Electronic structure of copper pnictides: Influence of different cations and pnictogens

We present calculated electronic structures and x-ray emission and absorption spectroscopy measurements of five copper pnictides. The x-ray emission and absorption spectra, which probe the occupied and unoccupied states, respectively, provide an empirical justification of the calculated electronic structure. We therefore verify the prediction that the copper 3$d$ states are fully occupied and buried deep in the valence band, and the Fermi surface of these materials is characterized by $s$,$p$ states. We show that generally the calculated electronic structure is in excellent agreement with our measurements. We finally examine the Fermi surfaces of these pnictides, and suggest that copper antimonides may exhibit efficient Fermi surface nesting similar to iron pnictides.

Figure 1

The calculated DOS (sum of both spin channels) versus energy for all copper pnictides studied herein, compared with that of copper metal. The Fermi energy is at 0 eV. The color scheme for the total DOS and the $d$- and $s$-symmetry partial DOS for copper are consistent between all panels. Because of the conduction band states have relatively weak intensity but are important for XAS, the unoccupied Cu 4$s$, 3$d$ states have been plotted twice, the second time scaled by a factor of 40.

Figure 2

The measured and calculated Cu $L$${}_3$ XES and XAS for all pnictides studied herein, compared with those for copper metal and CuO. In all cases the XES spectrum was excited at 1000 eV, far above resonance. The calculated XES spectra have been aligned with the measured spectra, with the calculated XAS spectra shifted accordingly. The vertical arrows indicate a small amount of 3$d$ holes in the conduction band from surface oxidation. The spectrum set (d), labeled as (As,Sb)${}_2$, corresponds to measured SrCu${}_2$(As${}_{0.84}$Sb${}_{0.16}$)${}_2$ and calculated SrCu${}_2$(As${}_{0.875}$Sb${}_{0.125}$)${}_2$ spectra, while the spectrum set (f), labeled as CaCu${}_{2-x}$As${}_2$, corresponds to measured CaCu${}_{1.7}$As${}_2$ and calculated CaCu${}_{1.75}$As${}_2$ spectra. All measured XAS spectra were acquired in surface-sensitive total electron yield (TEY) mode, save for the copper metal spectrum which was collected in bulk-sensitive total fluorescence yield (TFY) mode, due to surface oxidation. The Cu $L$${}_2$ XES band is situated near 949 eV, just off the right edge of the plot. Since the Cu $L$${}_2$ XES band shows no additional features in either the calculated or measured spectra, it has been omitted for clarity. The same is true for the Cu $L$${}_2$ XAS band, situated near 955 eV.

Figure 3

The measured Cu $L$${}_3$ XES (excited at 1000 eV, far above resonance) of several pnictides and CuO aligned to the Fermi level using the calculated spectra. In each case the solid “nonmetal” line corresponds to removal of as much of the measured Cu $L$${}_3$ XES of copper metal as possible (after the copper metal spectrum was also aligned to the Fermi level) without allowing the spectral intensity to become negative. The dotted “metal” line corresponds to the amount of the copper metal spectrum that was removed. The sum of the two lines reproduces the original measured spectra shown in Fig. 2. The thinner solid lines correspond to the “nonmetal” if the Fermi level of the copper pnictides was changed by $\pm 0.1$ eV relative to the Fermi level of Cu metal; this demonstrates that the trends shown here are not dependent on the energy alignment within the estimated margin of error.

Figure 4

The measured Cu $L$${}_{2,3}$ XES (excited at 1000 eV, far above resonance) for all pnictides studied herein, along with copper metal and CuO. The label (As,Sb) for spectrum (d) refers to SrCu${}_2$(As${}_{0.84}$Sb${}_{0.16}$)${}_2$. The inset shows the ratio of the integrated intensities of the $L$${}_2$ and $L$${}_3$ peaks, and the percentage of Cu $L$${}_3$ XES from Cu metal that can be subtracted from each pnictide Cu $L$${}_3$ XES spectrum, as described in Fig. 3.

Figure 5

The band structure for (a) CaCu${}_2$As${}_2$ (note the perfect stoichiometry of Cu in this case), (b) SrCu${}_2$As${}_2$, and (c) SrCu${}_2$Sb${}_2$ near the Fermi level (at 0 eV). Each band is shown in a different color. The high symmetry points for each Brillouin zone are labeled (according to the nomenclature found in Ref. 51), and the corresponding crystal momentum wave numbers have been suppressed for clarity. Note that the paths taken through the Brillouin zone for each system cross the $\Gamma$ point twice, so the dispersion outwards from the $\Gamma$ point in three different directions is shown. Note also that the $Z$ point is crossed twice in (a) and (b).

Figure 6

The Fermi surfaces for (a) CaCu${}_2$As${}_2$ (note the perfect stoichiometry of Cu in this case), (b) SrCu${}_2$As${}_2$, and (c) SrCu${}_2$Sb${}_2$. Each band is shown separately; the complete Fermi surface is shown to the bottom right for each system.