高純度超導體Ax(NH3)Fe2Se2 (A =鋇, 鍶 or 鈣)是經由氨熱法將鹼土金族鑲入belta-FeSe裡，製作出來的樣本是純超導，在過去245-phase的反鐵磁相會跟超導相一起共存，這是一個很頭痛的問題，因為反鐵磁相會影響超導性質的基本量測。 利用氨熱法製作的樣本之基礎超導性質像超導臨界溫度Tc (Tc = 39 K for A =鋇, 44 K for鍶 和43 K for鈣)，然而Tc的增高引起了大家對於Ax(NH3)Fe2Se2 (A = alkali 或 alkaliearth)產生了高度的興趣，不只是好奇為何Tc會增高，還包含了一連串超導性質與機制。 而經由X-ray的測量後，我們發現Bax(NH3)Fe2Se2與Srx(NH3)Fe2Se2的c-axis的變化，進而讓鐵硒原本的三維費米面轉化為近似二維費米面，除此之外，這篇論文還會提供基本的超導性質像超導低臨界場Bc1(0 K) (30 G for A =鋇 和24 G for A = 鍶) 和超導高臨界場Bc2(0 K) (13.4 T for A =鋇 和60.7 T for A =鍶)隱喻了Ax(NH3)Fe2Se2 (A =鋇, 鍶 or 鈣)是two-gap model、超導相干長度xi(0) (4.96 nm for A = Ba和2.33 nm for A = Sr)、Ginzburg-Landau parameters kappa(102.4 for A = Ba和266 for A = Sr)、超導穿透深度penetration depths lambda(0) (508.2 nm for A = Ba和620 nm for A = Sr)以及超導能隙分別為delta1 = 6.47 meV 和 delta2 = 1.06 meV。

High purity samples of Ax(NH3)Fe2Se2 (A = barium, strontium and calcium) superconductors were successfully synthesized by intercalating alkaliearth metals into tetragonal belta-FeSe by liquid ammonia. The ammonothermal method employed is known for capable of preparing single phase alkali/alkaliearth-intercalated iron-basic superconductors. The coexistence of antiferromagnitism in the alkali-containing iron-selenium superconductors synthesized by conventional thermal process is a huge barrier interfering detail studies of superconductivity. Beside the pure phase for superconductivity discussion, the enhancement of critical temperature Tc (Tc = 39 K for A = Ba, 44 K for Sr and 43 K for Ca) after alkali/alkaliearth intercalation into belta-FeSe attracts even more interests. In this work, the details of belta-FeSe preparation, ammonothermal intercalation process, lattice structure and magnetic properties measurements are described and used for physical parameters derivation.
The elongated c-axis and almost unchanged a-axis of Bax(NH3)Fe2Se2 and Srx(NH3)Fe2Se2, comparing with -FeSe, suggested an unchanged intra-Fe2Se2-layer structure and the Tc enhancement is due to a 3D to 2D-like Fermi surface transformation. The superconducting properties coherent lengths xi(0) (4.96 nm for A = Ba and 2.33 nm for A = Sr), Ginzburg-Landau parameters kappa(102.4 for A = Ba and 266 for A = Sr) and penetration depths lambda(0) (508.2 nm for A = Ba and 620 nm for A = Sr) obtained from the extrapolated lower and upper critical fields Bc1(0 K) (30 G for A = Ba and 24 G for A = Sr) and Bc2(0 K) (13.4 T for A = Ba and 60.7 T for A = Sr) indicates that both compounds are typical type-II superconductors. The temperature dependence of 1/lambda2(T) of Bax(NH3)Fe2Se2 deduced from the low field magnetic susceptibility shows a two-gap s-wave behaviour with superconducting gaps of delta1 = 6.47 meV and delta2 = 1.06 meV.

[1]T. M. McQueen, Q. Huang, V. Ksenofontov, C. Felser, Q. Xu, H. Zandbergen, Y. S. Hor, J. Allred, A. J. Williams, D. Qu, J. Checkelsky, N. P. Ong and R. J. Cava; Phys. Rev. B 79 (2009) 014522

[2]J. Paglione and R. L. Greene; High-temperature superconductivity in iron-based materials, Nature Phys. 6 (2010) 645

[3]A. A. Kordyuk; Low Temp. Phys. 38 (2012) 888

[4]K. W. Yeh, T. W. Huang, Y. L. Huang, T. K. Chen, F. C. Hsu, Phillip M. Wu, Y .C. Lee, Y. Y. Chu, C. L. Chen, J. Y. Luo, D. C. Yan and M. K. Wu; Europhys. Lett. 84 (2008) 37002

[5]J. Guo, S. Jin, G. Wang, S. Wang, K. Zhu, T. Zhou, M. He and X. Chen; Phys. Rev. B 82 (2010) 180520

[6]F. Ye, S. Chi, W. Bao, X. F. Wang, J. J. Ying, X. H. Chen, H. D. Wang, C. H. Dong and M. H. Fang; Phys. Rev. Lett. 107 (2011) 137003

[7]A. M. Zhang, T. L. Xia, K. Liu, W. Tong, Z. R. Yang and Q. M. Zhang; Sci. Rep. 3 (2013) 1216

[8]Z. W. Wang, Z. Wang, Y. J. Song, C. Ma, Y. Cai, Z. Chen, H. F. Tian, H. X. Yang, G. F. Chen and J. Q. Li; J. Phys. Chem. C 116 (2012) 17847

