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Angle-resolved photoemission study of nonsuperconductive Bi2Sr2Ca0.4Y0.6Cu2O8
PhysicaC 170 (1990) 416-418 North-Holland
Angle-resolved photoemission study of nonsuperconductive Bi2Sr2%J0.6Cu2Q3 T. Takahashi a, H. Matsuyama a, H. Katayama-Yoshida a, K. Seki b, K. Kamiya ’ and H. Inokuchi ’ a Department ofphysics. Tohoku Unwersity, Sendai 980, Japan b Department ofMaterials Science, Hiroshima University, Hiroshima 730, Japan ’ Institute for Molecular Science, Okazaki 444, Japan Received 1 August 1990
A comparative angle-resolved photoemission measurement has been performed on nonsuperconductive BiZSrZCa0.4Y0.6CuZ08 and superconductive Bi2Sr2CaCu20s to study the nature and origin of the electronic states near the Fermi level. It was found that hole-doping does not cause a rigid shift of the density of states relative to the Fermi level, but creates new electronic states in the vicinity of the Fermi level.
The existence of the Fermi-liquid states in the highhas been established by angle-resolved photoemission [ l-31, which observed dispersive bands crossing the Fermi level. The next step to approach the high-T, mechanism is to elucidate the nature and origin of the Fermi-liquid states. We performed a comparative angle-resolved photoemission study of superconductive Bi-Sr-Ca-Cu-0 and nonsuperconductive Bi-Sr-Ca-Y-Cu-0 where the number of doped holes are reduced by replacing divalent Ca with trivalent Y. By comparing the band structure of the superconductor and the non-superconductor, we found that the change of hole-concentration does not necessarily cause a rigid shift of the density of states relative to the Fermi level but instead the hole-doping creates new electronic states in the vicinity of the Fermi level in a quite similar manner to deep impurity states produced in semiconductors. We have already reported an angle-resolved photoemission study of Bi,SrzCaCuzOB with T,=85 K [ 11. The same experimental setup (spectrometer, photon energy, etc.) was employed for the non-superconductor sample to eliminate possible extrinsic effects on the shape and intensity of photoemission spectra. A single crystal of Bi-Sr-Ca-Y-Cu-0 typically 5 x 5 x 0.2 mm3 was grown by a self-flux tech-
T,superconductor
0921-4534/90/$03.50
nique. It is well established that Y atoms can be successively replaced with Ca atoms in Bi-Sr-Ca-Cu0 and thereby reduce the hole concentration and resultingly the T, [4,5 1. The composition of the grown crystal estimated from EPMA (electron-probe-micro-analysis) and its X-ray diffraction pattern (c-axis length, etc.) is Bi,Sr,Ca0,4Y0.6Cu208. The resistivity and magnetic-susceptibility measurements showed that the crystal does not become superconductive even at 4 K. Photoemission spectroscopy was performed with an angle-resolved photoemission spectrometer at the UVSOR, Institute for Molecular Science, Japan. The energy and angular resolutions were -0.2 eV and 5 2”, respectively. The crystal was cleaved in the spectrometer to obtain a fresh surface and kept at room temperature during measurements. No degradation of the sample surface was detected throughout the measurement. Figure 1 shows angle-resolved photoemission spectra of Bi,SrzCa0.4Y,.,CuzOB (solid lines) measured at fiw= 18 eV for a high-symmetry direction in the Brillouin zone IX, compared with those of BizSrzCaCuzOB (broken lines) [ 11. There are some similarities and differences between the two sets of the angle-resolved photoemission spectra. In the highbinding-energy-region (bands D, E and F), the energy position of peaks in the spectra is almost the
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
417
T. Takahashi et al. / Photoemission of Bi_Sr2Ca0.,Y0.6Cu208
TX direction
iiw=lBeV
--- Bi2SrLaCu208 - BiL%2Ca~Y~&u~O8
.::-.
...“,.,_,_““.
I I.
‘j;,. x=0.0 ;-....’.;.
I
IIImjIllllflJlli
15
Binding 3 6 Binding
4
2 G Energy(eV)
Fig. 1. Angle-resolved photoemission spectr ‘a of nonsuperconductive Bi2Sr2Cau4Y0 &u208 (solid lines) and superconductive BizSr2CaCuzOs (broken lines) measured at fiw= 18 eV for IX direction in the Brillouin zone.
