Photoemission Study of Yb₄Bi₃:
Unusual temperature dependence of the bulk Yb 4f spectra

We have performed a photoemission study of Yb₄Bi₃. The valence band spectra taken at hν = 125 eV have shown no multiplet structures associated with the trivalent Yb and revealed that the mean valence of the Yb ion is very close to 2.0. The divalent Yb 4f spectra have shown strong surface components besides weak but sharp bulk components. The surface 4f level shift is about 0.45 eV (300 K), 0.48 eV (150 K) and 0.54 (20 K). A clear energy shift of the bulk Yb 4f peaks toward E_F with decreasing temperature has been observed, although Yb₄Bi₃ shows neither phase transition nor crossover. Such an unusual peak shift of the bulk component may originate from a change of the lattice constant with decreasing temperature. The observed narrowing of the bulk 4f peaks is thought to be due to a phonon broadening (about 7k_BT).

Introduction

We have studied electronic structures of anti-Th₃P₄ type crystal Yb₄Bi₃ in comparison with Yb₄As₃. Yb₄As₃ is an interesting material because of its unusual physical properties. It undergoes a structural phase transition from a valence fluctuating (VF) state with a cubic phase to a charge ordered (CO) state with a trigonal phase at T_t~290K. The photoemission studies of Yb₄As₃ and Yb₄(As_1-xSb_x)₃ have been previously reported, where the spectra have shown remarkable temperature dependence resulting from the VF-CO transition [S. Suga et al., J. Phys. Soc. Jpn. vol.67, 3552 (1998).].
Yb₄Bi₃ has the same crystal structure as the cubic Yb₄As₃, but it does not show any phase transition. It is thought that the Yb 4f electronic state of Yb₄Bi₃ is different from that of Yb₄As₃. From a result of the magnetic susceptibility measurement, the Yb ions are considered to be close to 2+ (4f¹⁴). In order to investigate the detailed electronic state of Yb₄Bi₃, we have measured photoemission spectra of a single crystal Yb₄Bi₃. In our spectra, signals of Yb³⁺ state have not been observed, indicating that the Yb ions are divalent. Furthermore, the spectra show an unusual energy shift of the bulk Yb²⁺ 4f peaks with changing temperature although Yb₄Bi₃ does not show any phase transition.

Results and Discussion

Single crystalline Yb₄Bi₃ was supplied by Prof. A. Ochiai in Tohoku University. The photoemission measurement was done at a bending magnet beam line BL-3B of the Photon Factory (PF). The photoemission spectra were measured with using the SCIENTA SES200 hemispherical analyzer at a photon energy of hν = 125 eV, where the photoionization cross-section of the Yb 4f orbitals is approximately one hundred times as large as those of the Bi 6sp orbitals. The clean surface was obtained by scraping in situ with using a diamond file. The total energy resolution was set to about 90 meV. Temperature dependent spectra were repeatedly measured and the reproducibility of the spectra was surely confirmed.

Figure 1 shows the photoemission spectra in the whole valence-band region (0-29 eV) of Yb₄Bi₃ at 20, 150 and 300K. In all spectra, prominent doublet structures due to the Yb²⁺ 4f component (4f¹⁴->4f¹³) are observed from E_F to 4 eV. The complex structures are due to mixture of a bulk and a surface contribution. Bi 5d peaks are observed between 22 and 28 eV. Between Yb²⁺ 4f and Bi 5d structures, no clear structures are observed except for a weak and very broad structure in the wide energy range between 5 and 20 eV. No structures ascribable to the Yb²⁺ 4f state is recognized in this region. One can notice that the satellite and the Bi 5d spectra have no essential temperature dependence. As for the Yb²⁺ 4f components on the other hand, their line shape noticeably changes with temperature. Judging from Fig.1, it is known that the Yb²⁺ ions are dominant in Yb₄Bi₃. We conclude that the mean valence of the Yb ion in Yb₄Bi₃ is almost 2.0 and that the Yb ions are not valence fluctuating in contrast to the case of previously reported Yb₄As₃.

