摘要

二元硼化物自從超導體 MgB2因具備高達39K 的超導臨界溫度(Tc)被發現後而引起熱烈討論。掺雜是一種利用改變系統電子結構及費米能面態密度對來討論對超導溫度影響的常用方法。在我們的討論中，主要針對兩個MgB2 的相關系統 (1) Fe掺雜對於MgB2的影響 (2)Nb空缺對NbB2的影響。我們利用核磁共振儀對Fe掺雜於Mg1-xFexB2(x=0,0.003,0.006,0.012,0.03)做一系列的討論，奈特位移(K)、線形及自旋晶格鬆弛時間(T1)的量測。從11B NMR中央及核四極譜線寬度隨Fe濃度增加而變寬而推論Fe掺雜造成無序的影響。然而不隨溫度變化的中央譜線寬度證實Fe離子於MgB2並非磁性雜質。 除此之外，不隨Fe濃度改變的T1T值指出了B-2p的費米面態密度不受Fe取代Mg而影響。所有的實驗結果說明Tc的降低並非來自於電子費米面態密度的減少。根據這些討論，我們認為Tc的變化主要還是與電聲偶合隨掺雜影響較有關係。為了討論 Nb空穴如何影響NbB2系統的超導臨界溫度(Tc)，我們實現93Nb1-x(x=0,0.13,0.2,and0.26)及Seebeck係數值(S) 的量測。從93Nb自旋晶格鬆弛時間實驗結果推算出的費米面Nb-4d的態密度指出Nb0.74B2和Nb0.8B2有較大的態密度而Nb0.87B2的態密度為最小。以上的觀測與利用Nb空缺存在於Nb1-xB2所計算的能帶結構有一致性。此外，這些研究更說明Tc與Nb空缺所造成費米面附近的電子態密度變化無直接相關性。

Abstract

Diboride systems have aroused many investigations and offers after the superconductor MgB2 which was found to transform at the temperature about 39K. Substitution, is a normal method in solid state, using change of electronic structure and Fermi level discusses effects on the critical temperature of a system. In our study, there are two topics about MgB2 that we will presented: (1) Substitution of Fe for Mg in MgB2 (2) Nb deficiency in NbB2. We have conducted a systematic study on Fe-doped MgB2 alloys using 11B nuclear magnetic resonance spectroscopy. The Knight Shift, line shape, as well as spin-lattice relaxation time (T1) on individual composition have been identified. The central transition and quadrupole linewidths increase with Fe concentration, attributed to the disorder on the measurements. The temperature of the central transition linewidth further confirms that there is no magnetic moment associated with these dopes. In addition, the values of T1T remains unchanged with Fe content indicative of little or no effect on the B-2p Fermi level density of states (DOS) by substituting Fe in the Mg sublattice of MgB2. These observations clearly show that the drop of Tc cannot account for either by the electronic DOS reduction, which thus, revealed the importance of the phonon contributions to the change of Tc, presumably due to the decreased phonon coupling strength through Fe doping.
With the aim of providing experimental results for the Nb deficiency enhanced by superconducting temperature (Tc) in the Nb1-xB2 samples, a study on Nb1-xB2 (x=0, 0.13, 0.20, 0.26) by means of nuclear magnetic resonance (NMR) and Seebeck coefficient (S) measurements are carried out. From 93Nb NMR spin-lattice relaxation rates, we can deduce Nb 4d partial Fermi level density of states Nd(EF) for each individual composition. The results indicate that Nb0.74B2 and Nb0.8B2 possess large Nd(EF) while the lowest one appears in Nb0.87B2. Seebeck coefficient also shows smaller absolute values in Nb0.74B2 and Nb0.8B2, associated with higher Fermi level DOS in both compounds. These observations were found to be in good agreement with the prediction from band structure calculations based on the appearance of Nb vacancies in Nb1-xB2. In addition, the present study clearly revels that the observed Tc enhancement by Nb deficiency has no direct relevance to their electronic Fermi level DOS.

