- Email: [email protected]
Resonance effects in polarised near-infrared excited Raman spectra of YBa2Cu3O7−δ
PHYSICA
Physica B 194-196 (1994) 1575-1576 North-Holland
R E S O N A N C E E F F E C T S IN P O L A R I S E D N E A R - I N F R A R E D E X C I T E D R A M A N S P E C T R A OF Y B a 2 C u 3 0 7 - 8 . Henry L. Dewing a b, Bernd Giittler a, Piotr Przyslupski b and Rajen N. Basu a * a Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany. b Superconductivity IRC, Madingley Road, Cambridge, CB3 0HE, UK. Polarised near-infrared (1.1 eV) excited FT-Raman spectra of YBa2Cu307-8 have been measured for the In'st time. All the Raman active Ag phonon modes are observed, in agreement with the results of green (2.4 eV) excited Raman spectrometry. The calculated absolute scattering efficiencies of the phonons are different, however, due to changes in the influence of Raman resonance scattering. This provides an important tool with which to probe the electronic structure in this energy range. A comparison with theoretical predictions of the resonant Raman profiles using LDA calculations reveals significant deviations from the model. 1. I N T R O D U C T I O N Raman spectrometric studies of YBa2Cu307-8 (YBCO) have proved to be of great fundamental and practical interest as a probe for investigating key properties such as the gap energy and lattice dynamics, and also as a tool for quality control [14]. Investigating variations in the phonons' intensities as a function of the excitation energy (measuring resonance Raman profiles) also provides a unique insight into the electronic structure, which can be compared with the predictions of theoretical models [5-8]. The range of excitation energies used previously was 1.8-2.7 eV, but with the advent of FT-Raman spectrometry the restrictions caused by small intensities at low excitation energy have been largely overcome [4,9]. Here we present spectra measured using a 1.1 eV NIR laser as the excitation source, from which we deduced the phonons' Raman scattering efficiencies, effectively doubling the energy range over which a comparison with theoretical models can be made.
l
w
!
t
580 500 440 340 Y'(ZZ) ~F'
,..~k
,,I
t
150 120
t
Y'(X~)~'
2. E X P E R I M E N T A L
The NIR excited FT-Raman spectra were collected using a Bruker IFS 66 optical bench fitted with an FRA 106 FT-Raman accessory containing a Nd-YAG laser 0,=1064nm=l.16eV) as excitation source. Polarization of the spectra was achieved by fitting Glan-Thompson prism polarizers at the Jacquinot stop [10]. We measured a/b-twinned (001) and (110) plane epitaxial thin film samples of YBCO, which were grown by sputtering on LaAIO3 and SrTiO3 substrates and characterised by Tc and conventional green (2.41eV) excited Raman.
z(x-~z
800
.,,..,j,
700
600
500 400
300 200
j.,]
100
Wavenumber (cm-1) Fig 1. Near Infrared excited Fr-Raman Spectra of YBa2Cu307_8 in all polarisations.
* Permanent address: Central Glass and Ceramic Research Institute, Calcutta 700 032, India. 0921-4526/94/'$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)E1368-V
. .....
1576 3.
RESULTS
AND
DISCUSSION
The NIR FT-Raman spectra (Fig 1) show all the Ag modes seen in the green light excited spectra, at 120 150, 340, 440 and 500 cm-l[1-3]. The intensity of the mode near 500 may be slightly over-estimated as this phonon is extremely sensitive to the oxygen stoichiometry [4]. The weak intensity of the IR active phonon near 580, also observed in visible excited Raman, confirms that the oxygen in the CuO chains of the structure is well ordered. At higher excitation energies, the phonon bands near 150, 440 and 500 have stronger scattering in zz than in xx/yy, while the opposite is true for those near 120 and 340 [5-8]. It is evident, however, that the extreme anisotropy, especially of the 500 mode, is almost completely absent in our spectra. This anisotropy is caused by the strong influence of resonance scattering in the zz polarised visible light excited spectra. By contrast, the unchanged polarisation properties of the 120 and 340 modes over the entire range of excitation energies can be related to the underlying form of the Raman scattering tensors. Their predominantly xx/yy character is consistent with the assumption [1,2] that the asymmetric Fano line shape of these phonons, which we also observe, is caused by a coupling between the phonons and the charge carriers in the CuO2 planes. We find, however, that the ratio of the intensities in Big [z(x'y')z-] to Ag [z(xx)~] symmetry is different from that observed in green excited Raman. The Big character of the 340 mode is significantly reduced, while the 120 mode is found to have a non-vanishing intensity in z(x'y')~. We calculated the absolute scattering efficiencies of the various phonons by comparison with a CaF2 reference, following the procedure laid out in [7], using the ellipsometric data of [11]. Green Raman data on the same samples were collected and calculated as a comparison. We have plotted the results of these calculations with the data from [6] in Fig 2. We find good agreement between oar data and the results of the LDA band structure calculations for the zz polarised scattering efficiencies. It is, however, clear from our results that the predicted increase in resonance in xx/yy is greatly overestimated by these calculations: the proposed influence of chain-to-plane electron transitions predicted for yy polarisation is absent. The inconsistency between the LDA prediction and the experimental results at this energy, while they are in good agreement in the range 1.8 to 2.7 eV [6] provides an important clue for the modelling of the electronic structrure of this material.