[9]D. P. Shoemaker, D. Y. Chung, H. Claus, M. C. Francisco, S. Avci, A. Llobet and M. G. Kanatzidis; Phys. Rev. B 86 (2012) 184511

[10]W. Li, H. Ding, P. Deng, K. Chang, C. Song, K. He, L. Wang, X. Ma, J. P. Hu, X. Chen and Q. K. Xue; Nat. Phys. 8 (2012) 126

[11]F. Chen, M. Xu, Q. Q. Ge, Y. Zhang, Z. R. Ye, L. X. Yang, J. Jiang, B. P. Xie, R. C. Che, M. Zhang, A. F. Wang, X. H. Chen, D. W. Shen, J. P. Hu and D. L. Feng; Phys. Rev. X 1 (2011) 021020

[12]V. Ksenofontov, G. Wortmann, S. A. Medvedev, V. Tsurkan, J. Deisenhofer, A. Loidl and C. Felser; Phys. Rev. B 84 (2011) 180508

[13]R. H. Yuan, T. Dong, Y. J. Song, P. Zheng, G. F. Chen, J. P. Hu, J. Q. Li and N. L. Wang; Sci. Rep. 2 (2012) 221

[14]T. P. Ying, X. L. Chen, G. Wang, S. F. Jin, T. T. Zhou, X. F. Lai, H. Zhang and W. Y. Wang; Sci. Rep. 2 (2012) 426

[15]M. Burrard-Lucas, D. G. Free, S. J. Sedlmaier, J. D. Wright, S. J. Cassidy, Y. Hara, A. J. Corkett, T. Lancaster, P. J. Baker, S. J. Blundell and S. J. Clarke; Nature Mater. 12 (2013) 15

[16]A. Subedi, L. Zhang, D. J. Singh and M. H. Du; Phys. Rev. B 78 (2008) 134514

[17]R. Khasanov, K. Conder, E. Pomjakushina, A. Amato, C. Baines, Z. Bukowski, J. Karpinski, S. Katrych, H.-H. Klauss, H. Luetkens, A. Shengelaya and N. D. Zhigadlo; Phys. Rev. B 78 (2008) 220510

[18]T. P. Ying, X. L. Chen, G. Wang, S. F. Jin, X. F. Lai, T. T. Zhou, H. Zhang, S. J. Shen and W. Y. Wang; J. Am. Chem. Soc. 135 (2013) 2951

[19]S. Shibasaki, K. Ashida, Y. Takahei, K. Tomita, R. Kumai and T. Kambe; arXiv:condmat/ 1306.0979 (2008)

[20]M. Abdel-Hafiez, J. Ge, A. N. Vasiliev, D. A. Chareev, J. Van de Vondel, V. V. Moshchalkov and A. V. Silhanek; Phys. Rev. B 88 (2013) 174512

[21]P. K. Biswas, A. Krzton-Maziopa, R. Khasanov, H. Luetkens, E. Pomjakushina, K. Conder and A. Amato; Phys. Rev. Lett. 110 (2013) 137003

[22]A. Carrington and F. Manzano Physica C 385 (2003) 205

[23]P. K. Biswas, G. Balakrishnan, D. M. Paul, C. V. Tomy, M. R. Lees and A. D. Hillier; Phys. Rev. B 81 (2010) 092510

[24]Y. Y. Hsu, Y. B. Li, S. T. Jian, G. K. Li and M. C.Yang; Supercond. Sci. Technol. 29 (2016) 035005

[25]E. Kuzmann; Z. Homonnay; A. Vértes; I. Halász; J. Bánkuti and I. Kirschner; J. Radional. Nucl. Chem. Letters 136 2 (1989) 121

[26]J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu; nature 410 (2001) 63

[27]W. Schuster, H. Mikler and K.L. Komarek; Chem. 110 (1979) 1153

[28]Y. Kamihara, T. Watanabe, M. Hirano and H. Hosono; J. Am. Chem. Soc. 130 (2008) 3296

[29]H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano and H. Hosono;Nature 453 (2008) 376

[30]H. H. Wen, G. Mu, L. Fang, H. Yang and X. Zhu;epl 82 (2008) 17009

[31]X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen and D. F. Fang; Nature 453 (2008) 761

[32]Z. A. Ren, W. Lu, J. Yang, W. Yi, X. L. Shen, Z. C. Li, G. C. Che, X. L. Dong, L. L. Sun, F. Zhou and Z. X. Zhao; Chin. Phys. Lett. 25 (2008) 2215

[33]C. Wang, Y. K. Li, Z. W. Zhu, S. Jiang, X. Lin, Y. K. Luo, S. Chi, L. J. Li, Z. Ren, M. He, H. Chen, Y. T. Wang, Q. Tao, G. H. Cao and Z. A. Xu; Phys. Rev. B 79 (2009) 054521

[34]C. Wang, L. Li, S. Chi, Z. Zhu, Z. Ren, Y. Li, Y. Wang, X. Lin, Y. Luo, S. Jiang, X. Xu, G. Cao and Zhu'an Xu; epl 83 (2008) 67006

[35]Z. A. Ren, G. C. Che, X. L. Dong, J. Yang, W. Lu, W. Yi, X. L. Shen, Z. C. Li, L. L. Sun, F. Zhou and Z. X. Zhao; epl 83(2008) 17002

[36]M. Rotter, M. Tegel and D. Johrendt;Phys. Rev. Lett. 101 (2008) 107006