Kavli Affiliate: David A. Muller
| First 5 Authors: Lopa Bhatt, Abigail Y. Jiang, Eun Kyo Ko, Noah Schnitzer, Grace A. Pan
| Summary:
The discovery of high-temperature superconductivity in bulk La$_3$Ni$_2$O$_7$
under high hydrostatic pressure and, more recently, biaxial compression in
epitaxial thin films has ignited significant interest in understanding the
interplay between atomic and electronic structure in these compounds. Subtle
changes in the nickel-oxygen bonding environment are thought to be key drivers
for stabilizing superconductivity, but specific details of which bonds and
which modifications are most relevant remains so far unresolved. While direct,
atomic-scale structural characterization under hydrostatic pressure is beyond
current experimental capabilities, static stabilization of strained
La$_3$Ni$_2$O$_7$ films provides a platform well-suited to investigation with
new picometer-resolution electron microscopy methods. Here, we use multislice
electron ptychography to directly measure the atomic-scale structural evolution
of La$_3$Ni$_2$O$_7$ thin films across a wide range of biaxial strains tuned
via substrate. By resolving both the cation and oxygen sublattices, we study
strain-dependent evolution of atomic bonds, providing the opportunity to
isolate and disentangle the effects of specific structural motifs for
stabilizing superconductivity. We identify the lifting of crystalline symmetry
through modification of the nickel-oxygen octahedral distortions under
compressive strain as a key structural ingredient for superconductivity. Rather
than previously supposed $c$-axis compression, our results highlight the
importance of in-plane biaxial compression in superconducting thin films, which
suggests an alternative — possibly cuprate-like — understanding of the
electronic structure. Identifying local regions of inhomogeneous oxygen
stoichiometry and high internal strain near crystalline defects, we suggest
potential pathways for improving the sharpness and temperature of the
superconducting transition.
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