Electronic structure of the hole-doped delafossite oxides CuCr${}_{1-x}$Mg${}_x$O${}_2$

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr${}_{1-x}$Mg${}_x$O${}_2$ ($0\le x\le 0.03$) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation $+U$ ($\mathrm{LDA}+U$) scheme. Cr/Cu 3$p$-$3d$ resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr $3d$ character with a minor contribution from the Cu $3d$ through Cu $3d-$O $2p-$Cr $3d$ hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr $3d$ character. Cr $2p$ PES and $L$-edge XAS spectra exhibit typical Cr${}^{3+}$ features for all $x$, while the Cu $L$-edge XAS spectra exhibited a systematic change with $x$. This indicates now that the Cu valence is monovalent at $x=0$ and the doped hole should have Cu $3d$ character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu${}^{+}$ ($3d^{10}$) and Cu${}^{2+}$ ($3d^9$) like initial states in Cu $2p$-$3d$ resonant PES spectrum of at $x=0$, while Cu $2p$ PES spectra with no doubt shows the Cu${}^{+}$ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu $4s$ state, but also finite Cu $3d,4s-$Cr $3d$ charge transfer via O $2p$ states in the ground-state electronic configuration.

Figure 1

Valence-band spectra of CuCr${}_{0.98}$Mg${}_{0.02}$O${}_2$ taken with the photon energy around (a) the Cr 3$p$-$3d$ resonance region and (b) the Cu 3$p$-$3d$ resonance region. (c), (d) Constant initial-state spectra around the Cr 3$p$-$3d$ and Cu 3$p$-$3d$ resonances, respectively. Filled (open) symbols denote before (after) subtracting the background due to secondary electrons.

Figure 2

(a) Valence-band spectra of CuCrO${}_2$ taken with the photon energy around the Cr $2p$-$3d$ resonance region. The photon energies were determined by Cr $L$-edge XAS spectrum shown in Fig. 8. (b) On (576 eV) and off (571 eV) difference spectrum.

Figure 3

$\mathrm{LDA}+U$ band-structure calculations of CuCrO${}_2$. $U_{\mathrm{eff}}$ was set to 2 eV both for Cr and Cu $d$ states.

Figure 4

Comparison between the calculated DOS of CuCrO${}_2$ and the valence-band spectrum of CuCr${}_{0.98}$Mg${}_{0.02}$O${}_2$ taken at $h\nu =80.0$ eV.

Figure 5

(a) Wide-range valence-band spectra of CuCr${}_{0.98}$Mg${}_{0.02}$O${}_2$ in the energy range of the Cu satellite resonances. (b) Detailed satellite structures in (a). (c) CIS spectrum of the two satellite peaks at the binding energy ($E_{\mathrm{B}}$) of 13 and 15 eV.

Figure 6

Valence-band spectra of CuCr${}_{1-x}$Mg${}_x$O${}_2$ ($x=0,0.03$) taken with the photon energy around the Cu $2p$-$3d$ resonance region. (a) Cu $2p$-$3d$ resonant spectra of the $x=0$ sample. (b) Same as (a) around the prepeak energy region before the giant resonance. (c) Cu $2p$-$3d$ resonant spectra of $x=0.03$ sample in the same energy region as (b). (d) Cu $L_3$ XAS spectra of CuCr${}_{1-x}$Mg${}_x$O${}_2$ ($x=0$, 0.02, 0.03).

Figure 7

(a) Cu $2p$ core-level HX-PES spectra of CuCr${}_{1-x}$Mg${}_x$O${}_2$ ($x=0,0.03$). (b) Comparison of the $2p_{3/2}$ peak of $x=0$ and 0.03. Note that the background due to secondary electrons was subtracted by the Shirley method.

Figure 8

(a) Cr $2p$ core-level HX-PES spectra of CuCr${}_{1-x}$Mg$x$O${}_2$ ($x=0,0.02$). Note that the background due to secondary electrons was subtracted by the Shirley method in order to analyze the intensity change with $x$. (b) Cr $L_{2,3}$ XAS spectra of CuCr${}_{1-x}$Mg$x$O${}_2$ ($x=0,0.02,0.03$).

Figure 9

Near-$E_F$ valence-band spectra of CuCr${}_{1-x}$Mg${}_x$O${}_2$ ($x=0,0.03$) taken with 7.94 keV. The intensity is normalized with respect to the spectral weight sum from $-0.5$ to 4.0 eV.