10
05
EF
Energy
(eV)
Fig. 2. Comparison of representative angle-resolved photoemission spectra in the vicinity of the Fermi level between superconductor (x=0.0) and nonsuperconductor (x=0.6). Photoemission intensity is normalized to that of a main band at 3.5 eV (band Din&. 1).
r
same within f0.2 eV between the superconductor and the non-superconductor, whereas the relative intensity of peaks differs between the two compounds, especially for higher polar angles (0). In contrast with bands D, E and F, band C is apparently shifted toward the high-binding-energy-direction by about 1 eV when we assume bands C and C’ have the same origin. It is noted here that the intensity of band C’ showed a resonant enhancement at the photon energy of 18 eV similarly to band C [ 61, suggesting that both bands C and C’ have the same origin, namely of oxygen 2p. The most remarkable difference in the spectra between the superconductor and the non-superconductor is the photoemission intensity (namely the density of states) in the vicinity of the Fermi level. The intensity near the Fermi level (& - 1 eV) is remarkably reduced for the non-superconductor. This is clearly seen in fig. 2 which enlarges the Fermilevel region of some representative angle-resolved photoemission spectra. Figure 3 shows a comparison of the band structure determined by angle-resolved photoemission between the superconductor (open circles) and the non-
nonsuperconductive Band structures of Fig. 3. Bi2Sr2Ca,,4Y0.6Cu208 (solid circles) and superconductive Bi2Sr2CaCu208 (open circles) determined from angle-resolved photoemission, compared with a band-structure calculation for Bi,SrzCaCu,Os (thin solid lines) [ 71.
418
T. Takahashi et al. / Photoemission ofBi~r*Ca,,Y,,Cu,O,
superconductor (solid circles). A representative band-structure calculation for BizSr,CaCu,Os [ 7 ] is also shown by thin solid lines. As described above, the dispersive feature of bands in high-binding-energy-region (bands D, E and F) is very similar between the two compounds except for a slight energy shift ( _ 0.2 eV) toward the high-binding-energy-direction from the superconductor to the non-superconductor. The energy difference between bands C and C’ is about 0.8 eV, much larger than that of bands D, E and F. In the vicinity of the Fermi level of photoemission spectra for the non-superconductor, we did not observe any distinct structures corresponding to bands A and B for the superconductor, although the tail of band C’ extends very close to the Fermi level. The comparison described above provides a clue to the origin of the Fermi-liquid states in the highT, superconductor. If we stand for a simple rigid-band picture, the density of states (namely photoemission spectrum) would be uniformly shifted toward the high-binding-energy-direction relative to the Fermi level upon replacement of Ca with Y, since a trivalent Y atom donates an excess electron to the system. However, the experimental results shown above are obviously inconsistent with this simple picture. The experimental result shows that (i) hole- (or electron-)doping does not cause a rigid shift of the density of states relative to the Fermi level and instead (ii) hole-doping produces new electronic states in the vicinity of the Fermi level. The present experimental result is consistent with a previous X-ray absorption study of Bi2Sr2Ca, _xY,CuzOs [ 8 ] and also a comparative photoemission study of p-type (Laz_Sr,CuO,) and n-type (Ndz_,Ce,Cu04_,) superconductors [ 9,101, which reported that the Fermi level is positioned at almost the same energy for both p- and n-type superconductors. According to a simple rigid-band picture, a certain amount of energy shift corresponding to the band gap (about 2 eV ) should be observed for the Fermi-level position between the two different types of superconductors. The observed behavior of the electronic states near the Fermi level in the high-T, superconductor seems very similar to that of deep impurity states in semiconductors which are created in the vicinity of the
Fermi level upon doping. Variable-range-hoppingconduction observed in a slightly doped regime of high-T, superconductors resembles very much that of semiconductors. Thus, the present angle-resolved photoemission study indicates that the Fermi-liquid states in the high-T, superconductor originates from a kind of impurity states introduced by hole-doping into the Cu02 planes. The overlapping of the wavefunctions of these impurity states transform the localized states into extended states which have a finite energy dispersion across the Fermi level.
Acknowledgement This work was supported by a grant from the Ministry of Education, Culture and Science of Japan.
