Electronic Structure of Compressively Strained Bilayer Nickelate Thin Film

The discovery of superconductivity in bulk bilayer nickelates under high pressure, and its subsequent stabilization in compressively strained thin films at ambient pressure, has established a new platform for exploring high- <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mi>T</a:mi> <a:mi mathvariant="normal">c</a:mi> </a:msub> </a:mrow> </a:math> superconductivity beyond the cuprates. Central to this development is the prominent role of the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mn>3</d:mn> <d:msubsup> <d:mi>d</d:mi> <d:mi>z</d:mi> <d:mn>2</d:mn> </d:msubsup> </d:mrow> </d:math> orbital in shaping the low-energy electronic structure, imposing constraints on microscopic theories and fueling debate over the superconducting mechanism. Here we report a systematic angle-resolved photoemission spectroscopy study of compressively strained bilayer nickelate thin films spanning Ca doping, oxygen stoichiometry and film thickness. Despite variations in oxygen-vacancy disorder and surface termination, the electronic structure remains robust and exhibits a systematic strain-driven evolution consistent with an intermediate-correlation regime. In particular, we demonstrate a Ca-doping-induced electronic structure evolution that is consistent with Fermi-level crossing of the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mi>γ</f:mi> </f:math> band and is decoupled from the presence of superconductivity, suggesting that, while important for the <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mi>γ</h:mi> </h:math> band to be near the Fermi level, the debated <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mi>γ</j:mi> </j:math> crossing and the resulting Fermi pocket may not be a prerequisite for superconductivity. Together, our results establish key spectroscopic constraints on the minimal fermiology and correlation strength relevant to superconductivity in bilayer nickelates, providing an experimental foundation for microscopic theories of their pairing mechanism.