Electronic structure of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$

We study the electronic structure of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$, which is known to show an anomalous negative temperature coefficient of resistivity below room temperature, while the thermopower shows metallic behavior. We carry out hard x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and resonant PES across the Ce $3d\text{−}4f$ and Cu $2p\text{−}3d$ thresholds to investigate the role of Kondo screening in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$. The Ce $3d$ core-level PES shows the $f^0$, $f^1$, and $f^2$ features, and the Ce $3d\text{−}4f$ XAS shows corresponding features due to transitions into the $f^1$ and $f^2$ states, as in Kondo systems. The spectral feature assignments are confirmed by single-impurity Anderson model calculations and indicate a nearly trivalent-Ce configuration with small but finite mixed valency. The resonant PES valence band spectra across the Ce $3d\text{−}4f$ threshold show an intense Ce $4f^1$ resonance just below the Fermi level, while the Ce $4f^0$ feature is observed at a binding energy of 2.5 eV. The obtained $f$ partial density of states shows an unusual pseudogaplike dip at the Fermi level in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$. The results indicate the importance of Kondo screening and a pseudogap in the $f$ partial density of states for the anomalous transport and magnetic properties of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$.

Figure 1

The experimental Ce $3d$ core-level spectrum of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$ measured by HAXPES ($h$$\nu$ = 7935 eV) at $T=20$ K. The experimental spectrum is compared with the spectrum calculated using the single-impurity Anderson model, which confirms the Kondo screening derived $f^0$, $f^1$, and $f^2$ features of the spin-orbit split Ce $3d_{5/2}$ and $3d_{3/2}$ states. The individual $f^0$, $f^1$, and $f^2$ components and the Shirley background are also shown.

Figure 2

Comparison of the valence band spectra obtained using HAXPES ($h\nu =7935$ eV) at $T=20$ K, and soft x-ray PES ($h\nu =922.7$ eV) obtained at $T=22$ K.

Figure 3

(a) The Cu $2p$ core-level HAXPES spectrum measured at $T=20$ K indicative of a nominal Cu${}^{1+}$ valency. (b) The Cu $L$-edge XAS spectrum measured at $T=22$ K. The red arrows correspond to the peak positions obtained from the HAXPES Cu $2p$ spectrum. The labels a–e correspond to the photon energies used for the Cu $2p$-$3d$ resonant photoemission spectra shown in Fig. 4.

Figure 4

The giant resonance of the satellite seen in the Cu $2p$-$3d$ resonant PES positioned at $\sim$17 eV binding energy, obtained using photon energy $c=934.6$ eV. The off-resonance spectrum at photon energy $a=922.7$ eV, and the spectrum obtained at energy $b=931.6$ eV are also shown for comparison, plotted with a constant shift in intensity. The inset shows a narrower energy range spectra for photon energies a–e [marked in Fig. 3], plotted without any intensity shift but with a magnified intensity scale, in order to show the small changes in the intensity of the Cu $3d$ states in the main valence band.

Figure 5

The Ce $3d$-$4f$ ($M$-edge) x-ray absorption experimental spectrum (symbols) of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$ compared with single-impurity Anderson model calculations (line). Photon energies labeled A–G were used for measuring the Ce $3d$-$4f$ resonant PES spectra.

Figure 6

(Color online)) Ce $3d$-$4f$ resonant PES of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$ for energies labeled A–E and marked in Fig. 5. The results show a systematic increase in the $f$ partial density of states with an intense resonance maximum for incident photon energy $E$.

Figure 7

Comparison of experimental results with single-impurity Anderson model calculations for Ce $3d\text{−}4f$ resonant PES spectrum $E$. The comparison shows a good match between experiment and calculations carried out using the same parameters as for the Ce $3d$ core level (Fig. 1) and Ce $M$-edge XAS (Fig. 5).

Figure 8

(a) The near $E$${}_F$ on-resonant spectrum $E$ divided by the Fermi-Dirac function at $T=22$ K convoluted with a Gaussian function corresponding to the experimental energy resolution of 200 meV. The extracted $f$ partial density of states shows a dip at $E$${}_F$. (b) A similar analysis for the spectra $A$ to $F$ shows the systematic evolution of the dip feature at $E_F$. (c) In contrast, the off-resonant spectrum $A$ shows a typical metallic Fermi edge. A linear DOS multiplied by the Fermi-Dirac function and convoluted with the experimental Gaussian was used to fit the off-resonant spectrum $A$.

Figure 9

Plot of $T$$\chi$($T$) vs ln$T$ for $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$, using the $\chi$($T$) reported in Ref. [16]. The plot shows a faster than logarithmic decrease on reducing temperature down to about 10 K. At lower temperatures, a plateau region between 5–10 K is clearly seen, followed by a further reduction below 5 K.