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Phonon Raman scattering of Sr2RuO4
IWIlll ELSEVIER
Physica B 219&220 (1996) 222 224
Phonon Raman scattering of Sr2RuO4 M. Udagawa a'*, T. Minami a, N. Ogita a, Y. Maeno b, F. Nakamura b, T. Fujita b, J.G. Bednorz c, F. Lichtenberg c a Faculty oflntegrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan b Faculty of Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan IBM Research Division, Ziirich Research Laboratory, 8803 Riischlikon, Switzerland
Abstract Phonon Raman scattering spectra of Sr2RuO 4 have been measured in the temperature region between room temperature and 5 K. Three symmetry-allowed phonons have been observed at room temperature. Since similar spectra have been observed at low temperature, Sr2RuO 4 does not undergo structural phase transitions in the temperature region. The energy of the phonon modes has been calculated by a G F matrix method. The force constants along the c-axis of Sr2RuO 4 are larger than those for La2_ x SrxCuO4.
Recently, Maeno et al observed superconductivity of SrzRuO 4 below 0.97 K by the measurements of electric resistivity and magnetic susceptibility [1]. This observation is the first for the layered perovskite crystal without Cu ion. The crystal structure of SrzRuO4 is KENiF4 type. The distinct difference between SrzRuO4 and La:_xSrxCuO4 is the transition temperature of superconductivity: Tc of SrzRuO 4 is much lower than that for the cuprate superconductors. In this paper, to clarify the lattice dynamical properties of SrzRuO4, temperature and polarization dependencies of Raman scattering spectra for Sr2RuO 4 have been measured and normalmode calculation has been made by a G F matrix method.
Raman scattering spectra were measured by the following system. The incident beam was an Ar laser operated at 488 nm with an output power of 30 mW. The scattered light was analyzed by the triplemonochromator and detected by a diode-array multichannel detector. The temperature of the sample was changed from room temperature to 5 K. For the tetragonal K2NiF4 structure, four phonon modes are Raman-active; two A~g and two Eg modes, where Alg and Eg are irreducible representations for D4h. The Alg modes appear for the polarization geometries of (x,x) or (z,z), and Eg for (z,x), where x and z denote [1, 0, 0] and [0, 0, 1] axes, respectively. The Alg modes are vibrations of Sr or apical oxygen along the c-axis, and Eg those of Sr or apical oxygen along the a-axis.
2. Experimental
3. Results and discussion
The sample used in this study was a single crystal grown by a travelling-solvent floating-zone method.
The polarization dependence of Raman spectra of Sr2RuO4 at room temperature is shown in Fig. 1. We have clearly observed three peaks below 600 cm- 1. Three peaks can be assigned as phonons. The lower- energy Alg
I. Introduction
* Corresponding author.
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 7 0 2 - 4
223
M. Udagawa et al. /Physica B 219&220 (1996) 222 224
mode at 197 c m - 1 is a vibration due to Sr and the higher one at 540 c m - 1 due to apical oxygen along the c-axis. The Eg mode at 247 cm 1 is the vibration of apical oxygen along the a-axis. The peak at about 540 c m - 1 in the (z, x) spectra is an oblique mode of the higher-energy Aag mode due to the slight misalignments of the crystal axis setting. The Raman spectra at 5 K are similar to those at room temperature. Thus, it can be concluded that Sr2RuO4 does not undergo a structural phase transition in the temperature region. In order to clarify the lattice dynamical difference between Sr2RuO4 and La2-xSrxCuO4, we have calculated the energy of the phonon modes by a G F matrix method. In this calculation, we employed the following model reported by Maroni et al. [-2]. As the fitting parameters, the following longitudinal force constants are
I
1
I
I
i
Sr2RuO ~
I
f
i
I
R.T.
Alg
j.
:i
(Z,Z)
j~...
xo.4
kJ
k~
,~,,,.,.,..j~~
~
(x,x) 1
x
employed: f l is a force constant due to Ru-O bond along the a-axis; fz, the Ru-O (apical) bond along the c-axis; f3, the Sr-O (apical) along the c-axis; f4, the Sr-O (apical) along the [1, 1, 0] direction, and fs, the Sr-Ru, and f6 between Sr and O in RuO2 layer. As the target energies, three observed Raman modes and three A2, modes at 195, 373, and 481 c m - 1, measured by infrared reflectivity [,3], are employed. The obtained force constants, a-axis, and c-axis are listed in Table 1. We note that fl is mainly determined by the stretching mode of O on the RuO2 conduction plane along a-axis, which is an infrared-active mode. However, this mode cannot be observed for Sr2RuO4, because of high conductivity along the a-axis [-3]. The sum off2 and f3 is shown, since f2 and ./'3 cannot be obtained separately. The employed model is very simple, but qualitatively we can obtain the following features. The force constants off2 +f3 and f6 for Sr2RuO 4 are larger than those for Lal.85Sro.l 5CUO4. However, j~, which is the force constant of Sr-O (apical) bond along the a-axis in the block layer, is almost constant. Thus, the inter- atomic interaction along the c-axis of Sr2RuO4 is larger than that for Lal.ssSro.l~CuO4. The increase of f2 +f3 increases the energy of the higher-energy A~g mode for SrzRuO4. For Lal.85Sro.~sCuO4 the energy of this mode is about 430 c m - 1. Since the increased interatomic interaction along the c-axis decreases Tc for the Ce-doped NdzCuO4 system [-4], similar tendency is obtained for Sr2RuOa.
