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Electronic Raman scattering in single and double CuO2-layer TlBa(Ca)CuO superconductors
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PHYSICA ®
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ELSEVIER
Physica C 282-287 (1997) 1027-1028
Electronic Raman scattering in single and double Cu02-layer Tl-Ba-(Ca)-Cu-O superconductors Moonsoo Kang, G. Blumberg, and M. V. Klein a* aDepartment of Physics, University of Illinois at Urbana-Champaign 1110 W. Green St., Urbana, IL 61801, USA Low energy Raman continuum and the redistribution of the continuum to a peak ('26-peak') in the superconducting state have been studied in Tl-Ba-(Ca)-Cu-O superconductors with single CU02 layer (TI-2201) and double CU02 layer (TI-2212). The Coulomb screening effect is observed to be much stronger in TI-2201. The change from TI-2201 to TI-2212 of the normalized A 1g 26-peak intensity is identical within experimental error to that of normalized A 1g continuum intensity.
We report electronic Raman scattering study of Tl-Ba-(Ca)-Cu-O superconductors with single and double CU02 layers. The magnitude and the anisotropy of the super conducting gap were measured from those materials. The relative scattering intensities in A lg and BIg symmetries are compared to examine the effect of the Coulomb screening on single and double CU02 layer materials. The experiments were done on a Tl-2201 (ThBa2Cu06) single crystal with Tc = 85 K and a Tl-2212 (ThBa2CaCu20a) single crystal with T c = 102 K. The structures of Tl-2201 and Tl2212 are very similar except for the number of neighboring CU02 planes. We used a high energy (blue) excitation (2.73 eV) and a low energy (red) excitation (1.92 eV) from a Kr+ laser. The laser power was reduced to a level which does not increase the temperature of the illuminated spot significantly. To observe suppression of the continuum and appearance of the 26-peak in the superconducting state, we subtracted the spectra taken at temperatures above the Tc from the spectra at 4 K. The differences in Raman response (referred as Difference spectra herein after) are presented in Figure 1. Sharp features in difference spectra from Tl-2201 are due to temperature dependences 'Supported by NSF under DMR 91-20000 through the Science and Technology Center for Superconductivity. 0921-4534/97/$17.00 © Elsevier Science s.Y. All rights reserved. PH 80921-4534(97)00596-0
(a) Tl-2201
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(b) Tl-2212
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1000 1200
Raman shift (em· 1)
Figure 1. Differences of Raman response functions in super conducting (at 4 K) and normal (just above Tc) states in (a) Tl-2201 (single layer) and (b) Tl-2212 (double layer).
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M. Kang et al.lPhysica C 282-287 (1997) 1027-1028
Table 1 Normalized A lg 2~-peak and continuum intensities with respect to those in B lg . The last row shows the ratio of the Normalized Alg 2~-peak and continuum intensities between TI-2201 (single layer) and TI-2212 (double layer). TI-2201 TI-2212 TI-2201/TI-2212
0.44 0.94 0.47
0.33 0.71 0.46
of phonon peaks. Clear redistribution of the the continuum and the appearance of the 2~-peak are observed in both Alg and B lg spectra in both samples. In TI-2201, the 2~-peak positions are measured to be around 320 cm- l in A lg spectrum and 470 cm- l in B lg , which gives 2~/kBTc values of 5.4 in A lg and 8.0 in B lg . This is consistent with the results reported from similar samples[1,2]. The A lg and B lg 2~-peak position in TI-2212 are around 430 cm- l and 720 cm-t, respectively. This gives 2~/kBTc values of 6.1 (Alg) and 10.2 (B lg ), which are significantly bigger than the values from TI-2201. Those two distinctively different 2~ values measured in A lg and B lg are consistent with a highly anisotropic gap with a symmetry of the x 2 - y2 type. The large 2~/kBTc values in TI-2212 may indicate that this sample is slightly underdoped. From difference spectra, we measured the 2~ peak intensity by integrating the area above zero. The continuum intensities are determined by values at sufficiently higher frequencies well above the peak positions where the spectra are flat and the same above and below the critical temperature. We further normalized A lg continuum and 2~-peak intensities with respect to B lg intensities to compare the relative intensities of the A lg 2~-peak and continuum between TI-2201 and TI2212 as shown in Table 1. It is clear that the A lg 2~-peak is much weaker than the B lg peak in TI2201. In the TI-2212 sample, however, the A lg peak intensity is at least comparable to the B lg intensity. This kind of tendency is also observed in the case of the continuum intensities. Weak A lg 2~-peak and continuum intensities in TI-2201 can be explained as a result of strong Coulomb screening effect in a single CU02 layer material. Relatively stronger intensities in A lg
