在電子型高溫超導體Pr1.85Ce0.15CuO4+d (-0.003 £ d £ 0.03)系統中，我們在氧含量參數 d ~ 0.018時觀察到金屬絕緣體轉變。此系列樣本在之銅的K-邊緣X光吸收近邊緣結構(XANES)，得到幾乎不變的8980.8 eV吸收邊能量E0，表示銅(Cu)的名義價數比2還小，而這個結果跟由電中性平衡得到的在Pr1.85Ce0.15CuO3.997中的 ~1.84，和Pr1.85Ce0.15CuO4.03中的~1.91相當吻合。XANES的光譜所表現的是銅在3d軌域的電子組態，其中A1小峰對應到的是Cu(I)氧化態的3d10組態的佔有率，而A2對應到的則是Cu(II) 氧化態的3d10L (L 表示配位氧上的電洞)組態的佔有率。兩者能量差 DE(A2-A1) 的變化跟觀察到的金屬絕緣體轉變互相吻合，而且有一個從絕緣性的 Pr1.85Ce0.15CuO4.018 的2.42 eV 突然增加到有 15 K 超導溫度的低參雜 Pr1.85Ce0.15CuO4.015 的2.74 eV。
經由測量ab-平面上的電磁場穿透深度 lab(T) 隨溫度的變化，我們得以研究Pr1.85Ce0.15CuO4+d 和Pr2-xCexCuO4+d (x = 0.14, 0.15, 0.16) 兩系統的超導電子對的對稱性。當Pr1.85Ce0.15CuO4+d 樣本的氧含量參數從不足參雜增加到過量參雜，我們觀察到歸一化的穿透深度差 lab(T)/ lab(0) – 1從線性的溫度關係變成到二次方最後變到T2.39 的次方函數關係。相似的系統性變化由對溫度二次方過渡到2.67次方也在改變Ce含量的系統中關察到。因為T2可以很好的描述 lab(T)/ lab(0) – 1的溫度關係，超導電子對的對稱性在這兩系列的樣品基本上符合d-波的對稱性。而溫度關係的指數改變，表示其超導能隙方程有幾何上的改變。因此我們並沒有在此系統中觀察到文獻所說的d-

Metal-insulator transition near oxygen content parameter d ~ 0.018 was observed for electron-doped Pr1.85Ce0.15CuO4+d (-0.003 £ d £ 0.03) high-Tc superconductors. Cu K-edge X-ray absorption near-edge structure (XANES) studies with almost identical threshold edge energy E0 around 8980.8 eV indicates a Cu formal valence lower than 2, which is consistent the estimated Cu formal valence of ~1.84 for the 20.5 K superconductor Pr1.85Ce0.15CuO3.997 and ~1.91 for insulating Pr1.85Ce0.15CuO4.03. The XANES reflects the 3dn character where low energy peak A1 reflects the 3d10 occupation of Cu(I) oxidation state and A2 peak reflects the 3d10L (L for oxygen ligand hole) configuration for Cu(II) oxidation state. The variation of energy separation DE(A2-A1) is consistent with the metal-insulator transition and increases sharply from 2.42 eV for insulating Pr1.85Ce0.15CuO4.018 to 2.74 eV for 15 K underdoped superconductor Pr1.85Ce0.15CuO4.015.
The superconducting paring symmetry of the Pr1.85Ce0.15CuO4+d system and Pr2-xCexCuO4+d (x = 0.14, 0.15, 0.16) system were studied though the temperature dependence of the ab-plane electromagnetic penetration depth lab(T). A systematic change of the normalized penetration length difference lab(T)/ lab(0) – 1 from linear to quadratic then to a T2.39 power law for the oxygen content of Pr1.85Ce0.15CuO4+d varies from underdoped to overdoped. Similar systematic change from quadratic for under- and optimal-doped to T2.67 power law for overdoped sample was observed for the Ce varying system with d ~ 0. Systems basically can be described by the d-wave pairing symmetry, since the quadratic temperature dependence provides good fittings. The observed changes of power law exponent larger than 2 indicates a topological change of the gap function in the overdoped region. No direct evidence of s-wave paring symmetry was observed for the systems studied.

Reference
[1] J.G. Bedonrz and K.A. Muller, Z. Phys. B64, 189 (1986).
[2] Y. Tokura, H. Takage and S. Uchida, Nature 337, 345 (1989).
[3] H. Takagi, S. Uchida and Y. Tokura, Phys. Rev. Lett. 62, 1197 (1989
[4] J.T. Market, E.A. Early, T. Bjǿrnholm, S. Ghamaty, B. W. Lee, J.J. Neumeier, R.D. Price, C.L. Seaman and M.B. Maple, Physica C 158, 178 (1989)
[5] J.T. Market and M.B. Maple, Solid State Commun. 70, 145 (1989).
