為了明瞭晶格應變對鑭系214相高溫超導體的影響，我們製作了三組不同的薄膜樣品來進行觀察：(1)La2-xSrxCuO4 ( x=0.1~0.15 ) 鍍在LaSrAlO4(001)基板，(2) La2-xSrxCuO4 ( x=0.075~0.15 ) 鍍在LaSrAlO4(100)基板，(3) La2-xMxCuO4 ( M=Ba,Sr,Ca,Mg ; x=0.1 ) 鍍在LaSrAlO4(001)基板。比較(001)和(100)晶面取向的實驗結果發現，兩者形成相反的磊晶應變卻得到相類似的物理變化——超導態Tc增加，正常態R-T 曲線呈線性行為。因此推論單純地晶軸長度改變不是造成物理性質大幅變化的主因，而可能是有excess oxygen intercalation共同參與所造成的。另一方面，藉著不同取代離子造成的intrinsic strain來調節excess oxygen佔據的interstitial site大小，使得不同取代離子樣品的氧含量產生變化(氧含量依序為 Ba2+ □ Sr2+ > Ca2+ > Mg2+ )，更進一步證實excess oxygen對鑭系214相高溫超導體具有重要的影響力。

In order to make it clear for the strain effects of lanthanum-based 214 phase superconducting thin films, we prepared three kinds of thin films :
(1) La2-xSrxCuO4 ( x=0.1~0.15 ) deposited on LaSrAlO4(001) substrates,

(2) La2-xSrxCuO4 ( x=0.075~0.15 ) deposited on LaSrAlO4(100) substrates,

(3) La2-xMxCuO4 ( M=Ba,Sr,Ca,Mg ; x=0.1 ) deposited on LaSrAlO4(001) substrates.

Compared the results of films deposited on (001) and (100) orientation substrates, we found that two opposite substrate-induced epitaxial strains had a similar evolution of physical properties, which are enhanced Tc and linear normal-state R-T curve. Therefore, we speculated that only c-axis expansion was not the major factor altering physical properties so drastically. The excess oxygen intercalation may play an important role as well. On the other hand, the substitution of alkaline-earth element induced intrinsic strain in thin films, it may adjust the space of interstitial sites between two La-O layers occupied by excess oxygen, and thus changed the oxygen content in the film substituted for different alkaline-earth elements (oxygen content: Ba2+ □ Sr2+ > Ca2+ > Mg2+ ). This is another evidence that the excess oxygen is important in the physical properties of lanthanum-based 214 phase superconducting thin films.

