Electronic structure of an antiferromagnetic metal: CaCrO${}_3$

We report on the electronic structure of the perovskite oxide CaCrO${}_3$ using valence-band, core-level, and Cr $2p-3d$ resonant photoemission spectroscopy (PES). Despite its antiferromagnetic order, a clear Fermi edge characteristic of a metal with dominant Cr $3d$ character is observed in the valence-band spectrum. The Cr $3d$ single-particle density of states are spread over 2 eV, with the photoemission spectral weight distributed in two peaks centered at $~$1.2 and 0.2 eV below $E$${}_F$, suggestive of the coherent and incoherent states resulting from strong electron-electron correlations. Resonant PES across the Cr $2p-3d$ threshold identifies a “two-hole” correlation satellite and yields an on-site Coulomb energy $U~$ 4.8 eV. The metallic DOS at $E$${}_F$ is also reflected through the presence of a well-screened feature at the low binding energy side of the Cr $2p$ core-level spectrum. X-ray-absorption spectroscopy at Cr $L$${}_{3,2}$ and O $K$ edges exhibit small temperature-dependent changes that point toward a small change in Cr-O hybridization. The Cr $2p$ core-level spectrum can be reproduced using cluster model calculations that include a charge transfer from the metallic screening channel at $E$${}_F$. The overall results indicate that CaCrO${}_3$ is a strongly hybridized antiferromagnetic metal, lying in the regime intermediate to Mott-Hubbard and charge-transfer systems.

Figure 1

(a) Ca $2p$ and (b) O $1s$ core-level spectra obtained using 1200-eV excitation energy. The spectra can be fitted with asymmetric (Doniach- Šunjić) line-shape peaks.

Figure 2

(a) Soft x-ray VB spectrum of CaCrO${}_3$ obtained at 25 K. A clear metallic Fermi edge can be seen. The shaded portion was scanned using laser source and the corresponding VB spectrum is shown in (b). The feature at 0.2 eV is clearly seen. (c) Resolution-convoluted Fermi-Dirac function (solid line) fit to the experimental VB.

Figure 3

XAS measured as a function of temperature above and below antiferromagnetic ordering temperature $T_N=90$ K. (a) O $K$ edge and (b) Cr $L_{3,2}$ edge. Temperature-dependent changes are seen in both the spectra.

Figure 4

(a) The resonant PES VB spectra recorded using excitation photon energy across Cr $2p-3d$ transition. Dotted line shows the evolution of the satellite feature into the Auger feature. (b) CIS for the main Cr $3d$ peak at 1.2 eV and the satellite feature at 5.9 eV, normalized to the intensity obtained for $h\nu$ $=$ 571 eV spectrum and have arbitrary offset. (c) On-resonance VB reiterates the two-peak structure of Cr dominated states at $E_F$ with features at 1.2 and 0.2 eV.

Figure 5

Core-level spectra of (a) Cr $3s$ showing the doublet feature separated by 3.35 eV and (b) Cr $2p$ showing multiplet features. The calculated spectrum for Cr $2p$ is obtained using cluster model calculations including metallic screening channel at $E_F$. The charge-transfer energy $\Delta$ is set to 3 eV and the energy of metallic screening channel $\Delta {}^{*}$ is set to zero. (See text for more details.)

Figure 6

Cr $L_{3,2}$-edge XAS spectrum compared with the calculated spectra obtained using cluster model calculations with inclusion of metallic screening channel at $E_F$.