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Photoemission and ultraviolet inverse-photoemission studies of CrSe with NiAs-type crystal structure
Journal of Electron Spectroscopy
and Related Phenomena
78 (1996) 83-86
Photoemission and ultraviolet inverse-photoemission studies of CrSe with NiAs-type crystal structure M. Koyamaa, N. Happob, M. Tamurab, J. Haradab, T. Miharab, A. Furutab, M. Nakatakeb, H. Satob, M. Taniguchib and Y. Uedac aKure National College of Technology, Agaminami 2-2-l 1, Kure 737, Japan bDepartment of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739, Japan CTokuyama National College of Technology, Kume-Takajo 3538, Tokuyama 745, Japan
Valence-band and conduction-band electronic structures of NiAs-type CrSe have been investigated by means of resonant photoemission, ultraviolet photoemission and inverse-photoemission spectroscopies. Based on the comparison with the results of band-theory, features at - 1.5 eV and 1.6 eV relative to the valence-band maximum are assigned to emission from occupied Cr 3dl‘ and unoccupied Cr 3d&states with nearly localized character. The energy separation between two peaks provides the Cr 3d spin-exchage splitting energy of 3.lf 0.2 eV, in relatively good agreement with result of the band-structure calculation.
l.INTRODUCTION
band structure calculation [6]. Chromium Selenide CrSe is an antiferromagnetic semiconductor with NCel temperature of 94-320K [l-3]. Structural phase transition from NiAs-type to MnP-type takes place at about 575K owing to the Jabn-Teller effect [2], just like other transition metal chalcogenides. CrSe also exhibits anomaly in the magnetic susceptibility and electrical conductivity at the characteristic transition temperature Tt around 575K [l-4]. These properties are related with electronic band structures near the Fermi level EF [5]. Experimental information on the electronic structures of CrSe provides a clue to understand their properties. In this paper, we present the occupied Cr 3d partial density of states (DOS) investigated by resonant photoemission measurements at the Cr 3p3d excitation region and the Cr 3d spin-exchange splitting energy by in situ measurments of the ultraviolet photoemission and inverse-photoemission spectra. The inverse-photoemission spectrum is connected with the photoemission spectra at EF and the spin-exchange splitting energy is directly evaluated to be 3.1 f 0.2 eV. The experimental results are in relatively good agreement with the results of 0368-2048/%/$15.00
0 19% Elsevier Science B.V. All rights reserved
PII SO368 - 2048 (%) 02683-7
Z.EXPERIMENTAL
Samples were prepared as follows: An equal amount of Cr and Se was sealed in an evacuated quartz tube, which was heated at 900°C for a week. The reacted products were crushed and mixed well, and then heated again up to 1550°C in evacuated double quartz ampoules. It was kept for several hours at the temperature. Finally, it was cooled down to room temperature in a few days. The grown crystal was confirmed to form a homogeneous crystal phase with the NiAs-type structure by X-ray powder diffraction analysis. The lattice parameters a and c were 3.684 and 6.019& respectively. The resonant photoemission measurements were performed at BL- 11D of the Photon Factory in the National Laboratory for High Energy Physics (KEK-PF). Clean surfaces were obtained by scraping with a diamond file under ultrahigh vacuum of 4~10‘~~ Torr, and then the sample was transferred into the analysis chamber under pressure of 2x10-10 Torr. A combination of constant deviation
84
monochromator and double-stage cylindrical-mirror (DCMA) were used to measure the angle integrated photoemission spectra. Apparatus used for photoemission and inversephotoemission measurements are composed of the chambers for sample preparation, ultraviolet inversephotoemission spectroscopy and photoemission one, with base pressures of 2x10-10, 2x10-10 and 5~10‘~~ Torr, respectively. The inverse-photoemission measurements were performed using an low-energy electron gun of Erdmann-Zipf type and bandpass photon detector centered at 9.43 eV [6,7]. For photoemission experiments, He I (hv = 21.2 eV) and He II (hv= 40.8 eV) resonance lines were used as excitation light sources. After scraping, we measured in situ photoemission and inverse-photoemission spectra. All experiments were performed at room temperature.