1.N. Bloembergen., and T. Rowland, J Acta Met. 1 931 (1953).
2.C. H. Townes., C. Herring, and W. D. Knight., Phys. Rev. 77 852 (1950).
3.R. Kubo, and Obata, Y., J. Phys. Soc. Jpn. 11 547 (1956).
4.L. E. Orgel, J. Phys. Chem. Solids 21 123 (1961).
5.A. M. Clogston, V. Jaccarino, and Y. Yafet, Phys. Rev A650 134 (1964).
6.V. Jaccarino V., Proc. Col. Ampere 13 22 (1964).
7.College de France The principle of nuclear magnetism (Clarendon press Oxford 1961)
8.J. Korringa, Physica 16 601 (1950).
9.Y. Yafet and V. Jaccarino, Phys. Rev. 133, A 1630 (1964)
10.Y. Obata, J. Phys. Soc. Jpn. 18 1020 (1963).
11.J. Nagamatsu, N. Nakagawa, T. Muranaka et al, Nature 63 410 (2001).
12.F. Bouquent et al., Phys. Rev. Lett. 87 47001 (2001)
13.Y. Wang, et al., Physica C 355 179 (2001)
14.H. Kotegawa et al., Physica C 378-381 25 (2002)
15.G. Rubio-Bollinger, H. Suderrow, and S. Vieira, Phys. Rev. Lett. 86
5582 (2001).
16.G. Karapetrov et al., Phys. Rev. Lett 86 4373 (2001).
17.H. Schmidt et al., Phys. Rev. B 63 220504 (2001).
18.J. Kortus et al., Phys. Rev. Lett. 86 4656 (2001).
19.P. Canfield et al., Phys. Rev. Lett. 86 2423 (2001).
20.Y. Kong et al., Phys. Rev. B 64 020501 (2001)
21.K.-P. Bohnen, R.Heid, and B. Ranker, Phys. Rev. Lett. 86 5771(2001).
22.A. Y. liu et al., phys. Rev. Lett. 87 087005 (2001).
23.J. Karpinski, N. D. Zhigadlo, G. Schuck, S. M. Kazakov, B. Batlogg, K. Rogacki, R. Puzniak, J. Jun, E. Muller, P. Wagli, R. Gonnelli, D. Daghero, G. A. Ummarino, and V. A. Stepanov, Phys. Rev. B 71 174506 (2005), and references therein.
24. J. R. Cava, H. W. Zandbergen, and K. Inumaru, Physica C 385, 8(2003), and references therein
25. G. Papavassilion, M. Missas et al., Phys. Rev. B 66 140514 (2002).
26. S. Agrestini, C. Metallo et al., Phys. Rev. B 70 1345141 (2004).
27. K. Rogacki, B. Batlogg, J. Karpinski, N. D. Zhigadlo, G. Schuck,S. M. Kazakov, P. Wagli, R. Puzniak, A.Wisniewski, F. Carbone,
A. Brinkman, and D. van der Marel, cond-mat/0510227(unpublished).
28. S. Xu, Y. Moritomo, K. Kato, and A. Nakimura, J. Phys. Soc. Jpn.70 1889 (2001).
29. P. P. Singh and P. Jili Thomas Joseph, J. Phys.: Condens. Matter14 12441 (2002).
30. C. S. Lue, B. X. Xie, S. N. Horng, and J. H. Su, Phys. Rev. B 71195104 (2005).
31. G. Papavassiliou, M. Pissas, M. Karayanni, M. Fardis, S. Koutandos,and K. Prassides, Phys. Rev. B 66 140514(R) (2002).
32. S. Serventi, G. Allodi, C. Bucci, R. De Renzi, G. Guidi, E. Pavarini,P. Manfrinetti, and A. Palenzona, Phys. Rev. B 67 134518 (2003).
33. S. H. Baek, B. J. Suh, E. Pavarini, F. Borsa, R. G. Barnes, S. L.Bud’ko, and P. C. Canfield, Phys. Rev. B 66 104510 (2002).
34.E. Pavarini and I. I. Mazin, Phys. Rev. B 64 140504(R)(2001).35. C. S. Lue, Joseph Ross Jr., K. D. D. Rathnayaka, D. G. Naugle, S.Y. Wu, and W.-H. Li, J. Phys.: Condens. Matter 13 1585 (2001).
36. S. K. Chen and J. L. MacManus-Driscoll (private communication).
37. W. W. Simmons, W. J. O’Sullivan, and W. A. Robinson, Phys.Rev. 127 1168 (1962).
38. C. S. Lue, J. Y. Lin, and B. X. Xie, Phys. Rev. B 73 035125(2006).
39. J. Kortus, O. V. Dolgov, R. K. Kremer, and A. A. Golubov, Phys.Rev. Lett. 94 027002 (2005).
40. L. D. Cooley, A. J. Zambano, A. R. Moodenbaugh, R. F. Klie,Jin-Cheng Zheng, and Yimei Zhu, Phys. Rev. Lett. 95 267002 (2005).
41. W. L. McMillan, Phys. Rev. 167 331 (1968).
42. P. B. Allen and R. C. Dynes, Phys. Rev. B 12 905 (1975).
43. J. M. An and W. E. Pickett, Phys. Rev. Lett. 86 4366 (2001).
44. J. Kortus, I. I. Mazin, K. D. Belashchenko, V. P. Antropov, and L.L. Boyer, Phys. Rev. Lett. 86 4656 (2001).
45. S. Serventi, G. Allodi, C. Bucci, R. De Renzi, G. Guidi, E. Pavarini,P. Manfrinetti, and A. Palenzona, Phys. Rev. B 67 134518 (2003).
46. K. A. Yates, G. Burnell, N. A. Stelmashenko, D.-J. Kang, H. N.Lee, B. Oh, and M. G. Blamire, Phys. Rev. B 68 220512(R) (2003).
47. P. Postorino, A. Congeduti, P. Dore, A. Nucara, A. Bianconi, D.DiCastro, S. De Negri, and A. Saccone, Phys. Rev. B 65020507(R) (2001).
48. H. Rosner, W.E. Pickett, S.-L. Drechsler, A. Handstein, G. Behr, G. Fuchs, K. Nenkov, K.-H. Muller, and H. Eschrig, Phys. Rev. B 64 144516 (2001).
49. V. A. gasparov, N. S. Sidorov, I. I. Zver'kova, and M. P. Kulakov, JEPT Lett. 73 532 (2001).
50. H. Rosner, J. M. An, W. E. Pickett, and S. -L. Drechsler, Phys. Rev. B 66 024521 (2002).
51. Jin Nakamura, Shin-ya Nasubida, Eiki Kabasawa, Hisashi Yamazaki, Nobuyoshi Yamada, Kazuhiko Kuroki, Masamitsu Watanabe, Tamio Oguchi, Sergey Lee, Ayako Yamamoto, Set- suko Tajima, Yuji Umeda, Shin Minakawa, Noriaki Kimura, Haruyoshi Aoki, Shigeki Otani,
Shik Shin, Thomas A. Callcott, David L. Ederer, Jonathan D. Denlinger, and Rupert C. C. Perera, Phys. Rev. B 68, 064515 (2003).
52. C. Jariwala, A. Chainani, S. Tsuda, T. Yokoya, S. Shin, Y. Takano, K. Togano, S. Otani, and H. Kito, Phys. Rev. B 68 174506 (2003).
53. Prabhakar P. Singh, Phys. Rev. B 69 094519 (2004).
54. C. S. Lue and W. J. Lai, Phys. Status Solidi (b) 242 1108 (2005).
55. J. K. Hulm and B. T. Matthias, Phys. Rev. 82 273 (1951).
56. W. T. Ziegler and R. A. Young, Phys. Rev. 90 115 (1953).
57. A. S. Cooper, E. Corenzwit, L. D. Longinotti, B. T. Matthias, and W. H. Zachariasen, Proc. Nat. Acad. Sci. 67 313 (1970).