1. H. L. Kao, J. Kwo, R. M. Fleming, M. Hong and J. P. Mannaerts, In situ growth and properties of single-crystalline-like La2-xSrxCuO4 epitaxial films by off-axis sputtering, Appl. Phys. Lett. 59 (1991) P.2748
2. J. P. Locquet, J. Perret, J. Fompeyrine, E. Mächler, J.W. Seo and G. Van Tendeloo, Doubling the critical temperature of La1.9Sr0.1CuO4 using epitaxial strain, Nature 394 (1998) P.453
3. T. Nakano, M. Oda, C. Manabe, N. Momono, Y. Miura and M. Ido, Magnetic properties and electronic conduction of superconducting La2-xSrxCuO4 , Phys. Rev. B 49 (1994) P.16000
4. F. Nakamura, J. Hori, T. Goko, Y. Uno, N. Kikugawa and T.Fujita, Tc enhancement in La2-xSrxCuO4 under anisotropic pressure, J. Low temp. Physics 117(5/6) (1999) P.1145
5. P. G. Radaelli, J. D. Jorgensen, R. Kleb, B. A. Hunter, F. C. Chou and D.C. Johnston, Miscibility gap in electrochemically oxygenated La2CuO4+□ , Phys. Rev. B 49 (1994) P.6239
6. P. G. Radaelli, J. D. Jorgensen, A. J. Schultz, B. A. Hunter, J. L. Wagner, F. C. Chou and D.C. Johnston, Structure of the superconducting La2CuO4+□ phase (□□0.08,0.12) prepared by electrochemical oxidation, Phys. Rev. B 48 (1993) P.499
7. Z. G. Li, H. H. Feng, Z. Y. Yang, A. Hamed, S. T. Ting and P. H. Hor, Carrier-controlled doping efficiency in La2CuO4+□ , Phys. Rev. Lett. 77 (1996) P.5413
8. Weidong Si and X. X. Xi, Growth and properties of Pulse Laser Deposited superconducting La1.85Sr0.15CuO4 thin films, Applied superconductivity IEEE 9 (2) (1999) P.2406
9. H. Sato, A. Tsukada, M. Naito and A. Matsuda, La-214 thin films under epitaxial strain, Physica C 341~348 (2000) P.1767
10. 周午縱、梁維耀主編，高溫超導基礎研究，上海，上海科學技術出版社，1999，PP.21。unit-cell volume是從表2-1的a、c晶格常數換算來的(V=a2□c)。
11. H. Takagi, T. Ido, S. Ishibashi, M. Uota, S. Uchida and Y. Tokura, Superconductor-to-nonsuperconductor transition in (La1-xSrx)2CuO4 as investigated by transport and magnetic measurements,.Phy. Rev. B 40 (1989) P.2254
12. I. F. Kononyuk, L. A. Tikhonova, L. A. Makhnach and G. K. Zhavnerko, Electrical properties of the complex oxides La2-xMxCuO4 (M=Mg,Ca,Sr,Ba), Inorganic materials 19(6) (1983) P.841
13. T. Nagano, Y. Tomioka, Y. Nakayama, K. Kishio and K. Kitazawa, Bulk superconductivity in both tetragonal and orthorhombic solid solutions of (La1-xSrx)2CuO4-□ , Phy. Rev. B 48 (1993) P.9689
14. H. Sato, A. Tsukada, M. Naito and A. Matsuda, Absence of 1/8 anomaly in strained thin films of La2-xBaxCuO4+□ , Phy. Rev. B 62 (2000) P.R799
15. X. H. Zeng, Weidong Si, Z. M. Stum and X. X. Xi, Doping dependence of superconducting properties in Pulsed-Laser-Deposited La2-xSrxCuO4+□ (0≦x≦0.30) thin films, Applied superconductivity IEEE 11(1) (2001) P.3213
16. H. Sato, A. Tsukada, M. Naito and A. Matsuda, La2-xSrxCuOy epitaxial thin films ( x = 0 to 2 ): structure, strain, and superconductivity, Phy. Rev. B 61 (2000) P.12447
17. M. F. Hundley, J. D. Thompson, S. W. Cheong and Z. Fisk, Phase seperation in a high-pressure-oxygenated La2CuO4+□ crystal: evidence from anisotropic electronic transport and magnetic susceptibility, Phy. Rev. B 41 (1990) P.4062
18. H. Sato, M. Naito and H. Yamamoto, Superconducting thin films of La2CuO4+□ by oxygen doping using ozone, Physica C 280 (1997) P.178
19. I. Bozovic, G. Logvenov, I. Belca, B. Narimbetov and I. Sveklo, Epitaxial strain and superconductivity in La2-xSrxCuO4 thin films, Phy. Rev. Lett. 89 (2002) P.107001
20. B. Lorenz, Z. G. Li, T. Honma and P. H. Hor, Intrinsic tendency of electronic phase seperation into two superconducting states in La2-xSrxCuO4+□ , Phy. Rev. B 65 (2002) P.144522
21. F. Arrouy, J. P. Locquet, E. J. Williams, E. Mächler, R. Berger, C. Gerber, C. Monroux, J. C. Grenier and A. Wattiaux, Growth, microstructure, and electrochemical oxidation of MBE-grown c-axic La2CuO4 thin films, Phy. Rev. B 54 (1996) P.7512
22. A. M. Balagurov, V. Yu. Pomjakushin, V. G. Simkin and A. A. Zakharov, Neutron diffraction study of phase seperation in La2CuO4+□ single crystal, Physica C 272 (1996) P.277
23. Z. G. Li, H. H. Feng, Z. Y. Yang, A. Hamed, S. T. Ting and P. H. Hor, Carrier-controlled doping efficiency in La2CuO4+□ , Phy. Rev. Lett. 77 (1996) P.5413 La2CuO4+□