How compressive strain and oxygen content steer nickelate films toward high-temperature superconductivity

A peer-reviewed experimental study of (La,Pr)3Ni2O7−δ thin films found that compressive strain and optimization of oxygen content produce a similar microscopic evolution toward superconductivity. As either control was varied, electronic states associated with nickel 3dz2 and oxygen 2pz orbitals became progressively more delocalized, while long-range spin-density-wave order weakened. Short-range magnetic excitations survived after long-range coherence was strongly suppressed, although they became more strongly damped. Together, the observations identify an orbital-selective route associated with the transition from a non-superconducting parent state toward superconductivity in this Ruddlesden–Popper nickelate.

The researchers used X-ray absorption spectroscopy and resonant inelastic X-ray scattering to investigate thin films while varying epitaxial strain and oxygen content independently. Compressive strain is applied through the substrate on which the film is grown, while oxygenation changes the film’s composition. Both adjustments shifted the measured electronic and magnetic properties in broadly the same direction. This experimental setting is important because superconductivity in bulk Ruddlesden–Popper nickelates was initially realized under high pressure, whereas advances in thin-film growth have enabled superconducting nickelate films at ambient pressure through strain and oxygen control. The two independent tuning parameters allow the researchers to compare changes associated with lattice compression and oxygen content within the same material platform.

The clearest electronic change was a redistribution and broadening of spectral weight. With increasing compressive strain or optimized oxygen content, states linked to oxygen-derived 2pz orbitals and nickel 3dz2 orbitals became more delocalized. Spectral weight shifted toward oxygen-derived hole states, while features associated with initially localized nickel 3dz2 electrons grew broader and less pronounced. These measurements point to the increasing importance of an electronic pathway connecting adjacent nickel-oxygen layers through Ni 3dz2–O 2pz–Ni 3dz2 orbitals. The study associates the development of this delocalized interlayer channel with the evolution toward superconductivity, rather than establishing it as the sole cause of the superconducting state. Its systematic appearance under either tuning method nevertheless provides a concrete experimental benchmark for theories of nickelate superconductivity.

The magnetic transformation was more complex than a simple disappearance of magnetism. Long-range spin-density-wave order, pronounced in the non-superconducting state, progressively weakened as the films approached superconductivity. Both its measured intensity and its spatial correlation length decreased. This coordinated weakening supports competition between long-range magnetic order and the superconducting state in the studied films. Yet short-range magnetic excitations remained after long-range magnetic coherence was strongly suppressed. Those excitations became more strongly damped, but their characteristic energy bandwidth remained nearly unchanged. The findings therefore place an important constraint on microscopic explanations of nickelate superconductivity: an account must accommodate increased interlayer orbital delocalization, reduced long-range magnetic coherence, and the continued presence of short-range magnetic excitations with a nearly unchanged characteristic energy scale. By showing that strain and oxygen engineering can drive comparable orbital and magnetic changes in (La,Pr)3Ni2O7−δ films, the study offers a controlled experimental approach for probing the microscopic conditions associated with superconductivity in Ruddlesden–Popper nickelates.

Subject of Research: Experimental study of strain- and oxygen-controlled electronic and magnetic evolution in (La,Pr)3Ni2O7−δ nickelate films

Article Title: How compressive strain and oxygen content steer nickelate films toward high-temperature superconductivity

Article References: How compressive strain and oxygen content steer nickelate films toward high-temperature superconductivity. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: nickelate superconductors, Ruddlesden–Popper nickelates, thin films, compressive strain, oxygen content, orbital delocalization, spin-density waves, short-range magnetism, X-ray absorption spectroscopy, resonant inelastic X-ray scattering, compressive, strain