3.RESULTS AND DISCUSSION
3
4
25 E .-
z
2
C I,.,.I..*.I..,.I....I,..;;irll~ -25
Figure 1 shows a series of photoemission spectra for hv between 35 and 60 eV. Spectral intensities were normalized to the monochromator output by use of the photoelectron yield from Au film. Energy is defined with respect to the valence-band maximum (VBM) determined by extrapolation the leading edge of the highest valence-band peak to the base line. We find structures at -0.4, -1.5, -3.9 and 6.4 eV as shown by vertical arrows in the spectra taken at hv = 41.4 and 50.0 eV. It should be noticed that the peak at -1.5 eV is clearly seen at hv= 50.0 eV. With increasing hv from 35 to 60 eV, the intensity of the peak decreases gradually to a minimum at hv = 41.4 eV and then increases sharply to reach a maximum at hv = 5O:O eV, and then decreases again. Figure 2(a) shows constant initial-state spectra at selected initial-state energies (Ei) of the valence bands. It is noticeable that the intensity of the spectrum for Ei = -1.5 eV varies significantly with hv. The variation of the intensity observed between hv = 40 and 50 eV takes two minima and a maximum at hv = 41.4 and 43.1, and 50.0 eV, respectively. Figure 2(b) exhibits a total yield spectrum for hv between 35 and 60 eV. The structures between hv = 41 and 47 eV are due to the Cr 3p-3d core absorption and the structure around 55 eV originates form the Se 3d core absorption.
-20
-15
-10
-5
0
Energy (eV) Figure 1. A series of valence-band photoemission spectra of CrSe for hv in the Cr 3p-3d excitation region. The energy is defined relative to the VBM. The vertical arrows represent energy positions of structures.
The change of the intensity in photoemission spectra takes place over a specific energy range as a result of resonance due to an interference between the direct excitation process of the Cr 3d electrons(3p63d4+hv +3p63d3+@ and the discrete the Cr 3p-3d core excitation process followed by a su er Coster-Kroning decay (3p63d4+hv + 3pP3d5+3p63d3+Ef). The Cr 3d partial DOS was obtained by subtracting the spectrum measured at antiresonance (hv= 41.4 eV) from that taken just on resonance (hv= 50.0 eV). The result is shown as diff. in Fig. 3(a). A prominent peak is observed at -1.5 eV in the Cr 3d partial DOS as indicated by an arrow. Structures are also observed at -O--O.7 eV and -2.7--5.5 eV. Figure 3(b) shows the Cr 3d partial DOS calculated by
85
/...ri’
1
hv (eV)
-4.2 -
I
Photon energy (eV) I’...,....I....I....I....I...’ (b)
(b) Cr 3d DOS
t2gT
t2gk
F
I IJ
total yield
\
,...I....‘....‘....‘....‘....‘..., 1
I..l...l...‘...‘r.1’.I.‘...‘..,
Phc% ene$!y(eV)
60
Figure 2. (a) Plots of the photoemission intensities of selected valence bands of CrSe as a function of hv. Each valence band is specified by the initial energy Ei with respect to the VBM. (b) Total yield spectrum of CrSe for hv between 35 and 60 eV.
Dijkstra et al. for the antiferromagnetic phase of CrSe by means of the augmented spherical wave method [8]. We find prominent peaks in the Cr 3d partial DOS with t2 symmetry at -0.8 and 1.9 eV, respectively. l% ough the theoretical spectrum shift toward higher energy side than the experimental one, the spectral features are in agreement with those of the experimental Cr 3d partial DOS. In comparison with band-structure calculation, we assign the sharp peak at -1.5 eV in Fig. 3(a) to the nearly localized Cr
-10
-8
-6
-4
2
0
2
Energy (eV) Figure 3. (a) The experimental Cr 3d partial DOS spectrum of CrSe. The photoemission spectra measured at hv = 50.0 (on resonance) and 41.4 eV (antiresonance), respectively. (b) The Cr 3d partial DOS of CrSe calculated by Dijkstra er al.[8]. 3d states with t2g symmetry. The other structures around -4 eV originate from hybridization between the Cr 3d state with eg symmetry and Se 4p state. Figure 4(a) shows the valence-band photoemission spectra measured at hv= 21.2 eV and 40.8 eV and the conduction-band inversephotoemission spectrum. The photoemission and inverse-photoemission spectra are connected at EF, and energy is referred to the VBM. The photoemission spectra at hv = 21.2 and 40.8 eV exhibit three peaks at -1.5, -4.0 and -6.7 eV.
at 1.6 eV is assumed to be nearly localized unoccupied Cr 3d state with t2 symmetry. Thus, the energy separation between tl!e two main peaks provides the Cr 3d spin-exchange splitting energy of 3.1 + 0.2 eV. Figure 4(b) shows calculated total DOS for the antiferromagnetic phase of CrSe by Dijkstra et a1.[8]. As concerns the whole features of the calculated DOS, we can see relatively good agreement between the experimental and calculated DOS?. The clear peaks are seen at about -0.8 and 1.9 eV. The theoretical Cr 3d spin-exchange splitting energy of 2.7 eV is slightly smaller than the experimental energy splitting of 3.1 eV. ACKNOWLEDGMENTS 2.7eV
5
-10
-5
0
This work is partly supported by the Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture, Japan, Iketani Science and Technology Foundation, The Ogasawara Foundation for the Promotion of Science and Technology, The Murata Science Foundation and Shimazu Science Foundation. 5
10
15
Energy (eV) REFERENCES Figure 4. (a) Photoemission spectra measured at hv=40.8 and 21.2 eV, and inverse-photoemission spectrum of NiAs-type CrSe. (b) The total DOS of CrSe calculated by Dijkstra et a/.@].
Especially the peak at -1.5 eV is clearly seen at hv= 40.8 eV. The photoionization cross-section in the spectrum of the Cr 3d state is larger than that of the Se 4p state at h v= 40.8 eV [9]. Thus, the spectrum at hv = 40.8 eV is regarded as originating mainly from Cr 3d states. The prominent peak at -1.5 eV is also observed in the Cr 3d partial DOS in Fig. 3 and is due to nearly localized Cr 3d states with t2g symmetry. The conduction-band spectrum, on the other hand, shows a main peak at 1.6 eV together with a broad structure between 5 and 11 eV. The main peak
1. K. Masumoto, T. Hihara and T. Kamigaichi, J. Phys. Sot. Japan, 17 (1962) 1209. 2. K. Masumoto and T. Kamigaichi, J. Sci. Hiroshima Univ., A 29 (1965) 49. 3. I. Tsubokawa, J. Phys. Sot. J apan, 15 (1960) 2243. 4. K. Masumoto, J. Sci. Hiroshima Univ., A 27 (1964) 87. 5. K. Mochizuki and K. Kato, Solid State Physics, 21 (1986) 627. 6. K. Yokoyama, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi, Y. Ueda and M. Fujisawa, Rev. Sci Instrum., 64 (1993) 87. 7. Y. Ueda, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi and M. Fujisawa, Nucl. Instrum. Methods, A 330 (1993) 140. 8. J. Dijkstra, C. F. van Bruggen, C. Haas and R. A. de Groot, J. Phys. : Condens. Matter, 1 (1989) 9163. 9. J.J . Yeh and I. Lindau, At. Data Nucl. Data Tables, 32 (1985) 7.
and Related Phenomena
78 (1996) 83-86
Photoemission and ultraviolet inverse-photoemission studies of CrSe with NiAs-type crystal structure M. Koyamaa, N. Happob, M. Tamurab, J. Haradab, T. Miharab, A. Furutab, M. Nakatakeb, H. Satob, M. Taniguchib and Y. Uedac aKure National College of Technology, Agaminami 2-2-l 1, Kure 737, Japan bDepartment of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739, Japan CTokuyama National College of Technology, Kume-Takajo 3538, Tokuyama 745, Japan
Valence-band and conduction-band electronic structures of NiAs-type CrSe have been investigated by means of resonant photoemission, ultraviolet photoemission and inverse-photoemission spectroscopies. Based on the comparison with the results of band-theory, features at - 1.5 eV and 1.6 eV relative to the valence-band maximum are assigned to emission from occupied Cr 3dl‘ and unoccupied Cr 3d&states with nearly localized character. The energy separation between two peaks provides the Cr 3d spin-exchage splitting energy of 3.lf 0.2 eV, in relatively good agreement with result of the band-structure calculation.
