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

We report a Cu $K$- and $L_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$, with a focus on post-growth annealing effects. For the parent compound ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ $(x=0)$, 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 $R{E}_2{\mathrm{CuO}}_4$ $(\mathrm{RE}=\text{rare earth})$ 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.

Figure 1

Cu $K$-edge RIXS spectra for ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ with (a) $x=0$, (b) $x=0.05$, and (c) $x=0.16$. Filled and open symbols indicate the spectra of as-grown and annealed crystals, respectively. The spectra are normalized to the integrated intensity in the 3.5–6.0 eV range (see text) and offset vertically for clarity. (d) Comparison of spectra for three as-grown crystals at $\mathit{q}=(1/4,0)$. (e) Integrated intensity of the intraband excitations. Integrated intensity of the intraband excitations. The integration range is 0.7–1.4 eV, as indicated by gray bands in (a)–(d).

Figure 2

(a)–(d) Cu $L_3$-edge RIXS spectra normalized to the $dd$ intensity in the 1.0–3.5 eV range for as-grown and annealed crystals of ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$, as indicated. The spectra are vertically offset for clarity and the vertical scale of (a) is in the same units as in (b) to (d). For $x=0.16$ in (d), the intensity is multiplied by 1.3 to better highlight the relatively broad response. The in-plane momentum transfer is $\mathit{q}=(h,0)$. Except for $h=0.09$ ($h=0.08$) for the as-grown (annealed) $x=0$ crystal, spectra are compared at the same value of $h$.

Figure 3

(a)–(f) Fit results of Cu $L_3$-edge RIXS spectra of ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$. The spectra are vertically offset for clarity. Dots indicate the data, and gray and colored lines are components of the fits, as described in the text. For $h=0.08$ and 0.11 in (e) and (f), we also show the tail of $dd$ excitations. Black lines indicate the sums of all the components. Shaded areas represent the spectral shape of spin and charge excitations. The local minima indicated by arrows indicate the existence of the charge excitations at intermediate energies.

Figure 4

The peak positions of (a)–(c) spin and (d) charge excitations obtained from fits, as described in the text. Open and filled symbols indicate as-grown and annealed crystals, respectively. Dashed lines in (a)–(c) indicate the linear spin-wave fit for the Ce-free ($x=0$) as-grown crystal. (e) Integrated intensity of the Cu $L_3$-edge RIXS spectra in the energy range $(-0.13,0.13)$ eV, normalized to the $dd$ excitations. The arrow indicates the peak position of charge order expected from this Ce concentration [47]. The Ce concentrations are indicated in the respective figure panels.