spectra from TI-2212 shows that the Coulomb screening is not very effective in a double CU02 layer material. The ratio of normalized A lg 2~-peak and continuum intensities from TI-2201 to those from TI2212 are shown in the last row of Table 1. These ratios show that the A lg 2~-peak and continuum intensities in single layer material is only 46 % and 47 % of those in double layer material, respectively. These two very close values indicates that the A lg 2~-peak and continuum are screened in the same way and affected the same amount under a change in screening. Upon a close examination of the Coulomb screening mechanism[3]' it is natural to conclude that electronic Raman scattering for the continuum and the 2~-peak have a common vertex function or at least vertex functions with very similar k-dependence. This result is also consistent with a recent resonance study of the 2~-peak and the continuum in TI-2201 which shows the vertex functions for the two excitations have the same excitation energy dependence[2]. The Raman scattering vertex function describes how photons are coupled to a particular excitation. Thus, similarities in vertex functions for the continuum and the 2~-peak can be strong evidence that they are from the same origin, and that the 2~-peak comes from the redistribution of the continuum.
REFERENCES 1. R. Nemetschek et al., Phys. Rev. B 47 (1993) 3450. 2. Moonsoo Kang et al., Phys. Rev. Lett. 77 (1996) 4434. 3. M. V. Klein et al., Phys. Rev. B 29 (1984) 4976.
m
PHYSICA ®
~
ELSEVIER
Physica C 282-287 (1997) 1027-1028
Electronic Raman scattering in single and double Cu02-layer Tl-Ba-(Ca)-Cu-O superconductors Moonsoo Kang, G. Blumberg, and M. V. Klein a* aDepartment of Physics, University of Illinois at Urbana-Champaign 1110 W. Green St., Urbana, IL 61801, USA Low energy Raman continuum and the redistribution of the continuum to a peak ('26-peak') in the superconducting state have been studied in Tl-Ba-(Ca)-Cu-O superconductors with single CU02 layer (TI-2201) and double CU02 layer (TI-2212). The Coulomb screening effect is observed to be much stronger in TI-2201. The change from TI-2201 to TI-2212 of the normalized A 1g 26-peak intensity is identical within experimental error to that of normalized A 1g continuum intensity.
We report electronic Raman scattering study of Tl-Ba-(Ca)-Cu-O superconductors with single and double CU02 layers. The magnitude and the anisotropy of the super conducting gap were measured from those materials. The relative scattering intensities in A lg and BIg symmetries are compared to examine the effect of the Coulomb screening on single and double CU02 layer materials. The experiments were done on a Tl-2201 (ThBa2Cu06) single crystal with Tc = 85 K and a Tl-2212 (ThBa2CaCu20a) single crystal with T c = 102 K. The structures of Tl-2201 and Tl2212 are very similar except for the number of neighboring CU02 planes. We used a high energy (blue) excitation (2.73 eV) and a low energy (red) excitation (1.92 eV) from a Kr+ laser. The laser power was reduced to a level which does not increase the temperature of the illuminated spot significantly. To observe suppression of the continuum and appearance of the 26-peak in the superconducting state, we subtracted the spectra taken at temperatures above the Tc from the spectra at 4 K. The differences in Raman response (referred as Difference spectra herein after) are presented in Figure 1. Sharp features in difference spectra from Tl-2201 are due to temperature dependences 'Supported by NSF under DMR 91-20000 through the Science and Technology Center for Superconductivity. 0921-4534/97/$17.00 © Elsevier Science s.Y. All rights reserved. PH 80921-4534(97)00596-0
(a) Tl-2201
o II)
u
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:.a II)
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0
a
~
100 200 300 400 500 600 700 800
(b) Tl-2212
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a ~
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0
o
200
400
600
800
1000 1200
Raman shift (em· 1)
Figure 1. Differences of Raman response functions in super conducting (at 4 K) and normal (just above Tc) states in (a) Tl-2201 (single layer) and (b) Tl-2212 (double layer).
