic structure of the infinite-layer nickelate film. While lanthanide-cuprate La-Ba-Cu-O was the first superconducting compound synthesized [27], lanthanidenickelate was initially reported to not host superconductivity, but superconductivity was only realized in the neodymium-based counterpart, a rare-earth element with 4 f magnetism [14].…”

“…The M − T and M − H data were not presented in the early reports of the superconductivity observed in the infinite-layer nickelate thin films. Zeng et al provided a study on the thickness-dependent effect on the film's T c in both resistivity and susceptibility measurements [24]. The diamagnetic moment in M − T curve (measured at H c) below the superconducting transition is typically of ~10 −5 emu scale for a 2.5 × 5 mm 2 superconducting thin film of 8 nm thick.…”

Electronic structure of higher-order Ruddlesden-Popper nickelates

“…Subsequently, PrNiO 2 and LaNiO 2 were also found to be superconducting upon doping [8][9][10][11]. This series of discoveries led to an immense amount of experimental [12][13][14][15][16][17][18][19][20][21][22][23][24] and theoretical research . RNiO 2 (R = rare-earth) materials are the n = ∞ members of a larger structural series represented by the general formula R n+1 Ni n O 2n+2 where n is the number of NiO 2 planes along the c-axis.…”