l.INTRODUCTION
band structure calculation [6]. Chromium Selenide CrSe is an antiferromagnetic semiconductor with NCel temperature of 94-320K [l-3]. Structural phase transition from NiAs-type to MnP-type takes place at about 575K owing to the Jabn-Teller effect [2], just like other transition metal chalcogenides. CrSe also exhibits anomaly in the magnetic susceptibility and electrical conductivity at the characteristic transition temperature Tt around 575K [l-4]. These properties are related with electronic band structures near the Fermi level EF [5]. Experimental information on the electronic structures of CrSe provides a clue to understand their properties. In this paper, we present the occupied Cr 3d partial density of states (DOS) investigated by resonant photoemission measurements at the Cr 3p3d excitation region and the Cr 3d spin-exchange splitting energy by in situ measurments of the ultraviolet photoemission and inverse-photoemission spectra. The inverse-photoemission spectrum is connected with the photoemission spectra at EF and the spin-exchange splitting energy is directly evaluated to be 3.1 f 0.2 eV. The experimental results are in relatively good agreement with the results of 0368-2048/%/$15.00
0 19% Elsevier Science B.V. All rights reserved
PII SO368 - 2048 (%) 02683-7
Z.EXPERIMENTAL
Samples were prepared as follows: An equal amount of Cr and Se was sealed in an evacuated quartz tube, which was heated at 900°C for a week. The reacted products were crushed and mixed well, and then heated again up to 1550°C in evacuated double quartz ampoules. It was kept for several hours at the temperature. Finally, it was cooled down to room temperature in a few days. The grown crystal was confirmed to form a homogeneous crystal phase with the NiAs-type structure by X-ray powder diffraction analysis. The lattice parameters a and c were 3.684 and 6.019& respectively. The resonant photoemission measurements were performed at BL- 11D of the Photon Factory in the National Laboratory for High Energy Physics (KEK-PF). Clean surfaces were obtained by scraping with a diamond file under ultrahigh vacuum of 4~10‘~~ Torr, and then the sample was transferred into the analysis chamber under pressure of 2x10-10 Torr. A combination of constant deviation
84
monochromator and double-stage cylindrical-mirror (DCMA) were used to measure the angle integrated photoemission spectra. Apparatus used for photoemission and inversephotoemission measurements are composed of the chambers for sample preparation, ultraviolet inversephotoemission spectroscopy and photoemission one, with base pressures of 2x10-10, 2x10-10 and 5~10‘~~ Torr, respectively. The inverse-photoemission measurements were performed using an low-energy electron gun of Erdmann-Zipf type and bandpass photon detector centered at 9.43 eV [6,7]. For photoemission experiments, He I (hv = 21.2 eV) and He II (hv= 40.8 eV) resonance lines were used as excitation light sources. After scraping, we measured in situ photoemission and inverse-photoemission spectra. All experiments were performed at room temperature.
3.RESULTS AND DISCUSSION
3
4
25 E .-
z
2
C I,.,.I..*.I..,.I....I,..;;irll~ -25
Figure 1 shows a series of photoemission spectra for hv between 35 and 60 eV. Spectral intensities were normalized to the monochromator output by use of the photoelectron yield from Au film. Energy is defined with respect to the valence-band maximum (VBM) determined by extrapolation the leading edge of the highest valence-band peak to the base line. We find structures at -0.4, -1.5, -3.9 and 6.4 eV as shown by vertical arrows in the spectra taken at hv = 41.4 and 50.0 eV. It should be noticed that the peak at -1.5 eV is clearly seen at hv= 50.0 eV. With increasing hv from 35 to 60 eV, the intensity of the peak decreases gradually to a minimum at hv = 41.4 eV and then increases sharply to reach a maximum at hv = 5O:O eV, and then decreases again. Figure 2(a) shows constant initial-state spectra at selected initial-state energies (Ei) of the valence bands. It is noticeable that the intensity of the spectrum for Ei = -1.5 eV varies significantly with hv. The variation of the intensity observed between hv = 40 and 50 eV takes two minima and a maximum at hv = 41.4 and 43.1, and 50.0 eV, respectively. Figure 2(b) exhibits a total yield spectrum for hv between 35 and 60 eV. The structures between hv = 41 and 47 eV are due to the Cr 3p-3d core absorption and the structure around 55 eV originates form the Se 3d core absorption.
-20
-15
-10
-5
0
Energy (eV) Figure 1. A series of valence-band photoemission spectra of CrSe for hv in the Cr 3p-3d excitation region. The energy is defined relative to the VBM. The vertical arrows represent energy positions of structures.
The change of the intensity in photoemission spectra takes place over a specific energy range as a result of resonance due to an interference between the direct excitation process of the Cr 3d electrons(3p63d4+hv +3p63d3+@ and the discrete the Cr 3p-3d core excitation process followed by a su er Coster-Kroning decay (3p63d4+hv + 3pP3d5+3p63d3+Ef). The Cr 3d partial DOS was obtained by subtracting the spectrum measured at antiresonance (hv= 41.4 eV) from that taken just on resonance (hv= 50.0 eV). The result is shown as diff. in Fig. 3(a). A prominent peak is observed at -1.5 eV in the Cr 3d partial DOS as indicated by an arrow. Structures are also observed at -O--O.7 eV and -2.7--5.5 eV. Figure 3(b) shows the Cr 3d partial DOS calculated by
85
/...ri’
1
hv (eV)
-4.2 -
I
Photon energy (eV) I’...,....I....I....I....I...’ (b)
(b) Cr 3d DOS
t2gT
t2gk
F
I IJ
total yield
\
,...I....‘....‘....‘....‘....‘..., 1
I..l...l...‘...‘r.1’.I.‘...‘..,
Phc% ene$!y(eV)
60
Figure 2. (a) Plots of the photoemission intensities of selected valence bands of CrSe as a function of hv. Each valence band is specified by the initial energy Ei with respect to the VBM. (b) Total yield spectrum of CrSe for hv between 35 and 60 eV.