1028
M. Kang et al.lPhysica C 282-287 (1997) 1027-1028
Table 1 Normalized A lg 2~-peak and continuum intensities with respect to those in B lg . The last row shows the ratio of the Normalized Alg 2~-peak and continuum intensities between TI-2201 (single layer) and TI-2212 (double layer). TI-2201 TI-2212 TI-2201/TI-2212
0.44 0.94 0.47
0.33 0.71 0.46
of phonon peaks. Clear redistribution of the the continuum and the appearance of the 2~-peak are observed in both Alg and B lg spectra in both samples. In TI-2201, the 2~-peak positions are measured to be around 320 cm- l in A lg spectrum and 470 cm- l in B lg , which gives 2~/kBTc values of 5.4 in A lg and 8.0 in B lg . This is consistent with the results reported from similar samples[1,2]. The A lg and B lg 2~-peak position in TI-2212 are around 430 cm- l and 720 cm-t, respectively. This gives 2~/kBTc values of 6.1 (Alg) and 10.2 (B lg ), which are significantly bigger than the values from TI-2201. Those two distinctively different 2~ values measured in A lg and B lg are consistent with a highly anisotropic gap with a symmetry of the x 2 - y2 type. The large 2~/kBTc values in TI-2212 may indicate that this sample is slightly underdoped. From difference spectra, we measured the 2~ peak intensity by integrating the area above zero. The continuum intensities are determined by values at sufficiently higher frequencies well above the peak positions where the spectra are flat and the same above and below the critical temperature. We further normalized A lg continuum and 2~-peak intensities with respect to B lg intensities to compare the relative intensities of the A lg 2~-peak and continuum between TI-2201 and TI2212 as shown in Table 1. It is clear that the A lg 2~-peak is much weaker than the B lg peak in TI2201. In the TI-2212 sample, however, the A lg peak intensity is at least comparable to the B lg intensity. This kind of tendency is also observed in the case of the continuum intensities. Weak A lg 2~-peak and continuum intensities in TI-2201 can be explained as a result of strong Coulomb screening effect in a single CU02 layer material. Relatively stronger intensities in A lg
spectra from TI-2212 shows that the Coulomb screening is not very effective in a double CU02 layer material. The ratio of normalized A lg 2~-peak and continuum intensities from TI-2201 to those from TI2212 are shown in the last row of Table 1. These ratios show that the A lg 2~-peak and continuum intensities in single layer material is only 46 % and 47 % of those in double layer material, respectively. These two very close values indicates that the A lg 2~-peak and continuum are screened in the same way and affected the same amount under a change in screening. Upon a close examination of the Coulomb screening mechanism[3]' it is natural to conclude that electronic Raman scattering for the continuum and the 2~-peak have a common vertex function or at least vertex functions with very similar k-dependence. This result is also consistent with a recent resonance study of the 2~-peak and the continuum in TI-2201 which shows the vertex functions for the two excitations have the same excitation energy dependence[2]. The Raman scattering vertex function describes how photons are coupled to a particular excitation. Thus, similarities in vertex functions for the continuum and the 2~-peak can be strong evidence that they are from the same origin, and that the 2~-peak comes from the redistribution of the continuum.
REFERENCES 1. R. Nemetschek et al., Phys. Rev. B 47 (1993) 3450. 2. Moonsoo Kang et al., Phys. Rev. Lett. 77 (1996) 4434. 3. M. V. Klein et al., Phys. Rev. B 29 (1984) 4976.