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Structure, exchange interaction, and Madelung potential of R2CuO4 (R=Pr, Nd, Sm, Eu, Gd, and La)
Physica C 185-189 (1991) 973-974 North-Holland
STRUCTURE, EXCHANGE INTERACTION, AND MADELUNG POTENTIAL OF
R2CuO 4 (R -" Pr, Nd, Sm, Eu, Gd, and La)
Takuya UZUMAKI, Nobuo KAMEHARA, and Koiehi NIWA FUJITSU LABORATORIES LTD. 10-1 Morinosato-Wakarniya, Atsugi 243-01, Japan
Raman scattering measurements and crystal structure refinements of R2CuO4 (R = Pr, Nd, Sin, Eu, Gd, and La) and (Ca, Sr)Cu02 were carried out to investigate the relationship between the magnon and phonon energies and the Madelung potentials. Both the magnon and the phonon energies due to the breathing mode exhibit a systematic dependence on the Ct~,.Obond length. The behavior of the magnon energy agrees well with that ofphonon energy in T-phase materials. This l'ehavior is assumed to be due to the change in the charge-transfer gap energy. 1. INTRODUCTION To clarify the superconducting mechanism of high-Tc euprates, it is important to study the electronic state of the CuO2 plane, especially Cu: 3d(x2.y2)and O: 2pa orbitals. The crystal structure of several superconducting materials suggests that the Cu-O network, including the interatomic distances, is significant in determining the superconducting properties. Parent compounds of the high-Tc superconductors show antiferromagnetism. The magnon Raman scattering due to a superexchange interaction, J , can
configuration with a double monochrometer and an A.r-ion laser (k = 4880 A). The incident laser (40 mVO focused above the sample surface using a cylindrical lens. The Gd2CuO4 sample was sintered at 1100°C for 24 hours after calcining at 950°C for 12 hours. The sintering conditions of the other samples have been reported previouslyt. After measuring Raman scattering, the powder method of x-ray diffraction was used with RJeweld analysis to determine the structural parameters of the T-phase materials.
be measured in R2CuO4 (R = Pr, Nd, Sm, and Eu) and (Ca,Sr)CuO21. We suggested that J depends on the Cu-O bond length in these materials. The superexchange
3. RESULTS AND DISCUSSION The Raman spectrum of Gd2CuO4 is shown in Fig. t. Magnon scattering was observed at 2860 crn-1 and phonon
interaction occurs between Cu: 3d electron spin in the charge-transfer (CT) insulator. The CT-gap energy, Eg, was measured by the optical reflectivity2 of its several
scattering at 12t4 cm -1. This phonon mode has been designated the two-phonon scattering of the breathing
parent compounds and depends strongly on the Cu-O bond length. Ohta et al. 3 reported that the CT-gap energy correlates well with the difference in the Madelung site potential, AVM,between the Cu and O sites in the plane. In
I
,,~a
Qm
~7 ~
~
i
Gd2CuO 4
.5
this paper, we report the results of the Raman ~.,'attering measurements of magnca and phonon energies in R2CuO4 (R = Pr, Nd, Sm, Eu, Gd, and La) and (Ca,Sr)CuO2. In
I
Z
,# ~v
V
+
- 7~,:
iI
addition, we calculated the Madelung potentials by analyzing the crystal structures, so we can discuss the magnon energy, phonon energy, AVM, and Eg as they relate to the Cu-O bond length in 2-1-4 materials and (Ca,Sr)CuO2. 2. E X P E R I M E N T Raman scattering was measured in a backscattering
Raman shift (crnd) FIG. 1. Raman specL,'umin Gd2CuO4 a' ro~ra tempe~ture. A 40-roW Ar laser having a waveleng ~" of 4880 A was used.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.
7". Uzumaki et al, /R2CuO~(R=Pr, Nd, Sm, Eu, Gd, and La)
974
~
3200
'
'
'
'
'
~/ a
Gd I 2800
Sm
oo
et 2600 1.90
,
i 1.93
,
' ' 11500 ,~, • MaAnon I ,,-* O Phonon ] 'g~
o
,
Nd,Lai]
e~
120o g J
, 1.96
.
o_ P r l gh q~ / 1 1 0 0 1.99
Cu-O bond length (/k) FIG. 2. The Cu-O bond length dependence of two-magnon and phonon energy in T, T'-phase materials and (Ca,Sr)CuO2. 1.7
I
'
/
Sm
l~r 1.5
,
46.0
analysis of the T-phase materials 5. Figure 3 shows the AVM dependence of the CT-gap energy 2 in the T-phase. As reported by Ohta et al.3, E&clearly correlates with AVM. However, the Gd2CuO4 data deviates from the line. By expressing J as a fourth-order perturbation in CuO2 clustert, J mainly depends on the transfer energy, tpd, and the CT-gap energy. As shown in Fig. 3, the CT-gap energy of Gd2CuO4 is slightly larger than that of the value predicted using AVM. We think that the decrease in the
~ / / Eu
1.6
magnon and phonon energies decrease for Gd2CuO4. The difference in the Madelung site potentials, AVM,between
two-magnon energy of Gd2CuO4 can be explained by the increase in the gap energy, E8.
