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Ultrahigh-resolution angle-resolved photoemission study of LaX (X=S, Se, Te)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e121–e122
Ultrahigh-resolution angle-resolved photoemission study of LaX (X=S, Se, Te) M. Nakayamaa,*, T. Itob, H. Kumigashirab, H. Matsuib, H. Komatsub, T. Takahashib, H. Aokia, A. Ochiaia,1 a
Faculty of Sciences, Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan b Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Abstract Systematic study of the energy band structures for LaX (X=S, Se, Te) was carried out by ultrahigh-resolution angleresolved photoemission spectroscopy. The band gap between the conduction and valence bands was clearly observed in every LaX. The conduction and valence bands show the systematic changes from LaS to LaTe with regard to their energy positions and band widths. r 2003 Published by Elsevier B.V. PACS: 71.20.Eh; 79.60.Bm Keywords: La monochalcogenide; Energy band structure; ARPES
The rocksalt-type rare-earth monochalcogenides RX (R= rare earth; X=S, Se, Te) attract much attention because of their wide variety of anomalous physical properties such as the dense Kondo behavior in CeX (X=S, Se, Te) [1–3], the pressure-induced metal– insulator transition in SmS [4] and the anomalous valence fluctuating state in TmSe [5]. These interesting properties are originated from the interaction between the 4f and conduction electrons. Therefore an investigation of the electronic structures of RX in detail is necessary to understand their properties. In this paper, we report the first experimental observation of the enegy band stuructures of LaX (X=S, Se, Te), which are reference materials of RX, obtained by ultrahighresolution angle-resolved photoemission spectroscopy (ARPES) measurements. High-quality single crystals of LaX were prepared by the Bridgeman method using a tungsten heater furnace. *Corresponding author. Tel./fax: +81-22-217-6484. E-mail addresses: [email protected] (M. Nakayama), [email protected] (A. Ochiai). 1 Also correspond to. Tel.: +81-22-217-6628; fax: +81-22217-6480. 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.12.1254
Lattice constants of LaS, LaSe and LaTe are 5.852, ( respectively. The ARPES measure6.066 and 6:435 A; ments were performed with the He Ia resonance line using a SCIENTA SES-200 electron spectrometer at T ¼ 30 K: The clean (0 0 1) crystalline surfaces were obtained by in situ cleaving in an ultrahigh vacuum of better than 5 1011 Torr: Fig. 1 shows experimental band structures of LaS, LaSe and LaTe. Those were obtained by taking the second derivative of the ARPES spectra after smoothing and plotting the intensity in a square root scale as a function of the wave vector and binding energy. Dark parts correspond to the energy bands. We found that the energy bands of LaX are very similar to each other. The band structures of LaX are classified into two groups: conduction bands located in the energy range from the Fermi energy (EF ) to about 2 eV (see dotted line in Fig. 1) and valence bands which lie below the conduction bands. There is a clear energy gap between the conduction and valence bands. It is noted that the conduction and valence bands of LaX show systematic changes from LaS to LaTe with regard to their energy positions and band widths. The binding energy of the bottom of the conduction bands
ARTICLE IN PRESS e122
M. Nakayama et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e121–e122
Fig. 1. The experimental band structures of (a) LaS, (b) LaSe and (c) LaTe obtained by the ARPES measurements. Dark parts correspond to the energy bands. White dotted lines are guides for eyes.
decreases from LaS to LaTe. This trend is not due to a shift of the EF but due to narrowing of the band width because every LaX is an uncompensated metal with one electron per fomula unit [6]. On the other hand, the binding energy of the top of the valance bands increases with increasing weight of the chalcogen. Furthermore, we found its band width shows distinct broadening from LaS to LaTe. Thus, the trend of the valence bands is opposite to that of the conduction bands. These different trends may be attributed to the character of each band i.e. La 5d character of the conduction band and chalcogen p character of the valence band.
References [1] J. Schoenes, F. Hulliger, J. Magn. Magn. Mater. 63–64 (1987) 43.
