Nickelate superconductivity without rare-earth magnetism: (La,Sr)NiO$_{2}$ (2105.13494v1)

Abstract: The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO${2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO${2}$ [16,17], but not previously in (La,Sr)NiO${2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La${1-x}$Sr${x}$NiO${2}$ phase diagram can enter the regime of coherent low-temperature transport ($x$ = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ~ 9 K at $x$ = 0.20. Additionally, we observe the unexpected indication of a superconducting ground state in undoped LaNiO${2}$, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)${1-x}$Sr${x}$NiO${2}$ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

Insights into the Superconductivity of (La,Sr)NiO Films without Rare-Earth Magnetism

This paper explores the manifestation of superconductivity in (La,Sr)NiO films, challenging the long-held hypothesis that rare-earth magnetism is crucial for nickelate superconductivity. By optimizing the thin film synthesis process, extensive portions of the LaNiO composition space were discovered to exhibit superconducting transitions at low temperatures, with a maximal transition temperature (Tc) of around 9 K for La0.8Sr0.2NiO2.

A systematic paper of infinite layer nickelates across the lanthanide series revealed a similarly coherent electronic structure, with little qualitative difference in density-functional theory (DFT) predictions. However, previous observations of superconductivity in nickelates (e.g., (Nd,Sr)NiO) but not in (La,Sr)NiO prompted questions regarding the specific role of 4f-orbital hybridization in superconductivity. This research sheds light on the capacity of enhanced crystallinity and reduced disorder to unveil superconducting behavior in systems devoid of rare-earth magnetic moments.

Key Findings

The research demonstrated several findings critical for understanding superconductivity in (La,Sr)NiO systems:

1. Crystallinity and Resistivity: The paper emphasized a substantial reduction in the normal state resistivity with improved crystallinity, which facilitated superconductivity. Measurements show that high-quality film synthesis is essential, and must be below the Mott-Ioffe-Regel limit for superconductivity to occur.
2. Phase Diagram and Superconducting Dome: La1-xSrxNiO films exhibited a superconducting dome for doping levels between x = 0.14 and x = 0.20, surrounded by weakly insulating phases. Notably, even undoped LaNiO displayed signs of a superconductive state, possibly due to its self-doped electronic nature.
3. Resistivity Behavior: Unlike previous instances where LaNiO films demonstrated an a-axis to c-axis reorientation upon reduction, these optimized films maintained their orientation, possibly due to suppressed defect growth.
4. Structural and Electronic Features: Atomically uniform film growth without chemical segregation was verified via X-ray diffraction and electron microscopy techniques, which also highlighted notable changes in the lattice dynamics and rotation patterns in the film structure.
5. Hall Coefficient and Electronic Structure: Hall effect measurements indicated systematic shifts in electron-hole population distributions and compensation points across differing doping levels, suggesting an evolution in electronic structure not wholly captured by DFT models.

Implications and Future Perspectives

This paper provides significant insights into the synthesis and superconductivity of nickelates without involving rare-earth magnetic moments. Practically, improving materials crystallinity to achieve bulk superconductivity without reliance on rare-earth elements could lower production costs, making these systems more attractive for technological applications. Theoretically, these results necessitate a reevaluation of current models on superconductivity which presuppose the role of localized 4f-electrons in these systems.

Additionally, this work opens avenues for further paper into the complex interplay between crystallinity, electronic structure, and superconductivity, highlighting a substantial overlap with phenomena observed in other complex systems such as twisted bilayer graphene. It suggests potential studies on whether controlling disorder and extending this approach to other non-magnetic compounds could consistently result in superconductivity devoid of rare-earth contributions.

In conclusion, this research underscores the nuanced role that crystallinity and electronic heterogeneity play in governing superconductivity in La-Sr-Ni-O compounds, offering profound implications for the design and application of new superconductive materials.