I
T'-phase
~
phase materials. They agree especially well in that the
the Cu and O sites was calculated based on the Rietveld
-]1300
~
magnon and phonon energies are almost the same for T -
t
46.5
,
I
47.0
,
47.5
AVM (eV) FIG. 3. Relationship between the observed charge-transfer gap energy,E R, and the calculated AVM. AVM is the difference in Madelung site potentials between Cu and O in the plane. The values of E 8 were measured by optical reflectivity 2. mode4. Figure 2 shows the Cu-O bond length dependence of the two-magnon and the phonon energies in R2CuO4 (R = Pr, Nd, Sm, Eu, Gd, and La) and (Ca,Sr)CuO2, including some samples where La or Sm was substituted for Nd in the T-phase. Note that the magnon peak energy of (Ca,Sr)CuO2 is observed at a value consistent with the linear extrapolation of the relationship of T-phase materials, excluding Gd2CuO4 and La2CuO4 which are not on the line. Both the two-phonon energy of the breathing mode and the two-magnon energy depend strongly on the Cu-O bond length. The bond length relationships of the
4. CONCLUSIONS Both the magnon and phonon energies of R2CuO4 (R = Pr, Nd, Sin, Eu, Gd, and La) and (Ca,Sr)CuO2 depend on Cu-O bond length. In addition, the behavior of the magnon energy in T-phase nearly matches that of the phonon energy. The phonon was designated as the two-phonon scattering of the breathing mode. The decrease in the magnon energy of Gd2CuO4 is assumed to be due to the change in the CT-gap energy whose optical refiectivity was measured 2. REFERENCES
. T. Uzumaki, K. Yamanaka, N. Kamehara, and K. Niwa, Jpn. J. Appl. Phys., 29 (1990) Ll150. . T. Arima, K. Kikuchi, M. Kasuya, S. Koshihara, Y. Tokura, T. Ido, and S. Uchida, (to be published). . Y. Ohta, T. Tohyama, and S. Maekawa, Phys. Rev. Lett., 66 (1991) 1228. . $. Sugai, T. Kobayashi, and J. Akimitsu, Phys. Rev., B40 (1989) . T. Uzumaki, N. Kamehara, and K. Niwa, Jpn. L Appl. Phys., 30 (1991) L981. . T. Tohyama, and S. Maekawa, J. Phys. Soc. Jpn., 59 (1990) 1760.
STRUCTURE, EXCHANGE INTERACTION, AND MADELUNG POTENTIAL OF
R2CuO 4 (R -" Pr, Nd, Sm, Eu, Gd, and La)
Takuya UZUMAKI, Nobuo KAMEHARA, and Koiehi NIWA FUJITSU LABORATORIES LTD. 10-1 Morinosato-Wakarniya, Atsugi 243-01, Japan
Raman scattering measurements and crystal structure refinements of R2CuO4 (R = Pr, Nd, Sin, Eu, Gd, and La) and (Ca, Sr)Cu02 were carried out to investigate the relationship between the magnon and phonon energies and the Madelung potentials. Both the magnon and the phonon energies due to the breathing mode exhibit a systematic dependence on the Ct~,.Obond length. The behavior of the magnon energy agrees well with that ofphonon energy in T-phase materials. This l'ehavior is assumed to be due to the change in the charge-transfer gap energy. 1. INTRODUCTION To clarify the superconducting mechanism of high-Tc euprates, it is important to study the electronic state of the CuO2 plane, especially Cu: 3d(x2.y2)and O: 2pa orbitals. The crystal structure of several superconducting materials suggests that the Cu-O network, including the interatomic distances, is significant in determining the superconducting properties. Parent compounds of the high-Tc superconductors show antiferromagnetism. The magnon Raman scattering due to a superexchange interaction, J , can
configuration with a double monochrometer and an A.r-ion laser (k = 4880 A). The incident laser (40 mVO focused above the sample surface using a cylindrical lens. The Gd2CuO4 sample was sintered at 1100°C for 24 hours after calcining at 950°C for 12 hours. The sintering conditions of the other samples have been reported previouslyt. After measuring Raman scattering, the powder method of x-ray diffraction was used with RJeweld analysis to determine the structural parameters of the T-phase materials.
