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STM on Superconductors

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Scanning Tunneling Microscopy I

Part of the book series: Springer Series in Surface Sciences ((SSSUR,volume 20))

Abstract

Soon after the tunneling technique was introduced by Giaever in 1960 [8.1, 2], it became clear that this would become the method of choice to study the quasiparticle density of states in superconductors. Not only could the energy gap be measured with great accuracy, but detailed predictions from the microscopic Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity [8.3], such as the temperature dependence of the gap and depairing effects in magnetic fields, could also be verified. Tunneling studies on materials with a strong electron-phonon coupling, such as Pb, revealed direct information about the Eliashberg function α2(ω)F(ω), and subsequent modifications of the BCS theory could explain the enhanced energy gap in these strong coupling superconductors. With the prediction and observation of the possibility of pair tunneling [8.4] between two superconductors, a new field of low-temperature physics and practical applications was opened. Since then the field of superconductive tunneling spectroscopy has evolved to include many features, such as proximity effects, non-equilibrium states induced by large injection currents, inelastic tunneling spectroscopy etc. Because of the very strong non-linearity of the tunneling I-V characteristics, associated with the singularity in the density of states just above the gap, superconductive tunnel junctions have been used as microwave radiation detectors or mixers working with nearly 100% quantum efficiency. An overview of the various theoretical aspects and experimental applications of planar tunnel junctions can be found in [8.5–9].

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References

  1. I. Giaever: Phys. Rev. Lett. 5, 147 (1960)

    ADS  Google Scholar 

  2. I. Giaever: Phys. Rev. Lett. 5, 464 (1960)

    ADS  Google Scholar 

  3. J. Bardeen, L.N. Cooper, J.R. Schrieffer: Phys. Rev. 108, 1175 (1957)

    MathSciNet  ADS  MATH  Google Scholar 

  4. B.D. Josephson: Phys. Lett. 1, 251 (1962)

    ADS  MATH  Google Scholar 

  5. L. Solymar: Superconductive Tunneling and Applications (Chapman Hall, London 1972)

    Google Scholar 

  6. W.L. McMillan, J.M. Rowell: Tunneling and strong-coupling superconductivity, in Super conductivity, ed. by R.D. Parks (Dekker, New York 1969) p. 561

    Google Scholar 

  7. C.B. Duke: Tunneling in Solids, Solid State Physics, Vol. 10 (Academic, New York 1969)

    Google Scholar 

  8. E. Burstein, S. Lundqvist (eds.): Tunneling Phenomena in Solids (Plenum, New York 1969)

    Google Scholar 

  9. E.L. Wolf: Principles of Electron Tunneling Spectroscopy, International series of monographs on physics 71 (Oxford University Press, New York 1985)

    Google Scholar 

  10. G. Bednorz, K.A. Müller: Z. Phys. B 64, 189 (1986)

    ADS  Google Scholar 

  11. M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, C.W. Chu: Phys. Rev. Lett. 58, 908 (1987)

    ADS  Google Scholar 

  12. H. Maeda, Y. Tanaka, M. Fukutomi, T. Asano: Jpn. J. Appi. Phys. 27 L209 (1988)

    ADS  Google Scholar 

  13. J. Bardeen: Phys. Rev. Lett. 6, 57 (1961)

    ADS  Google Scholar 

  14. M.H. Cohen, L.M. Falicov, J.C. Phillips: Phys. Rev. Lett. 8, 316 (1962).

    ADS  MATH  Google Scholar 

  15. N.N. Bogoliubov: Nuovo Cimento 7, 794 (1958)

    Google Scholar 

  16. J.G. Valatin: Nuovo Cimento 7, 843 (1958)

    MathSciNet  Google Scholar 

  17. P.W. Anderson: In Lectures on the Many-Body Problem, ed. by E.R. Caianiello (Academic, New York 1964) p. 113

    Google Scholar 

  18. N. Garcia, F. Flores, F. Guinea: J. Vac. Sci. Technol. A 6, 323 (1988)

    ADS  Google Scholar 

  19. J.R. Schrieffer, J.W. Wilkins: Phys. Rev. Lett. 10, 17 (1963)

    ADS  MATH  Google Scholar 

  20. G.F. Blonder, M. Tinkham, T.M. Klapwijk: Phys. Rev. B 25, 4515 (1982)

    ADS  Google Scholar 

  21. A.F. Andreev: Zh. Eksp. Teor. Fiz. 46, 1823 (1964) [Sov. Phys. JETP 19, 1228 (1964)]

    Google Scholar 

  22. J.R. Kirtley: Tunneling measurements of the energy gap in high-Tc superconductors, to appear in Int. J. Mod. Phys. B

    Google Scholar 

  23. J.G. Simmons: J. Appi. Phys. 34, 1793 (1963)

    ADS  Google Scholar 

  24. H.R. Zeller, I. Giaever: Phys. Rev. 181, 789 (1969)

    ADS  Google Scholar 

  25. E. Ben-Jacob, Y. Gefen: Phys. Lett. 108A, 289 (1985)

    ADS  Google Scholar 

  26. D.V. Averin, K.K. Likharev: J. Low Temp. Phys. 62, 345 (1986)

    ADS  Google Scholar 

  27. K. Mullen, E. Ben-Jacob, R.C. Jacklevic, Z. Schuss: Phys. Rev. B 37, 98 (1988)

    ADS  Google Scholar 

  28. P.J.M. van Bentum, H. van Kempen, L.E.C van de Leemput, P.A.A. Teunissen: Phys. Rev. Lett. 60, 369 (1988)

    ADS  Google Scholar 

  29. P.J.M. van Bentum, R.T.M. Smokers, H. van Kempen: Phys. Rev. Lett. 60, 2543 (1988)

    ADS  Google Scholar 

  30. S.A. Elrod, A.L. de Lozanne, C.F. Quate: Appi. Phys. Lett. 45, 1240 (1984)

    ADS  Google Scholar 

  31. S.A. Elrod, A. Bryant, A.L. de Lozanne, S. Park, D. Smith, C.F. Quate: IBM J. Res. Dev. 30, 387 (1986)

    Google Scholar 

  32. J.R. Kirtley, S.I. Raider, R.M. Feenstra, A.P. Fein: Appi. Phys. Lett. 50, 1607 (1987)

    ADS  Google Scholar 

  33. M.A. Ramos, S. Vieira, A. Buendia, A.M. Baro: J. of Microsc. 152, 137 (1988)

    Google Scholar 

  34. R. Wilkins, M. Amman, R.E. Soltis, E. Ben-Jacob, R.C. Jaklevic: Phys. Rev. B 41, 8904 (1990)

    ADS  Google Scholar 

  35. R.T.M. Smokers, P.J.M. van Bentum, H. van Kempen: Physica B 165–166, 63 (1990)

    ADS  Google Scholar 

  36. H. Traeuble, U. Essman: J. Appi. Phys. 25, 273 (1969)

    Google Scholar 

  37. H.F. Hess, R.B. Robinson, J.V. Waszczak: Phys. Rev. Lett. 64, 2711 (1990)

    ADS  Google Scholar 

  38. H.F. Hess, R.B. Robinson, R.C. Dynes, J.M. Valles Jr., J.V. Waszczak: J. Vac. Sci. Technol. A8, 450 (1990)

    ADS  Google Scholar 

  39. H.F. Hess, R.B. Robinson, R.C. Dynes, J.M. Valles Jr., J.V. Waszczak: Phys. Rev. Lett. 62, 214 (1989)

    ADS  Google Scholar 

  40. A.W. Overhauser, L.L. Daemen: Phys. Rev. Lett. 62, 1691 (1989)

    ADS  Google Scholar 

  41. A.W. Overhauser, L.L. Daemen: Phys. Rev. B40, 10778 (1989)

    Google Scholar 

  42. J.D. Shore, M. Huang, A.T. Dorsey, J.P. Sethna: Phys. Rev. Lett. 62, 3089 (1989)

    ADS  Google Scholar 

  43. F. Gygi, M. Schlüter: Phys. Rev. Lett. 65, 1820 (1990)

    ADS  Google Scholar 

  44. F. Gygi, M. Schlüter: Phys. Rev. B41, 822 (1990)

    ADS  Google Scholar 

  45. R. Dittrich, C. Heiden: J. Vac. Sci. Technol. A6, 263 (1988)

    ADS  Google Scholar 

  46. R. Berthe, U. Hartmann, C. Heiden: preprint

    Google Scholar 

  47. M. Yoshimurka, K. Fujita, N. Ara, M. Kageshima, R. Shioda, A. Kawazu, H. Shigekawa, S. Hyodo: J. Vac. Sci. Technol. A8, 488 (1989)

    ADS  Google Scholar 

  48. C. Dai, C. Dai, C. Zhu, Z. Chen, G. Huang, X. Wu, D. Zhu, J.D. Baldeschwieler: J. Vac. Sci. Technol. A8, 484 (1989)

    Google Scholar 

  49. H. Bando, S. Kashiwaya, H. Tokumoto, H. Anzai, N. Kinoshita, K. Kajimura: J. Vac. Sci. Technol. A8, 479 (1990)

    ADS  Google Scholar 

  50. R. Fainchstein, J.C. Murphy: J. Vac. Sci. Technol., to be published

    Google Scholar 

  51. S. Vieira, M.A. Ramos, A. Buendia, A.M. Baro: Physica C 153–155, 1004 (1988)

    Google Scholar 

  52. D. Anselmetti, H. Heinzelmann, R. Wiesendanger, H. Jenny, H.-J. Güntherodt, M. Düggehn, R. Guggenheim: Physica B-C 153–155, 1000 (1988)

    Google Scholar 

  53. J.K. Grepstad, Ph. Niedermann, J.-M. Trescone, L. Antognazza, M.G. Karkut, O. Fisher: Physica C 153–155, 1453 (1988)

    Google Scholar 

  54. A.M. Okoniewski, J.E. Klemberg-Sapieha, A. Yelon: Appi. Phys. Lett. 53, 152 (1988)

    ADS  Google Scholar 

  55. M.D. Kirk, C.B. Eom, B. Oh, S.R. Spielman, M.R. Beasley, A. Kapitulnik, T.H. Geballe, C.F. Quate: Appi. Phys. Lett. 52, 2072 (1988)

    ADS  Google Scholar 

  56. S.L. Pryadkin, V.S. Tsoi: Pis’ma Zh. Eksp. Teor. Fiz. 49, 268 (1989) [JETP. Lett. 49, 305 (1989)]

    ADS  Google Scholar 

  57. LB. Al’tfeder, A.P. Volodin, I.N. Makarenko, S.M. Stivhov, A.V. Shubnikov: Pis’ma Zh. Eksp. Theor. Fiz. 50, 458 (1989) [JETP Lett. 50, 490 (1989)]

    ADS  Google Scholar 

  58. X.L. Wu, Z. Zhang, Y.L. Wang, C.M. Lieber: Science 248, 1211 (1990)

    ADS  Google Scholar 

  59. Z. Zhang, C.M. Lieber: to be pubhshed in J. Vac. Sci. Technol.

    Google Scholar 

  60. N.J. Zheng, U. Knipping, I.S.T. Tsong, W.T. Petuskey, J.C. Barry: J. Vac. Sci. Technol. A6, 457 (1988)

    ADS  Google Scholar 

  61. Ph. Niedermann, O. Fisher: J. Microsc. 152, 93 (1988)

    Google Scholar 

  62. M.C. Gallagher, J.G. Adler: Physica C 162–164, 1129 (1989)

    Google Scholar 

  63. M.D. Kirk, J. Nogami, A.A. Baski, D.B. Mitzi, A. Kapitulnik, T.H. Geballe, C.F. Quate: Science 242, 1673 (1988)

    ADS  Google Scholar 

  64. L.E.C. van de Leemput, P.J.M. van Bentum, F.AJ.M. Driessen, J.W. Gerritsen, H. van Kempen, L.W.M. Schreurs, P. Bennema: J. Microsc. 152, 103 (1988)

    Google Scholar 

  65. H. Heinzelmann, D. Anselmetti, R. Wiesendanger, H.-J. Güntherodt, E. Kaldis, A. Wisard: Appl. Phys. Lett. 53, 2447 (1988)

    ADS  Google Scholar 

  66. T. Nakamura, T. Kimura, T. Kimura, M. Ihara, L Umebu: J. Vac. Sci. Technol. A8, 472 (1990)

    ADS  Google Scholar 

  67. L.E.C. van de Leemput, P.J.M. van Bentum, F.A.J.M. Driessen, J.W. Gerritsen, H. van Kempen, L.W.M. Schreurs, P. Bennema: J. Crystal Growth, 98, 551 (1989)

    Google Scholar 

  68. P. Hartman: in An introduction to crystal growth. P. Hartman, ed. (North Holland, Amsterdam 1973) p. 367

    Google Scholar 

  69. P. Niedermann, H.J. Scheel, W. Sadowski: J. Appl. Phys. 65, 3274 (1989)

    ADS  Google Scholar 

  70. H.J. Scheel, P. Niedermann: J. Crystal Growth 94, 281 (1989)

    ADS  Google Scholar 

  71. A.L. de Lozanne: Proc. Tsukuba Seminar on High T Superconductivity, Tsukuba, Japan 1989, p. 161

    Google Scholar 

  72. P. Muralt, D.W. Pohl: Appl. Phys. Lett. 48, 514 (1986)

    ADS  Google Scholar 

  73. J.R. Kirtley, S. Washburn, M.J. Brady: Phys. Rev. Lett. 60, 1546 (1988)

    ADS  Google Scholar 

  74. A.D. Kent, L Maggio-Aprile, Ph. Niedermann, Ch. Renner, J.-M. Triscone, M.G. Karkut, L. Amtognazza, O. Fisher: Physica C 162–164, 1035 (1989)

    Google Scholar 

  75. A.D. Kent, L Maggio-Aprile, Ph. Niedermann, Ch. Renner, O. Fisher: J. Vac. Sci. Technol. A 8, 459 (1990)

    Google Scholar 

  76. A.D. Kent, L Maggio-Aprile, Ph. Niedermann, O. Fisher: Phys. Rev. B 39, 12363 (1989)

    ADS  Google Scholar 

  77. X.L. Wu, C.M. Lieber, D.S. Ginley, R.J. Baughman: Appl. Phys. Lett. 55, 2129 (1989)

    ADS  Google Scholar 

  78. D.G. Anselmetti: Thesis, University of Basel (1990)

    Google Scholar 

  79. L.L. Soethout: Thesis University of Nijmegen (1990)

    Google Scholar 

  80. LB. Al’tfeder, A.P. Volodin, V.A. Grazhulis, A.M. lonov, S.G. Karabashev: Pis’ma Zh. Eksp. Theor. Fiz. 50, 182 (1989) [JETP Lett. 50, 204 (1989)]

    ADS  Google Scholar 

  81. C.K. Shih, R.M. Feenstra, J.R. Kirtley, G.V. Chandrashekhar: Phys. Rev. B 40, 2682 (1989)

    ADS  Google Scholar 

  82. see for example N. Nücker, H. Romberg, X.X. Xi, J. Fink, B. Gegenhei, Z.X. Zhao: Phys. Rev. B 39, 6619 (1989)

    ADS  Google Scholar 

  83. see for example T. Takahashi, H. Katayama, Y. Okabe, S. Hosoya, K. Seki, H. Fujimoto, M. Sato, H. Inokuchi: Nature 334, 691 (1988)