E-rj
E,
I
I
(Z,X) x4 I
I 5/0
I
I
I
Acknowledgements
I
0 1000 ENERGY SHIFT (cm-~)
Fig. 1. Raman scattering spectra of S r 2 R u O 4 in the energy region between 0 and 1000cm-I at room temperature. The numbers shown in this figure denote the magnification rate of each spectrum.
We would like to express our sincere thanks to T. Katsufuji, M. Kasai, and Y. Tokura for providing data of infrared phonon modes. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas "Mechanism on Superconductivity" from the Ministry of Education, Science and Culture, Japan and also by the Cryogenic Center of Hiroshima University.
Table 1 Force constants of Sr2RuO4 and Lal.85Sro.lsCuO4 Crystal
Sr2RuO4 Lal .ssSr0.15CUO4
Force constants (mdyn/A) f~
5+5
Y,
f~
£
1.70"
2.09 1.70
0.21 0.20
0.52 0.55
0.64 0.24
a-axis (A)
c-axis (A)
3.872 3.779
12.74 13.226
"We assume that the energy of oxygen stretching mode on the CuO2 plane along the a-axis is 670 cm- 1.
224
M. Udagawa et al. / Physica B 219&220 (1996) 222-224
References I-1] Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T. Fujita, J.G. Bednorz and F. Lichtenberg, Nature 372 (1994) 532. 1-2] V.A. Maroni, T.O. Brun, M. Grimsditch and C.K. Loong, Phys. Rev. B 39 (1989) 4127.
[3] T. Katsufuji, M. Kasai and Y. Tokura, private communications. [4] M. Udagawa, Y. Nagaoka, N. Ogita, M. Masada, J. Akimitsu and K. Ohbayashi, Phys. Rev. B 49 (1994) 585.
Physica B 219&220 (1996) 222 224
Phonon Raman scattering of Sr2RuO4 M. Udagawa a'*, T. Minami a, N. Ogita a, Y. Maeno b, F. Nakamura b, T. Fujita b, J.G. Bednorz c, F. Lichtenberg c a Faculty oflntegrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan b Faculty of Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan IBM Research Division, Ziirich Research Laboratory, 8803 Riischlikon, Switzerland
Abstract Phonon Raman scattering spectra of Sr2RuO 4 have been measured in the temperature region between room temperature and 5 K. Three symmetry-allowed phonons have been observed at room temperature. Since similar spectra have been observed at low temperature, Sr2RuO 4 does not undergo structural phase transitions in the temperature region. The energy of the phonon modes has been calculated by a G F matrix method. The force constants along the c-axis of Sr2RuO 4 are larger than those for La2_ x SrxCuO4.
Recently, Maeno et al observed superconductivity of SrzRuO 4 below 0.97 K by the measurements of electric resistivity and magnetic susceptibility [1]. This observation is the first for the layered perovskite crystal without Cu ion. The crystal structure of SrzRuO4 is KENiF4 type. The distinct difference between SrzRuO4 and La:_xSrxCuO4 is the transition temperature of superconductivity: Tc of SrzRuO 4 is much lower than that for the cuprate superconductors. In this paper, to clarify the lattice dynamical properties of SrzRuO4, temperature and polarization dependencies of Raman scattering spectra for Sr2RuO 4 have been measured and normalmode calculation has been made by a G F matrix method.
Raman scattering spectra were measured by the following system. The incident beam was an Ar laser operated at 488 nm with an output power of 30 mW. The scattered light was analyzed by the triplemonochromator and detected by a diode-array multichannel detector. The temperature of the sample was changed from room temperature to 5 K. For the tetragonal K2NiF4 structure, four phonon modes are Raman-active; two A~g and two Eg modes, where Alg and Eg are irreducible representations for D4h. The Alg modes appear for the polarization geometries of (x,x) or (z,z), and Eg for (z,x), where x and z denote [1, 0, 0] and [0, 0, 1] axes, respectively. The Alg modes are vibrations of Sr or apical oxygen along the c-axis, and Eg those of Sr or apical oxygen along the a-axis.
2. Experimental
3. Results and discussion
The sample used in this study was a single crystal grown by a travelling-solvent floating-zone method.
