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Electronic structure of YbXCu4 (X = In, Cd, Mg) investigated by high-resolution photoemission spectroscopy

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aGraduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan, bFaculty of Engineering, Ehime University, Bunkyo-cho 3, Matsuyama 790-8577, Japan, cHiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, Higashi-Hiroshima 739-8526, Japan, dFaculty of Arts and Sciences, Hiroshima University, Kagamiyama 1-7-1, Higashi-Hiroshima 739-8521, Japan, eJapan Synchrotron Radiation Research Institute, SPring-8, Koto 1-1-1, Mikazuki-cho, Sayo, Hyogo 679-5143, Japan, fJapan Atomic Energy Research Institute, SPring-8, Koto 1-1-1, Mikazuki-cho, Sayo, Hyogo 679-5143, Japan, and gGraduate School of Engineering Science, Osaka University, Machikaneyama 1-3, Toyonaka 560-8531, Japan
*Correspondence e-mail: jinjin@hiroshima-u.ac.jp

(Received 27 January 2002; accepted 21 May 2002)

The valence-band electronic structure of YbXCu4 (X = In, Cd, Mg) has been investigated by means of temperature-dependent high-energy-resolution photoemission spectroscopy using a He I resonance line (hν = 21.22 eV) and synchrotron radiation (hν = 800 eV). Intensities of the structure due to the Yb2+ 4f7/2 states in the He I spectra of YbInCu4 and YbCdCu4 gradually increase with decreasing temperature from 300 to 10 K, and Yb2+ 4f7/2 structures are clearly observed as peaks near the Fermi level (EF) at 10 K. The enhancement of the Yb2+ 4f7/2 peak from 50 to 10 K is much greater for YbInCu4 than for YbCdCu4. On the other hand, the Yb2+ 4f7/2 states of YbMgCu4 are observed as a broad structure near EF. In the synchrotron radiation photoemission spectra of YbInCu4 and YbCdCu4, the structures due to the Yb2+ and Yb3+ 4f states are recognized at all temperatures. The intensity ratio Yb2+/Yb3+ gradually increases with decreasing temperature. The energy separations between the Yb2+ and Yb3+ 4f structures of YbInCu4 increase from 50 to 20 K. For YbMgCu4, on the other hand, almost only the Yb2+ structures are observed and little temperature dependence has been detected.

1. Introduction

Among the cubic C15b-type compounds YbXCu4 (X = In, Ag, Au, Cd, Mg, Tl, Pd, Zn), YbInCu4 has been the most extensively studied so far. This compound exhibits a first-order phase transition at Tv = 42 K (Felner et al., 1987[Felner, I., Nowik, I., Vaknin, D., Potzel, U., Moser, J., Kalvius, G. M., Wortmann, G., Schmiester, G., Hilscher, G., Gratz, E., Schmitzer, C., Pillmayr, N., Prasad, K. G., de Waard, H. & Pinto, H. (1987). Phys. Rev. B, 35, 6956-6963.]; Sarrao et al., 1996[Sarrao, J. L., Immer, C. D., Benton, C. L., Fisk, Z., Lawrence, J. M., Mandrus, D. & Thompson, J. D. (1996). Phys. Rev. B, 54, 12207-12211.]). In the high-temperature phase of YbInCu4, Yb is almost trivalent, displaying Curie–Weiss susceptibility with a paramagnetic moment near the free-ion value of 4.5μB. At Tv, the Yb valence is reduced to 2.8 (Cornelius et al., 1997[Cornelius, A. L., Lawrence, J. M., Sarrao, J. L., Fisk, Z., Hundley, M. F., Kwei, G. H. & Thompson, J. D. (1997). Phys. Rev. B, 56, 7993-8000.]) and the compound shows a temperature-independent Pauli paramagnetism below Tv. The lattice volume changes by 0.5% at Tv with no change in the crystal structure (Lawrence et al., 1996[Lawrence, J. M., Kwei, G. H., Sarrao, J. L., Fisk, Z., Mandrus, D. & Thompson, J. D. (1996). Phys. Rev. B, 54, 6011-6014.]). Furthermore, the Kondo temperature changes from TK+ ≃ 25 K in the high-temperature phases to TK− ≃ 400 K in the low-temperature phases (Lawrence et al., 1999[Lawrence, J. M., Osborn, R., Sarrao, J. L. & Fisk, Z. (1999). Phys. Rev. B, 59, 1134-1140.]). The mechanism of the valence transition has not clearly been revealed yet.

