Proc. Nati. Acad. Sci. USA Vol. 84, . 4681-4682, July 1987 Symposium Paper

This paper was presented at a symposium "Interfaces and Thin Films, " organized by John Armstrong, Dean E. Eastman, and George M. Whitesides, held March 23 and 24, 1987, at the National Academy of Sciences, Washington, D.C. Superconductivity above 90 K C. W. CHU Department of Physics and Space Vacuum Center, University of Houston, Houston, TX 77004; and Division of Materials Research, National Science Foundation, Washington, DC 20550

Superconductivity in the 30 K range was first observed (1) in zero) = 80 K on January 29, 1987, in a mixed Y-Ba-Cu-O the mixed La-Ba-Cu-O (LaBCO) compound system, and the (YBCO) system at an ambient pressure consisting of two importance of the layered K2NiF4 phase was stressed. phases, one black and one green. A few days later, the Independently, the Tokyo group (2) and our group (3) superconducting transition was pushed and narrowed reproduced the observations in late November 1986. At the down with Tc0 = 98 K and Tc = 94 K. The two phases were same time, we also reported (3) the observation of a sharp subsequently separated and ldentified by us (9): YBa2Cu3- resistance drop at 70 K, suggesting the possible existence of 06+x (black) and Y2BaCuO5 (green). We also succeeded (10) superconductivity above 70 K in LaBCO (which was later in making single-phase compounds and found the black proved to be correct by us). Unfortunately, we were unable phase, but not the green phase, to be superconducting above to stabilize the signal for a further check at that time. Later, 90 K with a 97-100% diamagnetic shift. the Tokyo group unambiguously attributed (4) the supercon- The linear temperature dependence of resistance was ductivity observed at 30-35 K to the single-layered K2NiF4 observed in oxide superconductors. A Curie-Weiss be- phase of LaBCO-i.e., (La0.85Bao.15)2Cu4-x. havior in the superconducting YBCO above Tc with a To explore the nature of this unusually high temperature negative Curie temperature of 6 K was also detected (11), superconductivity, we have subjected LaBCO compounds with demonstrating the existence of an antiferromagnetic interac- the K2NiF4 structure to high pressure. We found (3, 5) that the tion arising from the Cu ions in the compound. Magnetic superconducting transition temperature Tc is enhanced at an measurements (8, 12) showed that YBCO is a type II unprecedentedly high rate of 1 K/kbar (1 bar = 1 x 105 Pa), superconductor with an upper critical field of 200 Tesla at 0 about 100 times that for superconductors previously known. An K, the highest for superconductors existing to date. onset Tc (Tc) up to 57 K was achieved (P. H. Hor, . . Meng, Once the superconducting phase was known (9) as L. Gao, Z. J. Huang, and C.W.C., unpublished data) in LaBCO YBa2Cu306+., we decided to determine the active components under 12 kbar. This suggests that the mechanism responsible for responsible for the superconductivity in the YBCO compounds high-temperature superconductivity in oxides may be different by replacing Y by rare-earth and other elements. We found (13, from that arising from the conventional electron-phonon inter- 14) a whole new class of superconductors represented by action in ordinary superconductors. The results also suggest ABa2Cu306+x, where A = trivalent atomns-e.g., Y, La, Nd, that replacement of Ba by atoms with smaller radius-e.g., Sr Eu, Sm, Gd, Ho, Er, and Lu. That the superconductivity is and Ca-should also raise the Tc without the actual application almost independent of A is shown in Table 1 by the almost ofpressure. Indeed, (LaixSr,)2CuO4-, was observed to super- constant Tc0 and Tc1. Also given in Table 1 are the temperature conduct with a Tc - 35-48 K by various groups (6). Unfortu- Tdl, where the resistance deviates from its linear temperature nately, (La, _Ca,)2CuO4 only has a Tc - 20 K, even though Ca dependence, and lattice parameters. It is particularly interesting possesses a smaller radius than Sr. This immediately shows that that Gd3a2Cu306+, has a T. = 95 K and Tc1 = 92 K in contrast the mere reduction in interatomic distance cannot continue to to the strong magnetic moment associated with the Gd atoms. increase Tc to above 60 K in oxides with the K2NiF4 structure. This immediately led us to the conclusion (13, 14) that By examining the large volume of data accumulated by the superconductivity in this class of materials is confined to the end of December 1986, we found that (i) the Tc0 is always CuO2-Ba-CuO2+,-Ba-CuO2 layer assembly sandwiched be- reduced when the sample is made a pure single-layered- tween two A layers, with particular emphasis on the CU°2+x structure K2NiF4 phase with a sharp transition at 35 K, and layers. The CuO2 layers may serve as the shield for the CuO2+ (ii) the signs of superconductivity above 70 K always occur, layers from the A layers and, thus, help to retain the two- although unsystematically, in a mixed-phase compound con- dimensionality of the CuO2+x layers. sisting of structures in addition to K2NiF4. Therefore, we From the application point of view, the discovery of decided to search for Tc above 70 K in compounds with LaBa2Cu306+x to be superconducting above 90 K is extreme- structures different from K2NiF4. On January 12, 1987, we ly significant, since La2O3 is plentiful and cheap in compar- observed a large diamagnetic superconducting shift (about ison with Y203. Scientifically, it provides a solid basis for the 40% of a bulk superconductor) starting at 100 K in a mixed understanding of the occurrence of superconductivity in and Ba-rich LaBCO compound, a sufficient evidence for the different crystal structures-e.g., the K2NiF4 and LaBa2Cu3- existence of superconductivity above 77 K. Unfortunately, °6+x structures superconducting in the 30 and 90 K range, the signal disappeared the next day. Using the high-pressure respectively. As shown in Fig. 1, K2NiF4 has isolated CuO2 data as a guide, we finally detected unambiguously (7.8) layers, whereas the LaBa2Cu306+, has isolated CuO2-Ba- stable and reproducible superconducting signals starting at CuO2+x-Ba-CuO2 layer assemblies. Partial oxygen occupan- TC0 = 93 K and completing at Tc1 (where resistance becomes cy occurs in the center CuO2+x layers. Furthermore, the