References [ 1] T. Takahashi,
H. Matsuyama, H. Katayama-Yoshida, Y. Okabe, S. Hosoya, K. Seki, H. Fujimoto, M. Sato and H. Inokuchi, Nature 334 (1988) 691; ibid., Phys. Rev. B39 (1989) 6636. [ 21 R. Manzke, T. Buslap, R. Claessen and J. Fink, Europhys. Lett. 9 (1989) 477. [3] C.G. Olson, R. Liu,A.-B. Yang, D.W. Lynch, A.J. Arko, R.S. List, B.W. Veal, Y.C. Chang, P.Z. Jiang and A.P. Paulikas, Science 245 (1989) 731. [4] D.B. Mitzi, L.W. Lambardo, A. Kapitulnik, S.S. Laderman and R.D. Jacowitz, Phys. Rev. B41 ( 1990) 6564. [ 5 ] T. Tamegai, A. Watanabe, K. Koga, I. Ogura and Y. Iye, Jpn. J. Appl. Phys. 27 ( 1988) L1074. [6] T. Takahashi, H. Matsuyama, H. Katayama-Yoshida, Y. Okabe, S. Hosoya, K. Seki, H. Fujimoto, M. Sato and H. Inokuchi, Proc. 1st Int. Symp. High-T, Superconductivity (Nagoya, 1988), ed. K. Kitazawa and T. Ishiguro, p. 175. [ 7 ] S. Massidda, J. Yu and A.J. Freeman, Physica C 152 ( 1988) 251. [8] H. Matsuyama, T. Takahashi, H. Katayama-Yoshida, T. Kashiwakura, Y. Okabe, S. Sato, N. Kosugi, A. Yagishita, K. Tanaka, H. Fujimoto and H. Inokuchi, Physica C 160 (1989) 567. [9] J.W. Allen, C.G. Olson, M.B. Maple, J.-S. Kang, L.Z. Liu, J.-H. Park, R.O. Anderson, W.P. Ellis, J.T. Market, Y. Dalichaouch and R. Liu, Phys. Rev. Lett. 64 ( 1990) 595. [ lo] H. Namatame, A. Fujimori, Y. Tokura, M. Nakamura, K. Yamaguchi, A. Misu, H. Matsubara, S. Suga, H. Eisaki, T. Ito, H. Takagi and S. Uchida, Phys. Rev. B41 ( 1990) 7205.
Angle-resolved photoemission study of nonsuperconductive Bi2Sr2%J0.6Cu2Q3 T. Takahashi a, H. Matsuyama a, H. Katayama-Yoshida a, K. Seki b, K. Kamiya ’ and H. Inokuchi ’ a Department ofphysics. Tohoku Unwersity, Sendai 980, Japan b Department ofMaterials Science, Hiroshima University, Hiroshima 730, Japan ’ Institute for Molecular Science, Okazaki 444, Japan Received 1 August 1990
A comparative angle-resolved photoemission measurement has been performed on nonsuperconductive BiZSrZCa0.4Y0.6CuZ08 and superconductive Bi2Sr2CaCu20s to study the nature and origin of the electronic states near the Fermi level. It was found that hole-doping does not cause a rigid shift of the density of states relative to the Fermi level, but creates new electronic states in the vicinity of the Fermi level.
The existence of the Fermi-liquid states in the highhas been established by angle-resolved photoemission [ l-31, which observed dispersive bands crossing the Fermi level. The next step to approach the high-T, mechanism is to elucidate the nature and origin of the Fermi-liquid states. We performed a comparative angle-resolved photoemission study of superconductive Bi-Sr-Ca-Cu-0 and nonsuperconductive Bi-Sr-Ca-Y-Cu-0 where the number of doped holes are reduced by replacing divalent Ca with trivalent Y. By comparing the band structure of the superconductor and the non-superconductor, we found that the change of hole-concentration does not necessarily cause a rigid shift of the density of states relative to the Fermi level but instead the hole-doping creates new electronic states in the vicinity of the Fermi level in a quite similar manner to deep impurity states produced in semiconductors. We have already reported an angle-resolved photoemission study of Bi,SrzCaCuzOB with T,=85 K [ 11. The same experimental setup (spectrometer, photon energy, etc.) was employed for the non-superconductor sample to eliminate possible extrinsic effects on the shape and intensity of photoemission spectra. A single crystal of Bi-Sr-Ca-Y-Cu-0 typically 5 x 5 x 0.2 mm3 was grown by a self-flux tech-
T,superconductor
0921-4534/90/$03.50
nique. It is well established that Y atoms can be successively replaced with Ca atoms in Bi-Sr-Ca-Cu0 and thereby reduce the hole concentration and resultingly the T, [4,5 1. The composition of the grown crystal estimated from EPMA (electron-probe-micro-analysis) and its X-ray diffraction pattern (c-axis length, etc.) is Bi,Sr,Ca0,4Y0.6Cu208. The resistivity and magnetic-susceptibility measurements showed that the crystal does not become superconductive even at 4 K. Photoemission spectroscopy was performed with an angle-resolved photoemission spectrometer at the UVSOR, Institute for Molecular Science, Japan. The energy and angular resolutions were -0.2 eV and 5 2”, respectively. The crystal was cleaved in the spectrometer to obtain a fresh surface and kept at room temperature during measurements. No degradation of the sample surface was detected throughout the measurement. Figure 1 shows angle-resolved photoemission spectra of Bi,SrzCa0.4Y,.,CuzOB (solid lines) measured at fiw= 18 eV for a high-symmetry direction in the Brillouin zone IX, compared with those of BizSrzCaCuzOB (broken lines) [ 11. There are some similarities and differences between the two sets of the angle-resolved photoemission spectra. In the highbinding-energy-region (bands D, E and F), the energy position of peaks in the spectra is almost the
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
417
T. Takahashi et al. / Photoemission of Bi_Sr2Ca0.,Y0.6Cu208
TX direction
iiw=lBeV
--- Bi2SrLaCu208 - BiL%2Ca~Y~&u~O8
.::-.
...“,.,_,_““.
I I.
‘j;,. x=0.0 ;-....’.;.
I
IIImjIllllflJlli
15
Binding 3 6 Binding
4
2 G Energy(eV)
Fig. 1. Angle-resolved photoemission spectr ‘a of nonsuperconductive Bi2Sr2Cau4Y0 &u208 (solid lines) and superconductive BizSr2CaCuzOs (broken lines) measured at fiw= 18 eV for IX direction in the Brillouin zone.
10
05
EF
Energy
(eV)
Fig. 2. Comparison of representative angle-resolved photoemission spectra in the vicinity of the Fermi level between superconductor (x=0.0) and nonsuperconductor (x=0.6). Photoemission intensity is normalized to that of a main band at 3.5 eV (band Din&. 1).
r
same within f0.2 eV between the superconductor and the non-superconductor, whereas the relative intensity of peaks differs between the two compounds, especially for higher polar angles (0). In contrast with bands D, E and F, band C is apparently shifted toward the high-binding-energy-direction by about 1 eV when we assume bands C and C’ have the same origin. It is noted here that the intensity of band C’ showed a resonant enhancement at the photon energy of 18 eV similarly to band C [ 61, suggesting that both bands C and C’ have the same origin, namely of oxygen 2p. The most remarkable difference in the spectra between the superconductor and the non-superconductor is the photoemission intensity (namely the density of states) in the vicinity of the Fermi level. The intensity near the Fermi level (& - 1 eV) is remarkably reduced for the non-superconductor. This is clearly seen in fig. 2 which enlarges the Fermilevel region of some representative angle-resolved photoemission spectra. Figure 3 shows a comparison of the band structure determined by angle-resolved photoemission between the superconductor (open circles) and the non-
nonsuperconductive Band structures of Fig. 3. Bi2Sr2Ca,,4Y0.6Cu208 (solid circles) and superconductive Bi2Sr2CaCu208 (open circles) determined from angle-resolved photoemission, compared with a band-structure calculation for Bi,SrzCaCu,Os (thin solid lines) [ 71.