Dijkstra et al. for the antiferromagnetic phase of CrSe by means of the augmented spherical wave method [8]. We find prominent peaks in the Cr 3d partial DOS with t2 symmetry at -0.8 and 1.9 eV, respectively. l% ough the theoretical spectrum shift toward higher energy side than the experimental one, the spectral features are in agreement with those of the experimental Cr 3d partial DOS. In comparison with band-structure calculation, we assign the sharp peak at -1.5 eV in Fig. 3(a) to the nearly localized Cr
-10
-8
-6
-4
2
0
2
Energy (eV) Figure 3. (a) The experimental Cr 3d partial DOS spectrum of CrSe. The photoemission spectra measured at hv = 50.0 (on resonance) and 41.4 eV (antiresonance), respectively. (b) The Cr 3d partial DOS of CrSe calculated by Dijkstra er al.[8]. 3d states with t2g symmetry. The other structures around -4 eV originate from hybridization between the Cr 3d state with eg symmetry and Se 4p state. Figure 4(a) shows the valence-band photoemission spectra measured at hv= 21.2 eV and 40.8 eV and the conduction-band inversephotoemission spectrum. The photoemission and inverse-photoemission spectra are connected at EF, and energy is referred to the VBM. The photoemission spectra at hv = 21.2 and 40.8 eV exhibit three peaks at -1.5, -4.0 and -6.7 eV.
at 1.6 eV is assumed to be nearly localized unoccupied Cr 3d state with t2 symmetry. Thus, the energy separation between tl!e two main peaks provides the Cr 3d spin-exchange splitting energy of 3.1 + 0.2 eV. Figure 4(b) shows calculated total DOS for the antiferromagnetic phase of CrSe by Dijkstra et a1.[8]. As concerns the whole features of the calculated DOS, we can see relatively good agreement between the experimental and calculated DOS?. The clear peaks are seen at about -0.8 and 1.9 eV. The theoretical Cr 3d spin-exchange splitting energy of 2.7 eV is slightly smaller than the experimental energy splitting of 3.1 eV. ACKNOWLEDGMENTS 2.7eV
5
-10
-5
0
This work is partly supported by the Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture, Japan, Iketani Science and Technology Foundation, The Ogasawara Foundation for the Promotion of Science and Technology, The Murata Science Foundation and Shimazu Science Foundation. 5
10
15
Energy (eV) REFERENCES Figure 4. (a) Photoemission spectra measured at hv=40.8 and 21.2 eV, and inverse-photoemission spectrum of NiAs-type CrSe. (b) The total DOS of CrSe calculated by Dijkstra et a/.@].
Especially the peak at -1.5 eV is clearly seen at hv= 40.8 eV. The photoionization cross-section in the spectrum of the Cr 3d state is larger than that of the Se 4p state at h v= 40.8 eV [9]. Thus, the spectrum at hv = 40.8 eV is regarded as originating mainly from Cr 3d states. The prominent peak at -1.5 eV is also observed in the Cr 3d partial DOS in Fig. 3 and is due to nearly localized Cr 3d states with t2g symmetry. The conduction-band spectrum, on the other hand, shows a main peak at 1.6 eV together with a broad structure between 5 and 11 eV. The main peak
1. K. Masumoto, T. Hihara and T. Kamigaichi, J. Phys. Sot. Japan, 17 (1962) 1209. 2. K. Masumoto and T. Kamigaichi, J. Sci. Hiroshima Univ., A 29 (1965) 49. 3. I. Tsubokawa, J. Phys. Sot. J apan, 15 (1960) 2243. 4. K. Masumoto, J. Sci. Hiroshima Univ., A 27 (1964) 87. 5. K. Mochizuki and K. Kato, Solid State Physics, 21 (1986) 627. 6. K. Yokoyama, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi, Y. Ueda and M. Fujisawa, Rev. Sci Instrum., 64 (1993) 87. 7. Y. Ueda, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi and M. Fujisawa, Nucl. Instrum. Methods, A 330 (1993) 140. 8. J. Dijkstra, C. F. van Bruggen, C. Haas and R. A. de Groot, J. Phys. : Condens. Matter, 1 (1989) 9163. 9. J.J . Yeh and I. Lindau, At. Data Nucl. Data Tables, 32 (1985) 7.