[2] D. Ravot, P. Burlet, J. Rossat-Mignod, L. Tholence, J. Phys. 41 (1980) 1117. . [3] A. Donni, A. Furrer, P. Fisher, F. Hulliger, Physica B 186– 188 (1993) 541. [4] M. Ohashi, D. Kindo, N. Sato, T. Suzuki, T. Komatsubara, H. Takahashi, N. Mori, Rev. High Pressure Sci Technol. 7 (1998) 611. [5] P. Haen, F. Lapierre, J.M. Mignot, R. Tournier, F. Holtzberg, Phys. Rev. Lett. 43 (1979) 304. [6] M. Nakayama, et al., J. Magn. Magn. Mater., this issue, doi: 10.1016/j.jmmm.2003.12.634.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e121–e122
Ultrahigh-resolution angle-resolved photoemission study of LaX (X=S, Se, Te) M. Nakayamaa,*, T. Itob, H. Kumigashirab, H. Matsuib, H. Komatsub, T. Takahashib, H. Aokia, A. Ochiaia,1 a
Faculty of Sciences, Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan b Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Abstract Systematic study of the energy band structures for LaX (X=S, Se, Te) was carried out by ultrahigh-resolution angleresolved photoemission spectroscopy. The band gap between the conduction and valence bands was clearly observed in every LaX. The conduction and valence bands show the systematic changes from LaS to LaTe with regard to their energy positions and band widths. r 2003 Published by Elsevier B.V. PACS: 71.20.Eh; 79.60.Bm Keywords: La monochalcogenide; Energy band structure; ARPES
The rocksalt-type rare-earth monochalcogenides RX (R= rare earth; X=S, Se, Te) attract much attention because of their wide variety of anomalous physical properties such as the dense Kondo behavior in CeX (X=S, Se, Te) [1–3], the pressure-induced metal– insulator transition in SmS [4] and the anomalous valence fluctuating state in TmSe [5]. These interesting properties are originated from the interaction between the 4f and conduction electrons. Therefore an investigation of the electronic structures of RX in detail is necessary to understand their properties. In this paper, we report the first experimental observation of the enegy band stuructures of LaX (X=S, Se, Te), which are reference materials of RX, obtained by ultrahighresolution angle-resolved photoemission spectroscopy (ARPES) measurements. High-quality single crystals of LaX were prepared by the Bridgeman method using a tungsten heater furnace. *Corresponding author. Tel./fax: +81-22-217-6484. E-mail addresses: [email protected] (M. Nakayama), [email protected] (A. Ochiai). 1 Also correspond to. Tel.: +81-22-217-6628; fax: +81-22217-6480. 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.12.1254
Lattice constants of LaS, LaSe and LaTe are 5.852, ( respectively. The ARPES measure6.066 and 6:435 A; ments were performed with the He Ia resonance line using a SCIENTA SES-200 electron spectrometer at T ¼ 30 K: The clean (0 0 1) crystalline surfaces were obtained by in situ cleaving in an ultrahigh vacuum of better than 5 1011 Torr: Fig. 1 shows experimental band structures of LaS, LaSe and LaTe. Those were obtained by taking the second derivative of the ARPES spectra after smoothing and plotting the intensity in a square root scale as a function of the wave vector and binding energy. Dark parts correspond to the energy bands. We found that the energy bands of LaX are very similar to each other. The band structures of LaX are classified into two groups: conduction bands located in the energy range from the Fermi energy (EF ) to about 2 eV (see dotted line in Fig. 1) and valence bands which lie below the conduction bands. There is a clear energy gap between the conduction and valence bands. It is noted that the conduction and valence bands of LaX show systematic changes from LaS to LaTe with regard to their energy positions and band widths. The binding energy of the bottom of the conduction bands
ARTICLE IN PRESS e122
M. Nakayama et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e121–e122
Fig. 1. The experimental band structures of (a) LaS, (b) LaSe and (c) LaTe obtained by the ARPES measurements. Dark parts correspond to the energy bands. White dotted lines are guides for eyes.
decreases from LaS to LaTe. This trend is not due to a shift of the EF but due to narrowing of the band width because every LaX is an uncompensated metal with one electron per fomula unit [6]. On the other hand, the binding energy of the top of the valance bands increases with increasing weight of the chalcogen. Furthermore, we found its band width shows distinct broadening from LaS to LaTe. Thus, the trend of the valence bands is opposite to that of the conduction bands. These different trends may be attributed to the character of each band i.e. La 5d character of the conduction band and chalcogen p character of the valence band.
References [1] J. Schoenes, F. Hulliger, J. Magn. Magn. Mater. 63–64 (1987) 43.
[2] D. Ravot, P. Burlet, J. Rossat-Mignod, L. Tholence, J. Phys. 41 (1980) 1117. . [3] A. Donni, A. Furrer, P. Fisher, F. Hulliger, Physica B 186– 188 (1993) 541. [4] M. Ohashi, D. Kindo, N. Sato, T. Suzuki, T. Komatsubara, H. Takahashi, N. Mori, Rev. High Pressure Sci Technol. 7 (1998) 611. [5] P. Haen, F. Lapierre, J.M. Mignot, R. Tournier, F. Holtzberg, Phys. Rev. Lett. 43 (1979) 304. [6] M. Nakayama, et al., J. Magn. Magn. Mater., this issue, doi: 10.1016/j.jmmm.2003.12.634.