58. L. Leyarovska and E. Leyarovski, J. Less-Common Met. 67 249 (1979).
59. S. Okada, T. Atoda and I. Higashi, J. Solid State Chem. 68 61 (1987).
60. A. Yamamoto, C. Takao, T. Masui, M. Izumi, and S. Tajima, Physica C 383 197 (2002).
61. R. Escamilla, O. Lovera, T. Akachi, A. Duran, R. Falconi, F. Morales, and R. Escudero, J. Phys.: Condens. Matter 16 5979 (2004).
62. Carlos Angelo Nunes, Dariusz Kaczorowski, Peter Rogl, Marcia Regina Baldissera, Paulo At-sushi Suzuki, Gilberto Carvalho Coelho, Andriy Grytsiv, Gilles Andre, Francoise Bouree, and Shigeru Okada, Acta Materialia 53 3679 (2005).
63. R. Escamilla and L. Huerta, Supercond. Sci.Technol. 19 623 (2006).
64. Sogo Kuroiwa, Yoko Tomita, Akira Sugimoto, Toshikazu, and Jun Akimitsu, J. Phys. Soc. Jpn. 76 094705 (2007).
65. R. Khasanov, A. Shengelaya, A. Maisuradze, D. Di Castro, R. Escamilla, and H. Keller, Phys. Rev. B 77 064506 (2008).
66. Zhi-An Ren, Sogo Kuroiwa, Yoko Tomita, and Jun Akimitsu, Physica C 468 411 (2008).
67. A. L. Ivanovskii, Phys. Solid State 45 1829 (2003).
68. I. R. Shein and A. L. Ivanovskii, Phys. Rev. B 73 144108 (2006).
69. H. H. Farrell, Randall A. LaViolette, and T. M. Lillo, Physica C 449 1 (2006).
70. C. S. Lue, T. H. Su, B. X. Xie, S. K. Chen, J. L. MacManus-Driscoll, Y. K. Kuo, and H. D. Yang, Phys. Rev. B 73 214505 (2006).
71. R. J. Xiao, K. Q. Li, H. X. Yang, G. C. Che, H. R. Zhang, C. Ma, Z. X. Zhao, and J. Q. Li, Phys. Rev. B 73 224516 (2006).
72. H. Kotegawa, K. Ishida, Y. Kitaoka, T. Muranaka, N. Nakagawa, H. Takagiwa, and J. Akimitsu, Physica C 378-381 25 (2002).
73. C. S. Lue, T. H. Su, B. X. Xie, and C. Cheng, Phys. Rev. B 74 094101 (2006).
74. Y. Yafet and V. Jaccarino, Phys. Rev. 113 A1630 (1964).
75. Metallic Shifts in NMR, edited by G. C. Carter, L. H. Bennett, and D. J. Kahan (Pergamon, Oxford, 1977).
76. Shinji Wada, Ryozo Aoki, and Osamu Fujita, J. Phys. F 14 1515 (1984).
77. J. Korringa, Physica 16 601 (1950).
78. P. P. Singh, Solid State Communs. 125 323 (2003).
79. P. Vajeeston, P. Ravindran, C. Ravi, and R. Asokamani, Phys. Rev. B 63 045115 (2001).
80. I. R. Shein and A. L. Ivanovskii, Phys. Solid State 44 1833 (2002).
81. N.F. Mott and H. Jones, The Theory of the Properties of Metals (Clarendon Press Oxford 1936).
82. C. S. Lue and Y. K. Kuo, Phys. Rev. B 66 085121 (2002).
83. C. S. Lue, C. F. Chen, J. Y. Lin, Y. T. Yu, and Y. K. Kuo, Phys. Rev. B 75 064204 (2007).