Additional, previously overlooked or underestimated contributions, such as electron-electron or electronphonon interactions, must be considered [12]. Such interactions are central to many of the theories proposed to explain the high Tc.
_ 1 5 0 1 0 0 :S~ ~'5, Z0Z 0 c m",, J 50
v
~, 7
¢
ol
o 435cm. I
E 20
"f,>
- ~2
,
0.8
-
,--,.
0.4
10
o
Z uJ
o
- 150crn-I
4
U..
2 - '""
<
X <
/~" \ \
-
G
~ "
"
2
-- 1.0
',,
.
,
0.5
.
" 1 1 5 c m "I
4~_
',
\
,"
1.0
,~,
; ,._.. 1
:
'\
o ;--'~:r--~" , , , , 1.o 15 2.o 2.5 3.0 3.51.0 1.5 2.0 2.5
I 0.5
o
3.0
3.5
LASER ENERGY{eV)
Fig. 2 A diagram from [6] showing the resonant Raman profile of YBa2Cu30 7 as a function of the exciting laser energy, to which we have added our data points shown by the diamonds. The full and dashed curves are the resonance profdes predicted by LDA calculations. 1. C. Thomsen, M. Cardona, "Physical properties of high temperature superconductors I" (ed: D.M. Ginsberg) World Scientific, Singapore (1989) 2. R. Feile, Physica C 159 1 (1989) 3. J.R. Ferraro et al Appl. Spect. 44 351 (1990) 4. B Giittler et al Supercon Sci Tech. 4 199 (1991) 5. D. Kirillov et al. J. Appl. Phys. 66 977 (1989) 6. E.T. Heyen et al. Phys Rev Lett 65 3048 (1990) 7. E.T. Heyen, Dissertation, MPI, Stuttgart (1991) 8. O.V. Misochko et al. Physica C 185-189 1025 (1991) 9. R Zamboni et al Solid State Corn 70 813 (1989) 10 A Hoffmann et al J Raman Spec 22 497 (1991) 11 J Humlicek et al Solid State Com79 673 (1991) 12. W. E. Pickett Rev. Mod. Phys. 61 433 (1989)
Physica B 194-196 (1994) 1575-1576 North-Holland
R E S O N A N C E E F F E C T S IN P O L A R I S E D N E A R - I N F R A R E D E X C I T E D R A M A N S P E C T R A OF Y B a 2 C u 3 0 7 - 8 . Henry L. Dewing a b, Bernd Giittler a, Piotr Przyslupski b and Rajen N. Basu a * a Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany. b Superconductivity IRC, Madingley Road, Cambridge, CB3 0HE, UK. Polarised near-infrared (1.1 eV) excited FT-Raman spectra of YBa2Cu307-8 have been measured for the In'st time. All the Raman active Ag phonon modes are observed, in agreement with the results of green (2.4 eV) excited Raman spectrometry. The calculated absolute scattering efficiencies of the phonons are different, however, due to changes in the influence of Raman resonance scattering. This provides an important tool with which to probe the electronic structure in this energy range. A comparison with theoretical predictions of the resonant Raman profiles using LDA calculations reveals significant deviations from the model. 1. I N T R O D U C T I O N Raman spectrometric studies of YBa2Cu307-8 (YBCO) have proved to be of great fundamental and practical interest as a probe for investigating key properties such as the gap energy and lattice dynamics, and also as a tool for quality control [14]. Investigating variations in the phonons' intensities as a function of the excitation energy (measuring resonance Raman profiles) also provides a unique insight into the electronic structure, which can be compared with the predictions of theoretical models [5-8]. The range of excitation energies used previously was 1.8-2.7 eV, but with the advent of FT-Raman spectrometry the restrictions caused by small intensities at low excitation energy have been largely overcome [4,9]. Here we present spectra measured using a 1.1 eV NIR laser as the excitation source, from which we deduced the phonons' Raman scattering efficiencies, effectively doubling the energy range over which a comparison with theoretical models can be made.
l
w
!
t
580 500 440 340 Y'(ZZ) ~F'
,..~k
,,I
t
150 120
t
Y'(X~)~'
2. E X P E R I M E N T A L
The NIR excited FT-Raman spectra were collected using a Bruker IFS 66 optical bench fitted with an FRA 106 FT-Raman accessory containing a Nd-YAG laser 0,=1064nm=l.16eV) as excitation source. Polarization of the spectra was achieved by fitting Glan-Thompson prism polarizers at the Jacquinot stop [10]. We measured a/b-twinned (001) and (110) plane epitaxial thin film samples of YBCO, which were grown by sputtering on LaAIO3 and SrTiO3 substrates and characterised by Tc and conventional green (2.41eV) excited Raman.
z(x-~z
800
.,,..,j,
700
600
500 400
300 200
j.,]
100
Wavenumber (cm-1) Fig 1. Near Infrared excited Fr-Raman Spectra of YBa2Cu307_8 in all polarisations.