Direct investigation of the electronic structure of YbInCu4 has been carried out by means of temperature-dependent photoemission spectroscopy (Reinert et al., 1998[Reinert, F., Claessen, R., Nicolay, G., Ehm, D., Hüfner, S., Ellis, W. P., Gweon, G.-H., Allen, J. W., Kindler, B. & Assmus, W. (1998). Phys. Rev. B, 58, 12808-12816.]; Joyce et al., 1999[Joyce, J. J., Arko, A. J., Sarrao, J. L., Graham, K. S., Fisk, Z. & Riseborough, P. S. (1999). Philos. Mag. B, 79, 1-8.]; Moore et al., 2000[Moore, D. P., Joyce, J. J., Arko, A. J., Sarrao, J. L., Morales, L., Höchst, H. & Chuang, Y. D. (2000). Phys. Rev. B, 62, 16492-16499.]). On the other hand, the photoemission experiments for the other YbXCu4 compounds, such as YbCdCu4 and YbMgCu4, have not been performed so far. The purpose of the present study is to investigate the X-dependence of the electronic structure of YbInCu4, YbCdCu4 and YbMgCu4 by means of temperature-dependent high-energy-resolution photoemission spectroscopy. Valence electrons of elements X contribute to the Fermi level (EF) of YbXCu4 compounds and play an important role in determining a wide variety of their physical properties. A comparison of experimental results of YbInCu4 with those of YbCdCu4 and YbMgCu4 is fruitful for revealing the electronic structure peculiar to YbInCu4 and, furthermore, is expected to provide a clue to understanding the mechanism of the valence transition of YbInCu4.

YbCdCu4 and YbMgCu4 have so far been studied relatively less. Hiraoka et al. performed 113Cd NMR and 63Cu NQR experiments on YbCdCu4 and concluded that the Yb 4f states change gradually from the Fermi liquid to localized states from low to high temperature (Hiraoka et al., 1995[Hiraoka, K., Kojima, K., Hihara, T. & Shinohara, T. (1995). J. Magn. Magn. Mater. 140/144, 1243-1244.], 2000[Hiraoka, K., Murakami, K., Tomiyoshi, S., Hihara, T., Shinohara, T. & Kojima, K. (2000). Physica B, 281/282, 173-174.]). The Kondo temperatures TK of YbCdCu4 and YbMgCu4 are estimated to be ∼220 and ∼850 K, respectively, from the magnetic susceptibility experiments (Sarrao et al., 1999[Sarrao, J. L., Immer, C. D., Fisk, Z., Booth, C. H., Figueroa, E., Lawrence, J. M., Modler, R., Cornelius, A. L., Hundley, M. F., Kwei, G. H. & Thompson, J. D. (1999). Phys. Rev. B, 59, 6855-6866.]).

2. Experimental

High-resolution photoemission experiments on YbXCu4 (X = In, Cd, Mg) were carried out using a hemispherical analyzer (GAMMADATA-SCIENTA ESCA-200) with a He I resonance line (hν = 21.22 eV) from an intense He lamp (GAMMADATA-SCIENTA VUV-5010). Soft X-ray photoemission experiments using synchrotron radiation were carried out with hν = 800 eV at the BL-25SU beamline of SPring-8 (Saitoh et al., 2000[Saitoh, Y., Kimura, H., Suzuki, Y., Nakatani, T., Matsushita, T., Muro, T., Miyahara, T., Fujisawa, M., Soda, K., Ueda, S., Harada, H., Totsugi, M., Sekiyama, A. & Suga, S. (2000). Rev. Sci. Instrum. 71, 3254-3259.]). Typical values of total energy resolutions of the He I and synchrotron radiation photoemission experiments were below 10 meV (10 K) and 100 meV (20 K), respectively. The experiments were carried out from room temperature to 10 K (He I) or 20 K (synchrotron radiation). Clean surfaces of samples were prepared by scraping with a diamond file every 30–60 min.2 The binding energy of the photoemission spectra is defined with respect to EF of the respective samples.

The YbInCu4 and YbCdCu4 samples used for the present experiments were single crystals prepared by the flux growth method (Sarrao et al., 1996[Sarrao, J. L., Immer, C. D., Benton, C. L., Fisk, Z., Lawrence, J. M., Mandrus, D. & Thompson, J. D. (1996). Phys. Rev. B, 54, 12207-12211.]). The constituent elements with stoichiometric ratios in InCu or CdCu fluxes were put in an alumina crucible and sealed in an evacuated quartz ampoule. The sample was then heated to 1373 K and cooled slowly to 1073 K. After keeping at 1073 K for 20 h, the flux was removed. YbMgCu4 samples were polycrystals. An appropriate amount of the elements was melted in an arc-furnace. After the reaction, the product in the quartz ampoule was annealed at 873 K for two weeks. The crystal structures of all samples were confirmed to be the C15b-type by X-ray powder diffraction. For YbInCu4, the temperature width of the valence transition at Tv was below 2 K from the measurements of the magnetic susceptibility.