Figure 2 shows the spectra of Yb₄Bi₃ from E_F to 3.5 eV. It is noticed that the line shape of the surface components does not depend much upon the temperature, while the spectral weight of the bulk components is shifted toward the Fermi level (E_F) with decreasing temperature. The reproducibility of this behavior has been experimentally confirmed. In the case of Yb₄Bi₃ and Yb₄(As_0.88Sb_0.12)₃, the peak energy shift has been observed, however, the shift direction is towards higher binding energies with decreasing temperature and the value of the peak shift is about 50meV. As for Yb₄(As_0.71Sb_0.29)₃, which does not show the VF-CO transition, such a peak shift has not been observed. Since Yb₄Bi₃ neither shows any phase transition nor crossover, the observed bulk Yb²⁺ peak shift is unexpected. Such a phenomenon is, as far as we know, firstly seen by virtue of temperature regulation and high energy resolution of the experiment. At the same time, the bulk Yb²⁺ contributions become more noticeable at 20K, especially for the Yb²⁺ 4f_5/2 states, suggesting that narrowing of the peaks takes place with decreasing temperature in addition to the peak shift.

In order to quantitatively discuss the temperature dependence of the Yb²⁺ 4f spectra, we have performed the line-shape analysis using a least-squares fitting. All spectra are deconvoluted into the line-shapes of the 4f¹³ bulk and surface final states, consisting of the 4f_7/2 and 4f_5/2 doublet separated by about 1.3 eV. We have assumed an asymmetric line shape given by Mahan for all the bulk peaks. The widths of the spectra are mainly considered by the Gaussian- broadening. The asymmetry parameter a of the Yb²⁺ bulk components is optimized as 0.24-0.25, which is fairly independent of the measuring temperature and is consistent with that previously reported. Figure 3 shows the results of the deconvolution of Yb₄Bi₃ at 20, 150 and 300K. From the analysis, the peak binding energy of the bulk Yb²⁺ 4f_7/2 state is estimated as 0.37, 0.41 and 0.45 eV at 20, 150 and 300K, respectively. In the case of the bulk Yb²⁺ 4f_5/2 state, the peak is located at 1.66, 1.71 and 1.74 eV at 20, 150 and 300K. It is revealed that the value of the peak shift is of about -80 meV with decreasing temperature from 300 to 20K, where its temperature dependence is nearly linear. Therefore, this shift suggests the gradual change of the Yb 4f electronic state with temperature. Concerning the Yb²⁺ 4f_5/2 (4f_7/2) surface contribution, its peak energy is 2.20 (0.92), 2.17 (0.90) and 2.18 (0.91) eV at 20, 150 and 300K. The surface 4f peak shift with temperature is smaller than the bulk peak shift. The mean value of the surface core level shift of the 4f levels is estimated to be about 0.54, 0.48 and 0.45 eV at 20, 150 and 300K.
The lattice constant of Yb₄Bi₃ decreases with temperature as usual. The decreasing of temperature is thought to have a similar effect as adding pressure to a material. For decreasing the interatomic distance with decreasing the temperature, the electronic state of the Yb²⁺ 4f components becomes unstable and the Yb³⁺ states are favored. Therefore the Yb²⁺ bulk peaks may be shifted toward E_F at lower temperatures. Such an interpretation of the bulk 4f states seems to be rather different from the spectral behavior of Yb₄(As_1-xSb_x)₃. In the case of Yb₄As₃, the Yb²⁺ states in the CO state are more stable than those in the VF state. Thus the effect of the VF-CO transition makes the Yb²⁺ peak energy shift toward higher binding energies with decreasing temperature. These two mechanisms of the energy shifts may be canceled with each other under some circumstances. It has been pointed out that a short range CO state may occur in Yb₄(As_0.71Sb_0.29)₃. The negligible peak shift in Yb₄(As_0.71Sb_0.29)₃ seems to be explained by this cancellation mechanism.