The publication costs of this article were defrayed in part by page charge Abbreviations: LaBCO, La-Ba-Cu-O compound system; YBCO, payment. This article must therefore be hereby marked "advertisement" Y-Ba-Cu-O compound system; Tc, superconducting transition tem- in accordance with 18 U.S.C. §1734 solely to indicate this fact. perature; T., onset Tc; Tcl, TC where resistance becomes zero. 4681 Downloaded by guest on October 1, 2021 4682 Syhiposium Paper: Chu Proc. Natl. Acad. Sci. USA 84 (1987)

Table 1. ABa2Cu306+x properties 350 A Lattice parameters, 300 A T-01 K T.19 K TdI K a b c y*t 98 94 100 3.86 3.86 11.71 250 Lat 91 75 99 3.95 3.95 11.79 Ndt 91 70 93 3.89 3.89 11.73 5 200 Smt 94 82 135 3.88 3.88 11.73 0 Eut 94 88 160 3.86 3.86 11.74 I 150 Gdf 95 92 135 3.89 3.89 11.73 Hot 93 88 130 3.89 3.89 11.52 100 Ert 94 87 120 3.83 3.85 11.65 Lut§ 91 85 120 3.83 3.87 11.73 50 *Ref. 8. 0 tRef. 9. 70 90 110 130 150 170 190 210 230 250 tPresent work. §Ref. 23. T(K) FIG. 2. Signs of superconductivity up to 240 K in some of our LaBCO compounds can also be made (13, 14) insulating or Y-based compounds. magnetic, depending on the heat treatments. This greatly enhances the device potential of LaBCO. February 7, 1987. Unfortunately, they are not stable enough Our studies show that the following factors are important to survive the thermal cyclings for us to carry out further for the high-temperature superconductivity in these oxides: diagnostic checks. According to our results, they may be (i) the CuO2-Ba-CuO2,,-Ba-CuO2-layer assemblies with em- associated with phases with larger or better CuO2-layer phasis on the CuO2+5 layers, (ii) the quasi-two-dimensional assemblies, or different oxygen content. As mentioned ear- character of these layers, (iii) the antiferromagnetic interac- lier, it took about 2 months (November 25, 1986, to January tion in these layers, (iv) defects associated with oxygen 29, 1987) to stabilize the 90 K superconducting phase. It is not atoms, and (v) the ordering arrangement ofthe A and B atoms unforeseeable to take another few months to stabilize the in ABa2Cu306+5. In addition, the linear dependence of superconducting phase with a Tc at 240 K. occurrence of superconductivity resistance above Tc and the 1. Bednorz, J. G. & Muller, K. A. (1986) Z. Phys. B 64, 189-193. near the insulator/metal-phase boundary should also be 2. Uchida, S., Takagi, H., Kitazawa, K. & Tanaka, S. (1987) Jpn. J. considered if any model developments. In general, models Appl. Phys. Lett. 26, L1-L3. built on local deformations in the quasi-two-dimensional 3. Chu, C. W., Hor, P. H., Meng, R. L., Gao, L., Huang, H. Z. & Wang, Y. Q. (1987) Phys. Rev. Lett. 58, 405-407. system are able to explain qualitatively the observations. 4. Takagi, H., Uchida, S., Kitazawa, K. & