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Effect of gamma-irradiation on the electronic structure of (Bi1.65Pb0.35)Sr2Ca2Cu3O10 superconductor
Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 427–430 www.elsevier.nl / locate / elspec
Effect of gamma-irradiation on the electronic structure of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 superconductor 1 M. Faiz*, N.M. Hamdan
Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 8 August 2000; received in revised form 30 August 2000; accepted 6 September 2000
Abstract An XPS investigation was carried out on (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 superconductors, in an effort to study the effect of gamma-irradiation. There is no significant change in the binding energies of Bi 4f, Ca 2p, Cu 2p, O 1s, Pb 4f, and Sr 3p core levels up to the gamma dose of 50 Mrad. However, for the gamma dose of 60 Mrad, the Bi 4f region shows two sets of spin-orbit split levels, one set at about 1.3 eV lower and the other at about 1.7 eV higher binding energies than that of the low-dose samples. Furthermore, for this high-dose sample, (i) the O 1s region clearly shows the evidence of the removal of O by gamma irradiation and (ii) the Cu 2p region shows no satellite, which is typical of Cu 21 valence states and a low energy shoulder that is a signature of Cu 11 valence states, in contrast to the low-dose samples. These observations suggest a reduction of hole-concentration in the Cu–O planes through the O removal at a gamma dose between 50 and 60 Mrad. 2001 Elsevier Science B.V. All rights reserved. Keywords: XPS; High-T C superconductors; Gamma irradiation; Electronic structure; (Bi,Pb)-2223
1. Introduction For successful applications of high-T C superconductors in systems such as satellites operating in outer space, where the materials are exposed to high-energy ionizing radiation such as gamma rays, it is important to study how these materials behave under such exposure. Gamma-irradiation, like ion-, electron-, and neutron-irradiation [1–6], can produce crystal defects. Irradiation is used systematically to *Corresponding author. Tel.: 1966-3-860-2284; fax: 1966-3-8602293. E-mail address: [email protected] (M. Faiz). 1 Present address: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
produce defects as flux-pinning centers in these materials to improve their critical current densities (JC ) [7–10]. A higher JC is vital for the practical applications of superconductors. However, irradiation is also known to affect the other superconducting properties, such as critical temperature (T C ), of the materials [6]. The change in T C is often thought to be due to the breaking of the Cu–O bonds, though direct evidence for this idea is still lacking. Since superconductivity is strongly related to electronic structure, we have attempted to study in this work the change in the electronic structure of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 [(Bi,Pb)-2223] due to gamma-irradiation. Gamma-irradiation studies on thick YBCO films suggested that changes in superconducting properties
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00235-8
428
M. Faiz, N.M. Hamdan / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 427 – 430
are dominated by the irradiation-induced oxygen defects [10]. Similar studies on Bi-2212 have shown an increase in Cu and Bi valencies after a 64.8-Mrad gamma dose [11]. Bandyopadhyay et al. [12] suggested that the mechanisms of radiation damage in Bi-2212 and (Bi,Pb)-2223 are different. Therefore, in this work, we have concentrated on (Bi,Pb)-2223 to investigate the effect of gamma-irradiation on its electronic structure.
2. Experimental Samples with the nominal composition of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 [(Bi,Pb)-2223] were prepared by a modified co-precipitation technique [13]. Appropriate amounts of Bi 2 O 3 , PbO, CaCO 3 , SrCO 3 , and CuO with 99.9% purity were mixed and sintered in an alumina crucible in a temperature range of 820–8608C for a total of 400 h with several intermediate grinding. The pellets as obtained were quenched to room temperature. The standard fourprobe resistivity measurements showed an onset transition temperature of 110 K and a zero resistance at 105 K. XRD analysis showed a single Bi-2223 phase with the following lattice parameters: a5b5 ˚ and c537.0460.16 A. ˚ The details of 5.40860.015 A the preparation and the characterization of the samples are reported in Ref. [13]. Gamma-irradiation of the samples was done using a 60 Co source (1.25 MeV photons) at room temperature. Several samples were irradiated with various doses in the range of 0–60 Mrad at a dose rate of 6.8 Mrad / h. XPS spectra of the samples were obtained using a VG-ESCALAB MKII photoelectron spectrometer. The base pressure in the analysis chamber was maintained at 10 210 mbar. The samples were scraped with a stainless steel blade in order to get fresh clean surfaces just before analysis. Unmonochromatized Al K a photons were generated at an electrical power of 130 W and used for excitation. The kinetic energy of the photoelectrons was analyzed by a hemispherical energy analyzer with a multi-channeltron detector at 20 eV pass energy. A Shirley type background [14] was subtracted from each spectrum. Data were smoothed using a cubic function. The binding energy of adventitious carbon 1s core level, which is 284.6
eV [15], was used to correct for the energy shift due to surface charging. This is a well-accepted practice in the literature [16].
3. Results and discussion Fig. 1 shows the XPS spectra of (Bi,Pb)-2223 as a function of the gamma dose in the O 1s region. The peaks at about 528.0 and 530.8 eV are from the O sites in the Cu–O and Bi–O / Sr–O planes of the Bi-2223 phase, respectively [17]. The shoulder at about 533.0 eV may be from surface contamination or carbonate [18]. The figure clearly shows a dramatic drop in the intensity of the peaks related to the superconducting phase and a peak shift towards
Fig. 1. XPS spectra of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 as a function of the gamma dose in the O 1s region.
M. Faiz, N.M. Hamdan / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 427 – 430
429
lower binding energy for the sample with a 60-Mrad dose. This is evidence for the oxygen removal by a gamma-irradiation dose of 60 Mrad. Fig. 2 shows the XPS spectra of (Bi,Pb)-2223 as a function of the gamma dose in the Cu 2p region. The 2p 3 / 2 peak at about 932.8 eV with a satellite centered about 941.6 eV is ascribed to Cu 21 [18–23]. The spectra are similar up to a gamma dose of 50 Mrad. As the gamma dose is increased to 60 Mrad, the satellite disappears and a low energy shoulder, which is a signature of Cu 11 [18–23], appears. This means that the average Cu valency is decreased, and hence suggests that the hole concentration is reduced in the Cu–O plane, above the 50 Mrad gamma dose. This confirms the removal of O from the Cu–O planes. On the other hand, Wang et al. [11] found that in the
Bi-2212 system, after a 64.8-Mrad gamma dose the satellite did not disappear and a higher energy shoulder, which is a signature of Cu 31 [18–23], appeared. This means that the average Cu valency is increased after a 64.8-Mrad gamma dose. This comparison hints that the mechanisms of radiation damage in Bi-2212 and (Bi,Pb)-2223 are different as suggested by Bandyopadhyay et al. [12]. Fig. 3 shows the XPS spectra of (Bi,Pb)-2223 as a function of the gamma dose in the Bi 4f region. The spectra show the spin-orbit split components 4f 7 / 2 and 4f 5 / 2 at about 158.3 and 163.6 eV, respectively. These values are close to that of the Bi 31 valence states [15]. Again, the spectra are similar up to
Fig. 2. XPS spectra of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 as a function of the gamma dose in the Cu 2p region.
Fig. 3. XPS spectra of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 as a function of the gamma dose in the Bi 4f region.
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M. Faiz, N.M. Hamdan / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 427 – 430
gamma dose of 50 Mrad. As the gamma dose is increased to 60 Mrad, two sets of spin-orbit components appear. The binding energy of 4f 7 / 2 of set-1 is at about 157.0 eV and that of set-2 is at about 160.0 eV. Wang et al. [11] has also reported this effect in Bi-2212 after a 64.8-Mrad gamma-dose, in which both sets of the spin-orbit components shifted to higher binding energy. They have attributed the higher-energy set to Bi 51 valence states. The shift of the lower-energy set to lower binding energy, compared to the low-dose samples, in our results may imply a reduction in Bi 31 in order to compensate for the formation of Bi 51 . Note that an increase in Bi-valency also implies a reduction in hole-concentration in the Cu–O planes, since the Bi–O planes act as electron sinks for the Cu–O planes in these high-T C compounds. A phase transition due to irradiation has been ruled out in both Bi-2212 and (Bi,Pb)-2223 by previous studies [11,12]. In the same way, there is no significant change in the binding energies of Ca 2p, Pb 4f, and Sr 3p core levels up to the gamma dose of 50 Mrad.
4. Conclusion In summary, we have found that a reduction of Cu valency and removal of O in (Bi,Pb)-2223 at a gamma dose of between 50 and 60 Mrad. These observations suggest that the hole-concentration in the Cu–O planes of (Bi,Pb)-2223 is reduced through O removal at a gamma dose of between 50 and 60 Mrad. This can be taken as an evidence for the breaking of the Cu–O bonds mentioned in the introduction. The comparison of our results with that of Wang et al. [11] confirms that the mechanisms of radiation damage in Bi-2212 and (Bi,Pb)-2223 are different.
Acknowledgements The authors would like to acknowledge the support provided by the Physics Department, King Fahd University of Petroleum and Minerals for this study. Special thanks are due to Mr. A. Bulut and Mr. S. Marzoug for their assistance in sample irradiation.
References [1] L. Civale, A.D. Marwick, T.K. Worthington, M.A. Kirk, J.R. Thompson, L. Krusin-Elbanm, Y. Sun, J.R. Clem, F. Holtzberg, Phys. Rev. Lett. 67 (1991) 648. [2] M.P. Siegal, R.B. Van Dover, J.M. Phillips, E.M. Gyorgy, A.E. White, J.H. Marshall, Appl. Phys. Lett. 60 (1992) 2932. [3] K. Shiaishi, H. Itoh, T. Kato, Jpn. J. Appl. Phys. 30 (1991) L894. [4] Z. Drzrzga, H. Szymczuk, R. Szymczuk, Physica C 203 (1992) 335. [5] J.R. Cost, J.O. Willis, T.D. Thompson, D.E. Peterson, Phys. Rev. B 37 (1988) 2563. [6] P. Vasek, L. Smreka, J. Dominec, M. Pesek, O. Smreckova, D. Sykorova, Solid State Commun. 69 (1989) 23. [7] J. Bohandy, J. Suter, B.F. Kim, K. Moorjani, F.J. Adrian, Appl. Phys. Lett. 51 (1987) 2161. [8] B.A. Albis, N. Hamdan, A. Menard, H. Ozkan, Solid State Commun. 88 (1993) 237. [9] T. Kurihama, M. Yamashita, K. Tsuruta, T. Izumi, Physica C 219 (1994) 57. [10] A. Leyva, M. Mora, G. Martin, A. Martinez, Superond. Sci. Technol. 8 (1995) 816. [11] L.B. Wang, W.B. Wu, J.S. Zhu, X.M. Liu, X.G. Li, G.E. Zhou, Y.H. Zhang, Physica C 229 (1994) 263. [12] S.K. Bandyopadhyay, P. Barat, P. Sen, U. De, A. De, P.K. Mukhopadhyay, S.K. Kar, C.K. Majumdar, Physica C 228 (1994) 109. [13] N.M. Hamdan, PhD Thesis, Physics Department, Middle East Technical University, Ankara, Turkey, 1993. [14] A. Shirley, Phys. Rev. B 5 (1972) 4705. [15] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of XPS, Perkin-Elmer, Eden, Prairie, MN, USA, 1992. [16] D. Briggs, M.P. Seah, 2nd Edition, Practical Surface Analysis, Vol. 1, Wiley, New York, 1990. [17] A. Krol, C.S. Lin, Z.H. Ming, C.J. Sher, Y.H. Kao, C.T. Chen, F. Sette, Y. Ma, G.C. Smith, Y.Z. Zhu, D.T. Shaw, Phys. Rev. B 42 (1990) 2635. [18] T. Gourieux, G. Krill, M. Maurer, M.F. Ravet, A. Menny, H. Tolentino, A. Fontaine, Phys. Rev. B 37 (1988) 7516. [19] T.A. Faltens, W.K. Ham, S.W. Keller, K.J. Leary, J.N. Micaels, A.M. Stacy, H.-C. zur Loe, D.E. Morris, T.W. Barbee III, L.C. Bourne, M.L. Cohen, S. Hoen, A. Zettl, Phys. Rev. Lett. 59 (1987) 915. [20] D.D. Sarma, O. Strebel, C. Simmons, U. Neukirch, G. Kaindl, R. Hoppe, H.P. Muller, Phys. Rev. B 37 (1988) 9784. [21] A. Bianconi, M. DeSantis, A. Di Ciccio, A.M. Flank, A. Fontain, P. Legarde, H. Katayama-Yoshida, A. Kotani, A. Marcelli, Phys. Rev. B 38 (1988) 7196. [22] P. Kuiper, G. Kruizinga, J. Ghijsen, M. Grioni, P.J.W. Weijs, F.M.F. de Groot, G.A. Sawatzky, H. Verweii, L.F. Feiner, H. Petersen, Phys. Rev. B 38 (1988) 6483. [23] J. Fink, N. Nucker, H. Romberg, M. Alexander, S. Nakai, B. Scheerer, P. Adelman, D. Evert, Physica C 162–164 (1989) 1415.
Effect of gamma-irradiation on the electronic structure of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 superconductor 1 M. Faiz*, N.M. Hamdan
Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 8 August 2000; received in revised form 30 August 2000; accepted 6 September 2000
Abstract An XPS investigation was carried out on (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 superconductors, in an effort to study the effect of gamma-irradiation. There is no significant change in the binding energies of Bi 4f, Ca 2p, Cu 2p, O 1s, Pb 4f, and Sr 3p core levels up to the gamma dose of 50 Mrad. However, for the gamma dose of 60 Mrad, the Bi 4f region shows two sets of spin-orbit split levels, one set at about 1.3 eV lower and the other at about 1.7 eV higher binding energies than that of the low-dose samples. Furthermore, for this high-dose sample, (i) the O 1s region clearly shows the evidence of the removal of O by gamma irradiation and (ii) the Cu 2p region shows no satellite, which is typical of Cu 21 valence states and a low energy shoulder that is a signature of Cu 11 valence states, in contrast to the low-dose samples. These observations suggest a reduction of hole-concentration in the Cu–O planes through the O removal at a gamma dose between 50 and 60 Mrad. 2001 Elsevier Science B.V. All rights reserved. Keywords: XPS; High-T C superconductors; Gamma irradiation; Electronic structure; (Bi,Pb)-2223
1. Introduction For successful applications of high-T C superconductors in systems such as satellites operating in outer space, where the materials are exposed to high-energy ionizing radiation such as gamma rays, it is important to study how these materials behave under such exposure. Gamma-irradiation, like ion-, electron-, and neutron-irradiation [1–6], can produce crystal defects. Irradiation is used systematically to *Corresponding author. Tel.: 1966-3-860-2284; fax: 1966-3-8602293. E-mail address: [email protected] (M. Faiz). 1 Present address: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
produce defects as flux-pinning centers in these materials to improve their critical current densities (JC ) [7–10]. A higher JC is vital for the practical applications of superconductors. However, irradiation is also known to affect the other superconducting properties, such as critical temperature (T C ), of the materials [6]. The change in T C is often thought to be due to the breaking of the Cu–O bonds, though direct evidence for this idea is still lacking. Since superconductivity is strongly related to electronic structure, we have attempted to study in this work the change in the electronic structure of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 [(Bi,Pb)-2223] due to gamma-irradiation. Gamma-irradiation studies on thick YBCO films suggested that changes in superconducting properties
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00235-8
428
M. Faiz, N.M. Hamdan / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 427 – 430
are dominated by the irradiation-induced oxygen defects [10]. Similar studies on Bi-2212 have shown an increase in Cu and Bi valencies after a 64.8-Mrad gamma dose [11]. Bandyopadhyay et al. [12] suggested that the mechanisms of radiation damage in Bi-2212 and (Bi,Pb)-2223 are different. Therefore, in this work, we have concentrated on (Bi,Pb)-2223 to investigate the effect of gamma-irradiation on its electronic structure.
2. Experimental Samples with the nominal composition of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 [(Bi,Pb)-2223] were prepared by a modified co-precipitation technique [13]. Appropriate amounts of Bi 2 O 3 , PbO, CaCO 3 , SrCO 3 , and CuO with 99.9% purity were mixed and sintered in an alumina crucible in a temperature range of 820–8608C for a total of 400 h with several intermediate grinding. The pellets as obtained were quenched to room temperature. The standard fourprobe resistivity measurements showed an onset transition temperature of 110 K and a zero resistance at 105 K. XRD analysis showed a single Bi-2223 phase with the following lattice parameters: a5b5 ˚ and c537.0460.16 A. ˚ The details of 5.40860.015 A the preparation and the characterization of the samples are reported in Ref. [13]. Gamma-irradiation of the samples was done using a 60 Co source (1.25 MeV photons) at room temperature. Several samples were irradiated with various doses in the range of 0–60 Mrad at a dose rate of 6.8 Mrad / h. XPS spectra of the samples were obtained using a VG-ESCALAB MKII photoelectron spectrometer. The base pressure in the analysis chamber was maintained at 10 210 mbar. The samples were scraped with a stainless steel blade in order to get fresh clean surfaces just before analysis. Unmonochromatized Al K a photons were generated at an electrical power of 130 W and used for excitation. The kinetic energy of the photoelectrons was analyzed by a hemispherical energy analyzer with a multi-channeltron detector at 20 eV pass energy. A Shirley type background [14] was subtracted from each spectrum. Data were smoothed using a cubic function. The binding energy of adventitious carbon 1s core level, which is 284.6
eV [15], was used to correct for the energy shift due to surface charging. This is a well-accepted practice in the literature [16].
3. Results and discussion Fig. 1 shows the XPS spectra of (Bi,Pb)-2223 as a function of the gamma dose in the O 1s region. The peaks at about 528.0 and 530.8 eV are from the O sites in the Cu–O and Bi–O / Sr–O planes of the Bi-2223 phase, respectively [17]. The shoulder at about 533.0 eV may be from surface contamination or carbonate [18]. The figure clearly shows a dramatic drop in the intensity of the peaks related to the superconducting phase and a peak shift towards
Fig. 1. XPS spectra of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 as a function of the gamma dose in the O 1s region.
M. Faiz, N.M. Hamdan / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 427 – 430
429
lower binding energy for the sample with a 60-Mrad dose. This is evidence for the oxygen removal by a gamma-irradiation dose of 60 Mrad. Fig. 2 shows the XPS spectra of (Bi,Pb)-2223 as a function of the gamma dose in the Cu 2p region. The 2p 3 / 2 peak at about 932.8 eV with a satellite centered about 941.6 eV is ascribed to Cu 21 [18–23]. The spectra are similar up to a gamma dose of 50 Mrad. As the gamma dose is increased to 60 Mrad, the satellite disappears and a low energy shoulder, which is a signature of Cu 11 [18–23], appears. This means that the average Cu valency is decreased, and hence suggests that the hole concentration is reduced in the Cu–O plane, above the 50 Mrad gamma dose. This confirms the removal of O from the Cu–O planes. On the other hand, Wang et al. [11] found that in the
Bi-2212 system, after a 64.8-Mrad gamma dose the satellite did not disappear and a higher energy shoulder, which is a signature of Cu 31 [18–23], appeared. This means that the average Cu valency is increased after a 64.8-Mrad gamma dose. This comparison hints that the mechanisms of radiation damage in Bi-2212 and (Bi,Pb)-2223 are different as suggested by Bandyopadhyay et al. [12]. Fig. 3 shows the XPS spectra of (Bi,Pb)-2223 as a function of the gamma dose in the Bi 4f region. The spectra show the spin-orbit split components 4f 7 / 2 and 4f 5 / 2 at about 158.3 and 163.6 eV, respectively. These values are close to that of the Bi 31 valence states [15]. Again, the spectra are similar up to
Fig. 2. XPS spectra of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 as a function of the gamma dose in the Cu 2p region.
Fig. 3. XPS spectra of (Bi 1.65 Pb 0.35 )Sr 2 Ca 2 Cu 3 O 10 as a function of the gamma dose in the Bi 4f region.
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gamma dose of 50 Mrad. As the gamma dose is increased to 60 Mrad, two sets of spin-orbit components appear. The binding energy of 4f 7 / 2 of set-1 is at about 157.0 eV and that of set-2 is at about 160.0 eV. Wang et al. [11] has also reported this effect in Bi-2212 after a 64.8-Mrad gamma-dose, in which both sets of the spin-orbit components shifted to higher binding energy. They have attributed the higher-energy set to Bi 51 valence states. The shift of the lower-energy set to lower binding energy, compared to the low-dose samples, in our results may imply a reduction in Bi 31 in order to compensate for the formation of Bi 51 . Note that an increase in Bi-valency also implies a reduction in hole-concentration in the Cu–O planes, since the Bi–O planes act as electron sinks for the Cu–O planes in these high-T C compounds. A phase transition due to irradiation has been ruled out in both Bi-2212 and (Bi,Pb)-2223 by previous studies [11,12]. In the same way, there is no significant change in the binding energies of Ca 2p, Pb 4f, and Sr 3p core levels up to the gamma dose of 50 Mrad.
4. Conclusion In summary, we have found that a reduction of Cu valency and removal of O in (Bi,Pb)-2223 at a gamma dose of between 50 and 60 Mrad. These observations suggest that the hole-concentration in the Cu–O planes of (Bi,Pb)-2223 is reduced through O removal at a gamma dose of between 50 and 60 Mrad. This can be taken as an evidence for the breaking of the Cu–O bonds mentioned in the introduction. The comparison of our results with that of Wang et al. [11] confirms that the mechanisms of radiation damage in Bi-2212 and (Bi,Pb)-2223 are different.
Acknowledgements The authors would like to acknowledge the support provided by the Physics Department, King Fahd University of Petroleum and Minerals for this study. Special thanks are due to Mr. A. Bulut and Mr. S. Marzoug for their assistance in sample irradiation.
References [1] L. Civale, A.D. Marwick, T.K. Worthington, M.A. Kirk, J.R. Thompson, L. Krusin-Elbanm, Y. Sun, J.R. Clem, F. Holtzberg, Phys. Rev. Lett. 67 (1991) 648. [2] M.P. Siegal, R.B. Van Dover, J.M. Phillips, E.M. Gyorgy, A.E. White, J.H. Marshall, Appl. Phys. Lett. 60 (1992) 2932. [3] K. Shiaishi, H. Itoh, T. Kato, Jpn. J. Appl. Phys. 30 (1991) L894. [4] Z. Drzrzga, H. Szymczuk, R. Szymczuk, Physica C 203 (1992) 335. [5] J.R. Cost, J.O. Willis, T.D. Thompson, D.E. Peterson, Phys. Rev. B 37 (1988) 2563. [6] P. Vasek, L. Smreka, J. Dominec, M. Pesek, O. Smreckova, D. Sykorova, Solid State Commun. 69 (1989) 23. [7] J. Bohandy, J. Suter, B.F. Kim, K. Moorjani, F.J. Adrian, Appl. Phys. Lett. 51 (1987) 2161. [8] B.A. Albis, N. Hamdan, A. Menard, H. Ozkan, Solid State Commun. 88 (1993) 237. [9] T. Kurihama, M. Yamashita, K. Tsuruta, T. Izumi, Physica C 219 (1994) 57. [10] A. Leyva, M. Mora, G. Martin, A. Martinez, Superond. Sci. Technol. 8 (1995) 816. [11] L.B. Wang, W.B. Wu, J.S. Zhu, X.M. Liu, X.G. Li, G.E. Zhou, Y.H. Zhang, Physica C 229 (1994) 263. [12] S.K. Bandyopadhyay, P. Barat, P. Sen, U. De, A. De, P.K. Mukhopadhyay, S.K. Kar, C.K. Majumdar, Physica C 228 (1994) 109. [13] N.M. Hamdan, PhD Thesis, Physics Department, Middle East Technical University, Ankara, Turkey, 1993. [14] A. Shirley, Phys. Rev. B 5 (1972) 4705. [15] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of XPS, Perkin-Elmer, Eden, Prairie, MN, USA, 1992. [16] D. Briggs, M.P. Seah, 2nd Edition, Practical Surface Analysis, Vol. 1, Wiley, New York, 1990. [17] A. Krol, C.S. Lin, Z.H. Ming, C.J. Sher, Y.H. Kao, C.T. Chen, F. Sette, Y. Ma, G.C. Smith, Y.Z. Zhu, D.T. Shaw, Phys. Rev. B 42 (1990) 2635. [18] T. Gourieux, G. Krill, M. Maurer, M.F. Ravet, A. Menny, H. Tolentino, A. Fontaine, Phys. Rev. B 37 (1988) 7516. [19] T.A. Faltens, W.K. Ham, S.W. Keller, K.J. Leary, J.N. Micaels, A.M. Stacy, H.-C. zur Loe, D.E. Morris, T.W. Barbee III, L.C. Bourne, M.L. Cohen, S. Hoen, A. Zettl, Phys. Rev. Lett. 59 (1987) 915. [20] D.D. Sarma, O. Strebel, C. Simmons, U. Neukirch, G. Kaindl, R. Hoppe, H.P. Muller, Phys. Rev. B 37 (1988) 9784. [21] A. Bianconi, M. DeSantis, A. Di Ciccio, A.M. Flank, A. Fontain, P. Legarde, H. Katayama-Yoshida, A. Kotani, A. Marcelli, Phys. Rev. B 38 (1988) 7196. [22] P. Kuiper, G. Kruizinga, J. Ghijsen, M. Grioni, P.J.W. Weijs, F.M.F. de Groot, G.A. Sawatzky, H. Verweii, L.F. Feiner, H. Petersen, Phys. Rev. B 38 (1988) 6483. [23] J. Fink, N. Nucker, H. Romberg, M. Alexander, S. Nakai, B. Scheerer, P. Adelman, D. Evert, Physica C 162–164 (1989) 1415.