* Permanent address: Central Glass and Ceramic Research Institute, Calcutta 700 032, India. 0921-4526/94/'$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)E1368-V
. .....
1576 3.
RESULTS
AND
DISCUSSION
The NIR FT-Raman spectra (Fig 1) show all the Ag modes seen in the green light excited spectra, at 120 150, 340, 440 and 500 cm-l[1-3]. The intensity of the mode near 500 may be slightly over-estimated as this phonon is extremely sensitive to the oxygen stoichiometry [4]. The weak intensity of the IR active phonon near 580, also observed in visible excited Raman, confirms that the oxygen in the CuO chains of the structure is well ordered. At higher excitation energies, the phonon bands near 150, 440 and 500 have stronger scattering in zz than in xx/yy, while the opposite is true for those near 120 and 340 [5-8]. It is evident, however, that the extreme anisotropy, especially of the 500 mode, is almost completely absent in our spectra. This anisotropy is caused by the strong influence of resonance scattering in the zz polarised visible light excited spectra. By contrast, the unchanged polarisation properties of the 120 and 340 modes over the entire range of excitation energies can be related to the underlying form of the Raman scattering tensors. Their predominantly xx/yy character is consistent with the assumption [1,2] that the asymmetric Fano line shape of these phonons, which we also observe, is caused by a coupling between the phonons and the charge carriers in the CuO2 planes. We find, however, that the ratio of the intensities in Big [z(x'y')z-] to Ag [z(xx)~] symmetry is different from that observed in green excited Raman. The Big character of the 340 mode is significantly reduced, while the 120 mode is found to have a non-vanishing intensity in z(x'y')~. We calculated the absolute scattering efficiencies of the various phonons by comparison with a CaF2 reference, following the procedure laid out in [7], using the ellipsometric data of [11]. Green Raman data on the same samples were collected and calculated as a comparison. We have plotted the results of these calculations with the data from [6] in Fig 2. We find good agreement between oar data and the results of the LDA band structure calculations for the zz polarised scattering efficiencies. It is, however, clear from our results that the predicted increase in resonance in xx/yy is greatly overestimated by these calculations: the proposed influence of chain-to-plane electron transitions predicted for yy polarisation is absent. The inconsistency between the LDA prediction and the experimental results at this energy, while they are in good agreement in the range 1.8 to 2.7 eV [6] provides an important clue for the modelling of the electronic structrure of this material.
Additional, previously overlooked or underestimated contributions, such as electron-electron or electronphonon interactions, must be considered [12]. Such interactions are central to many of the theories proposed to explain the high Tc.
_ 1 5 0 1 0 0 :S~ ~'5, Z0Z 0 c m",, J 50
v
~, 7
¢
ol
o 435cm. I
E 20
"f,>
- ~2
,
0.8
-
,--,.
0.4
10
o
Z uJ
o
- 150crn-I
4
U..
2 - '""
<
X <
/~" \ \
-
G
~ "
"
2
-- 1.0
',,
.
,
0.5
.
" 1 1 5 c m "I
4~_
',
\
,"
1.0
,~,
; ,._.. 1
:
'\
o ;--'~:r--~" , , , , 1.o 15 2.o 2.5 3.0 3.51.0 1.5 2.0 2.5
I 0.5
o
3.0
3.5
LASER ENERGY{eV)
Fig. 2 A diagram from [6] showing the resonant Raman profile of YBa2Cu30 7 as a function of the exciting laser energy, to which we have added our data points shown by the diamonds. The full and dashed curves are the resonance profdes predicted by LDA calculations. 1. C. Thomsen, M. Cardona, "Physical properties of high temperature superconductors I" (ed: D.M. Ginsberg) World Scientific, Singapore (1989) 2. R. Feile, Physica C 159 1 (1989) 3. J.R. Ferraro et al Appl. Spect. 44 351 (1990) 4. B Giittler et al Supercon Sci Tech. 4 199 (1991) 5. D. Kirillov et al. J. Appl. Phys. 66 977 (1989) 6. E.T. Heyen et al. Phys Rev Lett 65 3048 (1990) 7. E.T. Heyen, Dissertation, MPI, Stuttgart (1991) 8. O.V. Misochko et al. Physica C 185-189 1025 (1991) 9. R Zamboni et al Solid State Corn 70 813 (1989) 10 A Hoffmann et al J Raman Spec 22 497 (1991) 11 J Humlicek et al Solid State Com79 673 (1991) 12. W. E. Pickett Rev. Mod. Phys. 61 433 (1989)