Post-growth annealing effects on charge and spin excitations in Nd$_{2-x}$Ce$_x$CuO$_4$

TL;DR

This study uses resonant inelastic x-ray scattering to investigate how post-growth annealing affects charge and spin excitations in Nd$_{2-x}$Ce$_x$CuO$_4$, revealing doping-related changes and similarities in high-energy spin excitations.

Contribution

It provides new insights into the effects of annealing on charge transfer and spin excitations, highlighting differences between high- and low-energy spin dynamics in electron-doped cuprates.

Findings

Charge-transfer gap present in as-grown Nd$_2$CuO$_4$

Electrons are doped into the system after annealing

High-energy spin excitations are similar in superconducting and non-superconducting samples

Abstract

We report a Cu K- and L$_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd$_{2-x}$Ce$_x$CuO$_4$, with focus on post-growth annealing effects. For the parent compound Nd$_2$CuO$_4$ ($x = 0$), a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of RE$_2$CuO$_4$ (RE = rare earth) can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For $x = 0.16$, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering.