[Paper Review] Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$

Bai Yang Wang, Yong Zhong+17 more|ArXiv.org|Apr 23, 2025

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This in-situ ARPES study investigates compressively strained La2PrNi2O7 thin films, comparing experimental band structure shifts to DFT predictions and revealing a strongly renormalized 3d_z2 band and nontrivial k_z dispersion of 3d_x2-y2, with implications for strain and pressure effects on superconductivity.

ABSTRACT

The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of compressively strained La$_2$PrNi$_2$O$_7$ films grown by oxide molecular beam epitaxy, and the ozone treated counterparts with an onset T$_c$ of 40 K, supplemented with results from pulsed laser deposition films with similar T$_c$. We resolve a systematic strain-driven electronic band shift with respect to that of bulk crystals, in qualitative agreement with density functional theory (DFT) calculations. However, the strongly renormalized flat 3$d_{z2}$ band shifts a factor of 5-10 smaller than anticipated by DFT. Furthermore, it stays ~70 meV below the Fermi level, contradicting the expectation that superconductivity results from the high density of states of this band at the Fermi level. We also observed a non-trivial k$_z$ dispersion of the cuprate-like 3$d_{x2-y2}$ band. Combined with results from both X-ray diffraction and DFT, we suggest that the strained films are under ~5 GPa effective pressure, considerably larger than the naïve expectation from the DFT relaxed structure. Finally, the ~70 meV energy position is intriguingly close to the collective mode coupling more prominently seen in thin films, in the energy range of both oxygen related phonons and the maximum of the spin excitation spectrum.

Motivation & Objective

• Motivate understanding of how compressive strain affects electronic structure in nickelate thin films related to superconductivity.
• Characterize band structure shifts under strain and compare with DFT predictions.
• Assess the role of specific Ni 3d orbital bands (dz2 and dx2-y2) in relation to superconductivity potential.
• Infer effective pressure in strained films from structural and electronic data.

Proposed method

• In-situ ARPES on oxide molecular beam epitaxy grown La2PrNi2O7 thin films and ozone-treated counterparts.
• Comparison of observed band shifts with density functional theory (DFT) calculations.
• Analysis of kz dispersion for the cuprate-like 3d_x2-y2 band.
• Integration of X-ray diffraction data with DFT to infer strain/pressure effects.
• Correlation of the ~70 meV energy position with possible collective mode coupling.

Experimental results

Research questions

• RQ1 How does compressive strain shift the electronic bands of La2PrNi2O7 relative to bulk crystals?
• RQ2 Does the strongly renormalized 3d_z2 band align with or contradict expectations for superconductivity in these films?
• RQ3 What is the nature of the kz dispersion of the 3d_x2-y2 band under strain?
• RQ4 What effective pressure do the strained films experience, and how does this relate to observed electronic structure changes?
• RQ5 Is the observed energy position near 70 meV connected to phonon or spin excitation modes relevant for superconductivity?

Key findings

• The strained La2PrNi2O7 films show a systematic strain-driven electronic band shift relative to bulk crystals, qualitatively agreeing with DFT.
• The strongly renormalized 3d_z2 band shifts 5–10 times smaller than DFT predictions and sits about 70 meV below the Fermi level.
• The 3d_x2-y2 band exhibits non-trivial kz dispersion consistent with cuprate-like character.
• Combined XRD and DFT data suggest the films are under an effective ~5 GPa pressure, higher than naive relaxed-structure estimates.
• The ~70 meV energy position is near the energy range of oxygen-related phonons and the spin excitation spectrum, implying possible coupling.