The polarization dependence of Raman spectra of Sr2RuO4 at room temperature is shown in Fig. 1. We have clearly observed three peaks below 600 cm- 1. Three peaks can be assigned as phonons. The lower- energy Alg
I. Introduction
* Corresponding author.
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 7 0 2 - 4
223
M. Udagawa et al. /Physica B 219&220 (1996) 222 224
mode at 197 c m - 1 is a vibration due to Sr and the higher one at 540 c m - 1 due to apical oxygen along the c-axis. The Eg mode at 247 cm 1 is the vibration of apical oxygen along the a-axis. The peak at about 540 c m - 1 in the (z, x) spectra is an oblique mode of the higher-energy Aag mode due to the slight misalignments of the crystal axis setting. The Raman spectra at 5 K are similar to those at room temperature. Thus, it can be concluded that Sr2RuO4 does not undergo a structural phase transition in the temperature region. In order to clarify the lattice dynamical difference between Sr2RuO4 and La2-xSrxCuO4, we have calculated the energy of the phonon modes by a G F matrix method. In this calculation, we employed the following model reported by Maroni et al. [-2]. As the fitting parameters, the following longitudinal force constants are
I
1
I
I
i
Sr2RuO ~
I
f
i
I
R.T.
Alg
j.
:i
(Z,Z)
j~...
xo.4
kJ
k~
,~,,,.,.,..j~~
~
(x,x) 1
x
employed: f l is a force constant due to Ru-O bond along the a-axis; fz, the Ru-O (apical) bond along the c-axis; f3, the Sr-O (apical) along the c-axis; f4, the Sr-O (apical) along the [1, 1, 0] direction, and fs, the Sr-Ru, and f6 between Sr and O in RuO2 layer. As the target energies, three observed Raman modes and three A2, modes at 195, 373, and 481 c m - 1, measured by infrared reflectivity [,3], are employed. The obtained force constants, a-axis, and c-axis are listed in Table 1. We note that fl is mainly determined by the stretching mode of O on the RuO2 conduction plane along a-axis, which is an infrared-active mode. However, this mode cannot be observed for Sr2RuO4, because of high conductivity along the a-axis [-3]. The sum off2 and f3 is shown, since f2 and ./'3 cannot be obtained separately. The employed model is very simple, but qualitatively we can obtain the following features. The force constants off2 +f3 and f6 for Sr2RuO 4 are larger than those for Lal.85Sro.l 5CUO4. However, j~, which is the force constant of Sr-O (apical) bond along the a-axis in the block layer, is almost constant. Thus, the inter- atomic interaction along the c-axis of Sr2RuO4 is larger than that for Lal.ssSro.l~CuO4. The increase of f2 +f3 increases the energy of the higher-energy A~g mode for SrzRuO4. For Lal.85Sro.~sCuO4 the energy of this mode is about 430 c m - 1. Since the increased interatomic interaction along the c-axis decreases Tc for the Ce-doped NdzCuO4 system [-4], similar tendency is obtained for Sr2RuOa.
E-rj
E,
I
I
(Z,X) x4 I
I 5/0
I
I
I
Acknowledgements
I
0 1000 ENERGY SHIFT (cm-~)
Fig. 1. Raman scattering spectra of S r 2 R u O 4 in the energy region between 0 and 1000cm-I at room temperature. The numbers shown in this figure denote the magnification rate of each spectrum.
We would like to express our sincere thanks to T. Katsufuji, M. Kasai, and Y. Tokura for providing data of infrared phonon modes. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas "Mechanism on Superconductivity" from the Ministry of Education, Science and Culture, Japan and also by the Cryogenic Center of Hiroshima University.
Table 1 Force constants of Sr2RuO4 and Lal.85Sro.lsCuO4 Crystal
Sr2RuO4 Lal .ssSr0.15CUO4
Force constants (mdyn/A) f~
5+5
Y,
f~
£
1.70"
2.09 1.70
0.21 0.20
0.52 0.55
0.64 0.24
a-axis (A)
c-axis (A)
3.872 3.779
12.74 13.226
"We assume that the energy of oxygen stretching mode on the CuO2 plane along the a-axis is 670 cm- 1.
224
M. Udagawa et al. / Physica B 219&220 (1996) 222-224
References I-1] Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T. Fujita, J.G. Bednorz and F. Lichtenberg, Nature 372 (1994) 532. 1-2] V.A. Maroni, T.O. Brun, M. Grimsditch and C.K. Loong, Phys. Rev. B 39 (1989) 4127.
[3] T. Katsufuji, M. Kasai and Y. Tokura, private communications. [4] M. Udagawa, Y. Nagaoka, N. Ogita, M. Masada, J. Akimitsu and K. Ohbayashi, Phys. Rev. B 49 (1994) 585.