Electronic states of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ studied by soft x-ray photoemission spectroscopy

We have carried out angle-integrated photoemission spectroscopy (AIPES) and angle-resolved photoemission spectroscopy (ARPES) experiments using soft x rays on single crystals of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ grown by the Bridgman method to investigate their electronic structures. The AIPES results showed that the Eu ions in ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ are in a divalent state and a nearly trivalent state, respectively, in accordance with the previously reported magnetic properties. The three-dimensional band structures and shapes of the Fermi surfaces of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ were studied by ARPES measurements. We found that the band structures near the Fermi level and Fermi surfaces of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ are very different from each other and are well reproduced by the band structure calculations based on density-functional theory for ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{YCu}}_2{\mathrm{Si}}_2$. This suggests that a charge transfer from the localized $4f$ states into the valence bands is responsible for the difference in the electronic states between ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$.

Figure 1

AIPES spectra measured at $h\nu$ = 800 eV for ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ samples grown by the Bridgman method and those grown by the In-flux method. These spectra are normalized by the integrated intensity from 5 to 2 eV, which roughly corresponds to the total intensity of the Cu $3d$ peak. The vertical bars represent the calculated $4f^6$ and $4f^5$ final-state multiplets cited from Refs. [12, 26], which correspond to the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ states, respectively. The calculated $4f^6$ final-state multiplet, which is located near $E_{\mathrm{F}}$, is shifted to match the ${\mathrm{Eu}}^{2+}$ components of the Bridgman-grown ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ sample, and the calculated $4f^5$ final-state multiplet seen below 6 eV is shifted to match the ${\mathrm{Eu}}^{3+}$ components of the Bridgman-grown ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ sample.

Figure 2

(a) Brillouin zone of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$, which have a body-centered tetragonal crystal structure. Intensity plots of ARPES spectra measured along the high-symmetry lines of (b) ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and (c) ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$. The ARPES spectra in the $\mathrm{\Gamma }\text{−}X\text{−}Z$ plane, shown by the green plane, in the Brillouin zone were obtained by ARPES measurements at constant photon energies of $h\nu =680$ and 710 eV for ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$, respectively. The ARPES spectra along the $\mathrm{\Gamma }\text{−}\left(\mathrm{\Lambda }\right)\text{−}Z$ line were obtained by $h\nu$-dependent ARPES from 670 to 740 and 694 to 778 eV for ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$, respectively. The red dashed lines are guides to the eyes.

Figure 3

ARPES spectra along the $\mathrm{\Gamma }\text{−}\left(\mathrm{\Sigma }\right)\text{−}Z$ line measured at $h\nu =680$ and 710 eV for (a) ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and (b) ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$. We normalized these spectra by the integrated intensity of each momentum distribution curve. The red dashed lines are guides to the eyes. Calculated band structures of (c) ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ and (d) ${\mathrm{YCu}}_2{\mathrm{Si}}_2$ along the $\mathrm{\Gamma }\text{−}\left(\mathrm{\Sigma }\right)\text{−}Z$ line. The color of each band represents the contributions of the Cu $3d$ and Sr $4d$ (Y $4d$) states for ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ (${\mathrm{YCu}}_2{\mathrm{Si}}_2$).

Figure 4

ARPES spectra of (a) ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and (b) ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ measured at $h\nu =680$ and 710 eV along the $X\text{−}\mathrm{\Gamma }\text{−}X$ line. We normalized these spectra by the integrated intensity of each momentum distribution curve. The red dashed lines are guides to the eyes. Corresponding calculated band structures of (c) ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ and (d) ${\mathrm{YCu}}_2{\mathrm{Si}}_2$. The color of each band represents the contributions of the Cu $3d$ and Sr $4d$ (Y $4d$) states for ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ (${\mathrm{YCu}}_2{\mathrm{Si}}_2$).

Figure 5

FS images of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ in the (a) $k_x\text{−}{k}_y$ and (c) $k_{xy}\text{−}{k}_z$ planes obtained by ARPES measurements at $h\nu =680\mathrm{eV}$ and $h\nu$-dependent ARPES measurements, respectively. The white dashed lines are guides to the eyes. Corresponding calculated FSs of ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ in the (b) $k_x\text{−}{k}_y$ and (d) $k_{xy}\text{−}{k}_z$ planes. (e) Three-dimensional shapes of the calculated FSs.

Figure 6

FS images of ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ in the (a) $k_x\text{−}{k}_y$ and (c) $k_x\text{−}{k}_z$ planes obtained by ARPES measurements at $h\nu =710\mathrm{eV}$ and $h\nu$-dependent ARPES measurements, respectively. The white dashed lines are guides to the eyes. Corresponding calculated FSs of ${\mathrm{YCu}}_2{\mathrm{Si}}_2$ in the (b) $k_x\text{−}{k}_y$ and (d) $k_x\text{−}{k}_z$ planes. (e) Three-dimensional shapes of the calculated FSs.