3. Results and discussion

Fig. 1[link] shows the photoemission spectra near EF of YbXCu4 (X = In, Cd, Mg) with the He I resonance line measured at 10 K. The spectra are normalized using intensities in the binding-energy region of 300–700 meV, where we assume a dashed line from 350 to 550 meV for YbInCu4 and YbCdCu4. The prominent peaks near EF in the spectra of YbInCu4 and YbCdCu4 are ascribed to the Yb2+ 4f7/2 states. The peak intensity of YbInCu4 is stronger than that of YbCdCu4, suggesting that the Yb ion in YbInCu4 is more divalent than that in YbCdCu4 at 10 K. The peak energies of YbInCu4 and YbCdCu4 are around 50 and 30 meV, respectively. On the other hand, the Yb2+ 4f7/2 states in YbMgCu4 are extremely broad compared with those in YbInCu4 and YbCdCu4, which implies that the electronic structure near EF of YbMgCu4 is substantially different from YbInCu4 and YbCdCu4, and the Yb2+ 4f7/2 states have a large dispersion due to the hybridization between the Yb2+ 4f7/2 and conduction-band states.

[Figure 1]
Figure 1
He I photoemission spectra near EF of YbXCu4 (X = In, Cd, Mg) measured at 10 K.

Fig. 2[link] shows the temperature dependence of the Yb2+ 4f7/2 peak of YbInCu4 and YbCdCu4. Roughly speaking, the peak intensity is reduced with increasing temperature and the peak almost disappears at 300 K for both compounds. Similar temperature dependence has also been observed for YbAgCu4 (Weibel et al., 1993[Weibel, P., Grioni, M., Malterre, D., Dardel, B., Bear, Y. & Besnus, M. J. (1993). Z. Phys. B91, 337-341.]). One notices, however, the slight differences between the spectra of both compounds. For YbCdCu4, the energy position of the Yb2+ 4f7/2 peak gradually shifts to the deeper side with increasing temperature. On the other hand, the peak energy for YbInCu4, which is around 50 meV at 10 K, first shifts toward the EF side at 50 K and then to the deeper side above 100 K, in contrast with the results of YbCdCu4. Furthermore, the enhancement of the peak intensity from 50 to 10 K for YbInCu4 is much greater than that for YbCdCu4.

[Figure 2]
Figure 2
Temperature-dependent He I photoemission spectra near EF of (a) YbInCu4 and (b) YbCdCu4.

In order to estimate the peak energy and intensity with accuracy, the photoemission spectra are fitted with the summation of the Yb2+ 4f7/2 conduction bands and the background due to secondary electrons. We assume the Yb2+ 4f7/2 feature with the asymmetric Doniach–Sunjic line shape and conduction bands with constant density of states at all temperatures. The background contribution is estimated from the integrated method. Finally, we convolute the obtained curve with the Gaussian function to represent the experimental resolution, taking into account the thermal effect using the Fermi–Dirac function. The derived curves reproduce well the experimental results for all temperatures, and for YbCdCu4 the curve-fitting procedure also works successfully.

From the curve-fitting procedure, the peak energies of YbInCu4 and YbCdCu4 at 10 K are estimated to be 46 and 31 meV, respectively. According to the single-impurity Anderson model, these values correspond to kBTK (Blyth et al., 1993[Blyth, R. I. R., Joyce, J. J., Arko, A. J., Canfield, P. C., Andrews, A. B., Fisk, Z., Thompson, J. D., Bartlett, R. J., Risebourough, P., Tang, J. & Lawrence, J. M. (1993). Phys. Rev. B, 48, 9497-9507.]) and provide TK ≃ 534 and 360 K. The deeper peak energy for YbInCu4 is thus qualitatively explained by the higher TK: ∼430 K for YbInCu4 (Sarrao et al., 1996[Sarrao, J. L., Immer, C. D., Benton, C. L., Fisk, Z., Lawrence, J. M., Mandrus, D. & Thompson, J. D. (1996). Phys. Rev. B, 54, 12207-12211.]) and ∼220 K for YbCdCu4 (Sarrao et al., 1999[Sarrao, J. L., Immer, C. D., Fisk, Z., Booth, C. H., Figueroa, E., Lawrence, J. M., Modler, R., Cornelius, A. L., Hundley, M. F., Kwei, G. H. & Thompson, J. D. (1999). Phys. Rev. B, 59, 6855-6866.]). The energy shift about 3 meV to the deeper side from 50 to 10 K for YbInCu4 is also qualitatively explained by the change of TK at Tv.

Fig. 3[link] shows the peak intensities of the Yb2+ 4f7/2 lines, derived from the fitting procedure for the photoemission spectra, as a function of temperature. One notices that the peak intensity of YbCdCu4 increases continuously with decreasing temperature. For YbInCu4 from 300 to 50 K the intensity also increases continuously, while from 50 to 10 K the intensity is remarkably enhanced compared with YbCdCu4. The drastic enhancement is peculiar to YbInCu4 and would reflect the change of the electronic structure of YbInCu4 at Tv. For YbMgCu4, little temperature dependence has been observed.

[Figure 3]
Figure 3
Intensities of the Yb2+ 4f7/2 peak in the photoemission spectra of YbInCu4 and YbCdCu4 as a function of temperature.

The photoemission spectra using the He I resonance line discussed above provide information only on the Yb2+ 4f states. In addition, at hν = 21.22 eV (He I) the photoionization cross section of the Yb 4f states is significantly small compared with the other states forming the conduction bands such as the In 5p and Yb 5d states (Yeh & Lindau, 1985[Yeh, J. J. & Lindau, I. (1985). Atom. Data Nucl. Data Tables, 32, 1-155.]). In order to observe the Yb 4f states clearly and to investigate the temperature-dependence of the Yb3+ states as well as the Yb2+ states, we have measured the synchrotron radiation photoemission spectra at hν = 800 eV for YbXCu4 (X = In, Cd, Mg). At hν = 800 eV, contributions to the photoemission spectra from the other valence electrons, except for the Cu 3d and Cd 4d states, are almost negligible. The ratios of the photoionization cross sections are [Cu 3d]/[Yb 4f] ≃ 0.14 and [Cd 4d]/[Yb 4f] ≃ 0.22 (Yeh & Lindau, 1985[Yeh, J. J. & Lindau, I. (1985). Atom. Data Nucl. Data Tables, 32, 1-155.]).

Fig. 4[link] shows the synchrotron radiation photoemission spectra of YbXCu4 (X = In, Cd, Mg) measured at 20 K. The remarkable structures at 3–5.5 eV in all the spectra are assigned to the Cu 3d states, and the structure around 10 eV in YbCdCu4 are assigned to the Cd 4d states. In the spectrum of YbInCu4 the prominent doublet peaks due to the Yb2+ 4f7/2 and 4f5/2 states are observed at 0.1 and 1.4 eV, respectively, which probe the Yb2+ 4f states in the bulk. On the other hand, one also notices some structures in the Yb2+ 4f region at 0–3 eV other than the doublet peaks. A structure at 2.7 eV and a weak shoulder at the shallower binding-energy side of the Yb2+ 4f5/2 peaks, which are denoted by `S' in the figure, are known to be contributions from the surface layer. The weak structures just below the Yb2+ 4f7/2 and 4f5/2 peaks (dashed lines in the figure) also come from the second surface layer. The Yb3+ 4f states contribute to the spectrum from 5.5 to 12 eV as multiplet structures. These are well reproduced by atomic calculations (Gerken, 1983[Gerken, F. (1983). J. Phys. F, 13, 703-713.]). It should be noted that the photoemission spectra of YbInCu4 and YbCdCu4 are quite similar including the relative intensity of the Yb2+ and Yb3+ 4f structures. The Yb ions in YbInCu4 and YbCdCu4 are in the mixed divalent and trivalent states.

[Figure 4]
Figure 4
Synchrotron radiation photoemission spectra of YbXCu4 (X = In, Cd, Mg) measured at 20 K.

On the other hand, the spectrum of YbMgCu4 is completely different from the other two compounds. The Yb2+ 4f peaks at 0.2 and 1.5 eV are considerably broad, consistent with the He I photoemission spectrum. In addition, the intensity of the Yb3+ 4f multiplet structures is considerably weak, indicating that the Yb ion in YbMgCu4 is in the almost divalent state. These experimental results for the Yb valence of YbXCu4 (X = In, Cd, Mg) are in qualitative agreement with those of the Yb LIII-edge absorption experiments, although the intensity of the Yb3+ 4f structures in the photoemission spectrum of YbMgCu4 is too small (Felner et al., 1987[Felner, I., Nowik, I., Vaknin, D., Potzel, U., Moser, J., Kalvius, G. M., Wortmann, G., Schmiester, G., Hilscher, G., Gratz, E., Schmitzer, C., Pillmayr, N., Prasad, K. G., de Waard, H. & Pinto, H. (1987). Phys. Rev. B, 35, 6956-6963.]; Sarrao et al., 1999[Sarrao, J. L., Immer, C. D., Fisk, Z., Booth, C. H., Figueroa, E., Lawrence, J. M., Modler, R., Cornelius, A. L., Hundley, M. F., Kwei, G. H. & Thompson, J. D. (1999). Phys. Rev. B, 59, 6855-6866.]).