be measured in R2CuO4 (R = Pr, Nd, Sm, and Eu) and (Ca,Sr)CuO21. We suggested that J depends on the Cu-O bond length in these materials. The superexchange
3. RESULTS AND DISCUSSION The Raman spectrum of Gd2CuO4 is shown in Fig. t. Magnon scattering was observed at 2860 crn-1 and phonon
interaction occurs between Cu: 3d electron spin in the charge-transfer (CT) insulator. The CT-gap energy, Eg, was measured by the optical reflectivity2 of its several
scattering at 12t4 cm -1. This phonon mode has been designated the two-phonon scattering of the breathing
parent compounds and depends strongly on the Cu-O bond length. Ohta et al. 3 reported that the CT-gap energy correlates well with the difference in the Madelung site potential, AVM,between the Cu and O sites in the plane. In
I
,,~a
Qm
~7 ~
~
i
Gd2CuO 4
.5
this paper, we report the results of the Raman ~.,'attering measurements of magnca and phonon energies in R2CuO4 (R = Pr, Nd, Sm, Eu, Gd, and La) and (Ca,Sr)CuO2. In
I
Z
,# ~v
V
+
- 7~,:
iI
addition, we calculated the Madelung potentials by analyzing the crystal structures, so we can discuss the magnon energy, phonon energy, AVM, and Eg as they relate to the Cu-O bond length in 2-1-4 materials and (Ca,Sr)CuO2. 2. E X P E R I M E N T Raman scattering was measured in a backscattering
Raman shift (crnd) FIG. 1. Raman specL,'umin Gd2CuO4 a' ro~ra tempe~ture. A 40-roW Ar laser having a waveleng ~" of 4880 A was used.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.
7". Uzumaki et al, /R2CuO~(R=Pr, Nd, Sm, Eu, Gd, and La)
974
~
3200
'
'
'
'
'
~/ a
Gd I 2800
Sm
oo
et 2600 1.90
,
i 1.93
,
' ' 11500 ,~, • MaAnon I ,,-* O Phonon ] 'g~
o
,
Nd,Lai]
e~
120o g J
, 1.96
.
o_ P r l gh q~ / 1 1 0 0 1.99
Cu-O bond length (/k) FIG. 2. The Cu-O bond length dependence of two-magnon and phonon energy in T, T'-phase materials and (Ca,Sr)CuO2. 1.7
I
'
/
Sm
l~r 1.5
,
46.0
analysis of the T-phase materials 5. Figure 3 shows the AVM dependence of the CT-gap energy 2 in the T-phase. As reported by Ohta et al.3, E&clearly correlates with AVM. However, the Gd2CuO4 data deviates from the line. By expressing J as a fourth-order perturbation in CuO2 clustert, J mainly depends on the transfer energy, tpd, and the CT-gap energy. As shown in Fig. 3, the CT-gap energy of Gd2CuO4 is slightly larger than that of the value predicted using AVM. We think that the decrease in the
~ / / Eu
1.6
magnon and phonon energies decrease for Gd2CuO4. The difference in the Madelung site potentials, AVM,between
two-magnon energy of Gd2CuO4 can be explained by the increase in the gap energy, E8.
I
T'-phase
~
phase materials. They agree especially well in that the
the Cu and O sites was calculated based on the Rietveld
-]1300
~
magnon and phonon energies are almost the same for T -
t
46.5
,
I
47.0
,
47.5
AVM (eV) FIG. 3. Relationship between the observed charge-transfer gap energy,E R, and the calculated AVM. AVM is the difference in Madelung site potentials between Cu and O in the plane. The values of E 8 were measured by optical reflectivity 2. mode4. Figure 2 shows the Cu-O bond length dependence of the two-magnon and the phonon energies in R2CuO4 (R = Pr, Nd, Sm, Eu, Gd, and La) and (Ca,Sr)CuO2, including some samples where La or Sm was substituted for Nd in the T-phase. Note that the magnon peak energy of (Ca,Sr)CuO2 is observed at a value consistent with the linear extrapolation of the relationship of T-phase materials, excluding Gd2CuO4 and La2CuO4 which are not on the line. Both the two-phonon energy of the breathing mode and the two-magnon energy depend strongly on the Cu-O bond length. The bond length relationships of the
4. CONCLUSIONS Both the magnon and phonon energies of R2CuO4 (R = Pr, Nd, Sin, Eu, Gd, and La) and (Ca,Sr)CuO2 depend on Cu-O bond length. In addition, the behavior of the magnon energy in T-phase nearly matches that of the phonon energy. The phonon was designated as the two-phonon scattering of the breathing mode. The decrease in the magnon energy of Gd2CuO4 is assumed to be due to the change in the CT-gap energy whose optical refiectivity was measured 2. REFERENCES
. T. Uzumaki, K. Yamanaka, N. Kamehara, and K. Niwa, Jpn. J. Appl. Phys., 29 (1990) Ll150. . T. Arima, K. Kikuchi, M. Kasuya, S. Koshihara, Y. Tokura, T. Ido, and S. Uchida, (to be published). . Y. Ohta, T. Tohyama, and S. Maekawa, Phys. Rev. Lett., 66 (1991) 1228. . $. Sugai, T. Kobayashi, and J. Akimitsu, Phys. Rev., B40 (1989) . T. Uzumaki, N. Kamehara, and K. Niwa, Jpn. L Appl. Phys., 30 (1991) L981. . T. Tohyama, and S. Maekawa, J. Phys. Soc. Jpn., 59 (1990) 1760.