[6] A.C.W.P. James, S.M. Zahurak and D.W. Murphy, Nature 338, 240 (1989).
[7] E.A. Early, N.Y. Ayoub, J. Beille, J.T. Market and M.B. Maple, Physica C 160, 320 (1989).
[8] D.J. Van Harlingen, Rev. Mod. Phys. 67, 515 (1995).
[9] Muller-Buschbaum, H. Angew. Chem. Int. Edn Engl. 16, 674.
[10] P.G. Radaelli, J. D. Jorgenson, A.J. Schultz, J.L. Peng, and R.L. Greene, Phys. Rev. B 49, 15322 (1994).
[11] P. Ghigna, G. Spinolo, A. Filipponi, A.V. Chadwick, and P. Hanmer, Physica C 246, 345 (1995).
[12] Z. Tan, J.I. Budnick, C.E. Bouldin, J.C. Woicik, S-W. Cheong, A.S. Cooper, G.P. Espinosa, Z. Fisk, Phys. Rev. 42, 1037 (1990)
[13] E.E. Alp, S.M. Mini, M. Ramanathan, B. Dabrowski, D.R. Richards, and D. G. Hinks, Phys. Rev. B, 40, 2617 (1989)
[14] G. Liang, Y. Guo, D. Badresingh, X. Wu, Y. Tang, M. Croft, J. Chen, A. Sahiner, B-h O, J. T. Markert, Phys. Rev. B 51, 1258 (1995).
[15] P. Ghigna, G. Spinolo, M. Scavini, G. Chiodelli, G. Flor , A.V. Chadwick, Physica C 268, 150 (1996)
[16] G. Liang, M. Li, J.T. Markert, and L.V. Wang, Intl. J. Modern Phys. B 12, 3299 (1998)
[17] H. Oyanagi, Y. Yokoyama, H. Yamaguchi, Y. Kuwahara, T. Katayama, Y. Nishihara, Phys. Rev. B 42, 10136 (1990)
[18] J.R. Kirtley, C.C. Tsuei, J.Z. Sun, C.C. Chi, Locks see Yu-Jahnes, A. Gupta, M. Rupp and M.B. Ketchen, Nature 373, 225 (1995)
[19] C.C. Tsuei, J.R. Kirtley, M. Rupp, J.Z. Sun, C.C. Chi, A. Gupta, Locks See Yu-Jahnes, M.B. Ketchen, Physica C 263, 232 (1996).
[20] F. Gross, B.S. Chandrasekhar, D. Einzel, K. Andres, p.J. Hirschfeld, H.R. Ott, J. Beuers, Z. Fisk and J.L. Smith, Z. Phys. B 64, 175 (1986).
[21] H.M. Hazen et al., Phys. Rev. Lett. 60, 1174 (1988).
[22] J.A. Skinta, T.R. Lemberger, T. Greibe, M. Naito, Phys. Rev. Lett. 88, 207003 (2002)
[23] A. Biswas, P. Fournier, M.M. Qazilbash, V.N. Smolyaninova, H. Balci, R.L. Greene, Phys. Rev. Lett. 88, 207004 (2002)
[24] J.A. Skinta, M-S. Kim, T.R. Lemberger, T. Greibe, M. Naito, Phys. Rev. Lett. 88, 207005 (2002)
[25] D. Shoenber, Syperconducticity (Cambridge University Press, CamBridge, 1954) page 164.
[26] J.F. Annett, N. Goldenfeld and S.R. Renn, in Physical Properties of High Temperature Superconductors II, edited by D.M. Ginsberg (World Scientific, Singapore, 1990), pp. 571.
[27] C. Panagopoluos, T. Xiang, Phys. Rev. Lett. 81, 2336 (1998).
[28] E. H. Appelman, L. R. Morss, A. M. Kini, U. Geiser, A. Umezawa, G. W. Crabtree, K. D. Carlson, Inorg. Chem. 26, 3273 (1987).
[29] A. Bianconi, A. Marcelli, H. Dexpert, R. Karnatak, A. Kotani, T. Jo, J. Petiau, Phys. Rev. B 35, 806 (1987).
[30] H. C. Chiang, Y. Y. Hsu, B. C. Chang, B. N. Lin, T. I. Hsu, H C. Ku, J. Appl. Phys. 89, 7660 (2001).
[31] H. C. Ku, Y. Y. Hsu, B. C. Chang, B. N. Lin, H. C. Chiang, T. J. Yang, Physica C 364-365, 285 (2001).
[32] B. C. Chang, Y. Y. Hsu, H. C. Ku, Physica B 312-313, 59 (2002).
[33] E. Dagotto, Rev. Mod. Phys. 66, 763 (1994).
[34] D. J. van Harlingen, Rev. Mod. Phys. 67, 515 (1995).