418
T. Takahashi et al. / Photoemission ofBi~r*Ca,,Y,,Cu,O,
superconductor (solid circles). A representative band-structure calculation for BizSr,CaCu,Os [ 7 ] is also shown by thin solid lines. As described above, the dispersive feature of bands in high-binding-energy-region (bands D, E and F) is very similar between the two compounds except for a slight energy shift ( _ 0.2 eV) toward the high-binding-energy-direction from the superconductor to the non-superconductor. The energy difference between bands C and C’ is about 0.8 eV, much larger than that of bands D, E and F. In the vicinity of the Fermi level of photoemission spectra for the non-superconductor, we did not observe any distinct structures corresponding to bands A and B for the superconductor, although the tail of band C’ extends very close to the Fermi level. The comparison described above provides a clue to the origin of the Fermi-liquid states in the highT, superconductor. If we stand for a simple rigid-band picture, the density of states (namely photoemission spectrum) would be uniformly shifted toward the high-binding-energy-direction relative to the Fermi level upon replacement of Ca with Y, since a trivalent Y atom donates an excess electron to the system. However, the experimental results shown above are obviously inconsistent with this simple picture. The experimental result shows that (i) hole- (or electron-)doping does not cause a rigid shift of the density of states relative to the Fermi level and instead (ii) hole-doping produces new electronic states in the vicinity of the Fermi level. The present experimental result is consistent with a previous X-ray absorption study of Bi2Sr2Ca, _xY,CuzOs [ 8 ] and also a comparative photoemission study of p-type (Laz_Sr,CuO,) and n-type (Ndz_,Ce,Cu04_,) superconductors [ 9,101, which reported that the Fermi level is positioned at almost the same energy for both p- and n-type superconductors. According to a simple rigid-band picture, a certain amount of energy shift corresponding to the band gap (about 2 eV ) should be observed for the Fermi-level position between the two different types of superconductors. The observed behavior of the electronic states near the Fermi level in the high-T, superconductor seems very similar to that of deep impurity states in semiconductors which are created in the vicinity of the
Fermi level upon doping. Variable-range-hoppingconduction observed in a slightly doped regime of high-T, superconductors resembles very much that of semiconductors. Thus, the present angle-resolved photoemission study indicates that the Fermi-liquid states in the high-T, superconductor originates from a kind of impurity states introduced by hole-doping into the Cu02 planes. The overlapping of the wavefunctions of these impurity states transform the localized states into extended states which have a finite energy dispersion across the Fermi level.
Acknowledgement This work was supported by a grant from the Ministry of Education, Culture and Science of Japan.
References [ 1] T. Takahashi,
H. Matsuyama, H. Katayama-Yoshida, Y. Okabe, S. Hosoya, K. Seki, H. Fujimoto, M. Sato and H. Inokuchi, Nature 334 (1988) 691; ibid., Phys. Rev. B39 (1989) 6636. [ 21 R. Manzke, T. Buslap, R. Claessen and J. Fink, Europhys. Lett. 9 (1989) 477. [3] C.G. Olson, R. Liu,A.-B. Yang, D.W. Lynch, A.J. Arko, R.S. List, B.W. Veal, Y.C. Chang, P.Z. Jiang and A.P. Paulikas, Science 245 (1989) 731. [4] D.B. Mitzi, L.W. Lambardo, A. Kapitulnik, S.S. Laderman and R.D. Jacowitz, Phys. Rev. B41 ( 1990) 6564. [ 5 ] T. Tamegai, A. Watanabe, K. Koga, I. Ogura and Y. Iye, Jpn. J. Appl. Phys. 27 ( 1988) L1074. [6] T. Takahashi, H. Matsuyama, H. Katayama-Yoshida, Y. Okabe, S. Hosoya, K. Seki, H. Fujimoto, M. Sato and H. Inokuchi, Proc. 1st Int. Symp. High-T, Superconductivity (Nagoya, 1988), ed. K. Kitazawa and T. Ishiguro, p. 175. [ 7 ] S. Massidda, J. Yu and A.J. Freeman, Physica C 152 ( 1988) 251. [8] H. Matsuyama, T. Takahashi, H. Katayama-Yoshida, T. Kashiwakura, Y. Okabe, S. Sato, N. Kosugi, A. Yagishita, K. Tanaka, H. Fujimoto and H. Inokuchi, Physica C 160 (1989) 567. [9] J.W. Allen, C.G. Olson, M.B. Maple, J.-S. Kang, L.Z. Liu, J.-H. Park, R.O. Anderson, W.P. Ellis, J.T. Market, Y. Dalichaouch and R. Liu, Phys. Rev. Lett. 64 ( 1990) 595. [ lo] H. Namatame, A. Fujimori, Y. Tokura, M. Nakamura, K. Yamaguchi, A. Misu, H. Matsubara, S. Suga, H. Eisaki, T. Ito, H. Takagi and S. Uchida, Phys. Rev. B41 ( 1990) 7205.