The energy separation between the centre of gravity of the Yb2+ and Yb3+ 4f structures roughly provides f + Uff, where f and Uff represent the energy level of an f electron and the averaged Coulomb interaction energy between the Yb 4f electrons in the compounds, respectively. One notices that f + Uff is largest in YbInCu4 among the three compounds and decreases as X goes from In to Cd to Mg. The smallest f + Uff values of YbMgCu4 would qualitatively be understood from the small Uff value because of the Yb 4f states hybridizing with the conduction-band states, which are also supported from the broad Yb2+ 4f7/2 peak in both He I and synchrotron radiation photoemission spectra of YbMgCu4.

Fig. 5[link] shows the temperature-dependent synchrotron radiation photoemission spectra of YbInCu4 measured at 20, 50, 100, 200 and 300 K. The Yb2+ 4f7/2 and 4f5/2 peak intensities increase continuously with decreasing temperature down to 20 K, in agreement with the results of the He I photoemission spectra. On the other hand, the intensity of the Yb3+ 4f structures decreases. A remarkable change in the spectra between 20 and 50 K owing to the valence transition is not clearly observed. The photoemission spectra of YbCdCu4 exhibit a similar temperature dependence as YbInCu4, indicating that the electronic structures of the two compounds are similar. For YbMgCu4, on the other hand, little temperature dependence has been observed.

[Figure 5]
Figure 5
Temperature-dependent synchrotron radiation photoemission spectra of YbInCu4.

Fig. 6[link] shows the synchrotron radiation photoemission spectra of YbInCu4 measured at 20 and 50 K, in comparison with those of YbCdCu4. The binding-energy region of the Cu 3d states is removed from the figure. It should be noted that the energy separation between the Yb2+ and Yb3+ 4f structures, i.e. f + Uff, increases by about 0.2 eV in the spectrum of YbInCu4 at 20 K, while that of YbCdCu4 is almost unchanged. Since the Uff value is considered to be almost unchanged, this suggests that the f value changes critically through the valence transition of YbInCu4.

[Figure 6]
Figure 6
Comparison of the synchrotron radiation photoemission spectra of YbInCu4 and YbCdCu4 measured at 20 and 50 K.

In summary, although the temperature-dependent He I and synchrotron radiation photoemission spectra of YbInCu4 and YbCdCu4 are similar, the enhancement of the Yb2+ 4f7/2 peak from 50 to 10 K for YbInCu4 is greater than that for YbCdCu4, and the energy separation between the Yb2+ and Yb3+ 4f structures of YbInCu4 changes between 20 and 50 K. These results reflect the valence transition of YbInCu4. On the other hand, the electronic structure of YbMgCu4 is different from YbInCu4 and YbCdCu4, and little temperature dependence has been observed.

Footnotes

Presented at the `International Workshop on High-Resolution Photoemission Spectroscopy of Correlated Electron Systems' held at Osaka, Japan, in January 2002.

2For YbInCu4, the photoemission spectra for the sample surfaces prepared by scraping and cleavage are compared by Reinert et al. (1998[Reinert, F., Claessen, R., Nicolay, G., Ehm, D., Hüfner, S., Ellis, W. P., Gweon, G.-H., Allen, J. W., Kindler, B. & Assmus, W. (1998). Phys. Rev. B, 58, 12808-12816.]). The temperature dependence of the spectra is the same for both sample surfaces, though the peak intensity is slightly reduced for the scraped surface. In this paper we limit the discussion to the temperature dependence and X dependence of the photoemission spectra. Experiments for the cleaved surfaces are in progress.

Acknowledgements

The authors are grateful to the Cryogenic Center, Hiroshima University, for liquid helium. They also thank F. Nagasaki, Y. Nishikawa, H. Fujino, T. Narimura and Y. Ueda for their assistance in the photoemission experiments. The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No.2000B0438-NS-np and No.2001A0261-NS-np). This work is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

References

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