    ADS  Google Scholar 

  84. M. Tanaka, T. Takahashi, H. Katayama-Yoshida, S. Yamazaki, M. Fujinami, Y. Okabe, W. Mizutani, M. Ono, K. Kajimura: Nature 339, 691 (1989)

    ADS  Google Scholar 

  85. M. Tanaka, S. Yamalzaki, M. Fujinami, T. Takahashi, H. Katayama-Yoshida, W. Mizutani, K. Kajimura, M. Ono: J. Vac. Sci. Technol. A8, 475 (1990)

    ADS  Google Scholar 

  86. T. Hasegawa, M. Nantoh, H. Suzuki, N. Motohira, K. Kishio, K. Kitazawa: Physica B165/166, 1563 (1990)

    Google Scholar 

  87. Z. Zhang, Y.L. Wang, X.L. Wu, J.-L. Huang, C.M. Lieber: Phys. Rev. B42, 1082 (1990)

    ADS  Google Scholar 

  88. C.J. Chen, C.C. Tsuei: Solid State Commun. 71, 33 (1989)

    ADS  Google Scholar 

  89. H.R. Flensberg, P. Hedegard, M. Brix: Phys. Rev. B38, 841 (1988)

    ADS  Google Scholar 

  90. P.W. Anderson, Z. Zou: Phys. Rev. Lett. 60, 132 (1988)

    ADS  Google Scholar 

  91. see for example J.F. Zasadsinski, N. Tralshawala, D.G. Hinks, B. Dabrowski, A. Mitchell, D.R. Richards: Physica C158, 519 (1989)

    ADS  Google Scholar 

  92. J.R. Kirtley, C.C. Tsuie, S.L Park, C.C. Chi, J. Rozen, M.W. Shafer: Phys. Rev. B35, 7216

    Google Scholar 

  93. P.J.M. van Bentum, H.F.C. Hoevers, H. van Kempen, L.E.C. van de Leemput, M.J.M.F. de Nivelle, L.W.M. Schreurs, R.T.M. Smokers, P.A.A. Teunissen: Physica C 153–155, 1718 (1986)

    Google Scholar 

  94. J.R. Kirtley, C.C. Tsuei, S.L Park, C.C. Chi, J. Rozen, M.W. Schäfer: Phys. Rev. 35, 7216 (1987)

    ADS  Google Scholar 

  95. J.R. Kirtley, D.J. Scalapino: To be published

    Google Scholar 

  96. N. Bulut, D. Hone, D.J. Scalapino, N.E. Bickers: Phys. Rev. B41, 1797 (1990)

    ADS  Google Scholar 

  97. A. Millis, H. Monier, D. Pines: To be published

    Google Scholar 

  98. S.L. Cooper, M.V. Klein, B.G. Pazol, T.P. Rice, D.M. Ginzburg: Phys. Rev. B37, 5920 (1988)

    ADS  Google Scholar 

  99. P.J.M. van Bentum, G. Martinez, P. Lejay: Physica C 162–164, 1237 (1989)

    Google Scholar 

  100. C.M. Varma, P.B. Littlewood, S. Schmitt-Rink, E. Abrahams, A.E. Ruckenstein: Phys. Rev. Lett. 63, 1996 (1989)

    ADS  Google Scholar 

  101. P.J.M. van Bentum, L.E.C van de Leemput, L.W.M. Schreurs, P.A.A. Teunissen, H. van Kempen: Phys. Rev. B36, 843 (1987)

    ADS  Google Scholar 

  102. J.-C. Wan, A.M. Goldman, J. Maps: Physica C 153–155, 1377 (1988)

    Google Scholar 

  103. R. Wilkins, M. Amman, R.E. Soltis, E. Ben-Jacob, R.C. Jaklevic: Phys. Rev. B41, 8904 (1990)

    ADS  Google Scholar 

  104. M.A. Ramos, S. Vieira: Physica C 162–164, 1045 (1989)

    ADS  Google Scholar 

  105. S. Vieira, M.A. Ramos, M. Vallet-Regi, J.M. Gonzalez-Calbet: Phys. Rev. B38, 9295 (1988)

    ADS  Google Scholar 

  106. J.R. Kirtley, R.M. Feenstra, A.P. Fein, S.L Raider, W.J. Gallagher, R. Sandstrom, T. Dinger, M.W. Shafer, R. Koch, R. Laibowitz, B. Bumble: J. Vac. Sci. Technol. A6, 259 (1988)

    ADS  Google Scholar 

  107. J. Geerk, X.X. Xi, G. Linker: Z. Phys. B-Cond. Mat. 73, 329 (1988)

    ADS  Google Scholar 

  108. J. Geerk, G. Linker, G. Meyer, Q. Li, R.-L. Wang, X.X. Xi: Physica C 162–164, 837 (1989)

    Google Scholar 

  109. M. Gurvitch, J.J. Valles Jr., A.M. Cucolo, R.C. Dynes, J.G. Garno, L.F. Schneemeyer, J.V. Wasczak: Phys. Rev. Lett. 63, 1008 (1989)

    ADS  Google Scholar 

  110. S. Vieira, J.G. Rodrigo, M.A. Ramos, N. Agrait, K.V. Rao, Y. Makino, J.L. Costa: J. Appi. Phys. 67, 5026 (1990)

    ADS  Google Scholar 

  111. S. Vieira, J.G. Rodrigo, M.A. Ramos, K.V. Rao, Y. Makino: Phys. Rev. B40, 11403 (1989)

    ADS  Google Scholar 

  112. M.C. Gallagher, J.G. Adler: J. Microsc. 152, 123 (1988)

    Google Scholar 

  113. M.C. Gallagher, J.G. Adler, J. Jung, J.P. Franck: Phys. Rev. B37, 7846 (1988)

    ADS  Google Scholar 

  114. M. Tachiki, S. Takahashi, F. Steglich, H. Adrian: Z. Phys. — Condensed Matter 80, 161 (1990)

    ADS  Google Scholar 

  115. J.C. Phillips: Phys. Rev. B41, 8968 (1990)

    ADS  Google Scholar 

  116. R.C. Dynes, J.P. Garno, G.B. Hertel, T.P. Orlando: Phys. Rev. Lett. 53, 2437–1984).

    Google Scholar 

  117. A.P. Fein, J.R. Kirtley, M.W. Shafer: Phys. Rev. B37, 9738 (1988)

    ADS  Google Scholar 

  118. A.P. Volodin, M.S. Khaikin: Sov. Phys. JETP Lett. 46, 508 (1989)

    Google Scholar 

  119. J.S. Tsai, I. Takeuchi, J. Fujitu, T. Yoshitake, S. Miura, S. Tanaka, T. Terashima, Y. Bando, K. Lijima, K. Yamamoto: Physica C 153–155, 1385 (1988)

    Google Scholar 

  120. I. Igushi, H. Watanabe, Y. Kasai, T. Mochiku, A. Sugishita, E. Yamak: Physica B148, 322 (1987)

    Google Scholar 

  121. Mark Lee, D.B. Mitzi, A. Kapitulnik, M.R. Beasley: Phys. Rev. B39, 801 (1989)

    ADS  Google Scholar 

  122. A. Edgar, C.J. Adkins, S.J. Chandler: J. Phys. C20, L1009 (1987)

    ADS  Google Scholar 

  123. B.A. Aminov, L.I. Leoniuk, K. Sethupathi, M.V. Sudakova, Ya.G. Ponomarev, M.V. Pedyash, D.K. Petrov, H. Rahimov, L. Rosta: To be published

    Google Scholar 

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van Bentum, P.J.M., van Kempen, H. (1992). STM on Superconductors. In: Güntherodt, HJ., Wiesendanger, R. (eds) Scanning Tunneling Microscopy I. Springer Series in Surface Sciences, vol 20. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-97343-7_8

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