Enhancement of metallicity by Na doping in La$_3$Ni$_2$O$_{7+{\delta}}$

TL;DR

This study investigates how sodium doping in La$_3$Ni$_2$O$_{7+{\delta}}$ affects its structure and electronic properties, revealing phase transitions and enhanced metallicity, which provide insights into tuning electronic phases in layered nickelates.

Contribution

It provides a comprehensive analysis of Na doping effects on La$_3$Ni$_2$O$_{7+{\delta}}$, including structural transitions and electronic behavior, advancing understanding of doping-induced phase control.

Findings

Na doping induces a structural transition from '327' to '4310' phase.

Na substitution enhances metallicity and suppresses density wave transition.

Pressure further suppresses the density wave transition without affecting low-temperature insulating behavior.

Abstract

The observation of high-$T_c$ superconductivity in bilayer nickelate La$_3$Ni$_2$O$_7$ under high pressure provides a new venue for exploring novel unconventional superconductors and elucidating the mechanism of high-$T_c$ superconductivity. Subsequently, numerous chemical substitution studies have been reported, aiming to stabilize superconductivity at ambient pressure, or significantly reduce the pressure threshold required for its occurrence. Here, we report the comprehensive study on sodium (Na) doping in the Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_{7+{\delta}}$, where Na$^+$ substitutes for La$^{3+}$ at the A-site with varying doping concentrations. The structural, thermal, magnetic, and electronic transport properties of as-synthesized polycrystalline samples were systematically investigated. X-ray diffraction (XRD) analysis reveals that Na doping induces a structural transition from the '327' Amam phase to the '4310' Bmab phase when $x\geq0.075$, which is further corroborated by thermogravimetric analysis (TGA) measurements. Substitution of La$^{3+}$ with Na$^+$ gives rise to a gradual expansion of the '327' phase lattice. Meanwhile, resistivity measurements indicate that the density wave (DW) transition is marginally suppressed and metallicity is significantly enhanced. Upon the application of pressure, DW transition can be further suppressed, whereas the low-$T$ insulating behaviors remain insensitive to pressure. These results offer critical insights into the roles of elemental substitution and charge carrier doping in steering the competing electronic phases in layered nickelates.