Comparative Raman study of Ruddlesden-Popper nickelates and the monolayer-trilayer polymorph

TL;DR

This study uses Raman spectroscopy to compare the phononic and electronic properties of Ruddlesden-Popper nickelates, especially monolayer-trilayer structures, revealing unique features linked to their electronic structure and phase identification.

Contribution

It provides a detailed Raman spectral analysis that distinguishes different RP nickelate phases and uncovers unique electronic and phononic characteristics of ML-TL structures.

Findings

Identified Raman features for phase discrimination.

Discovered unique Raman responses in ML-TL due to electronic structure.

Linked Raman differences to self-doping and confinement effects.

Abstract

Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including bilayer (BL) La$_3$Ni$_2$O$_7$, trilayer (TL) La$_4$Ni$_3$O$_{10}$, and structural polymorphs composed of monolayer-bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL lattice architecture.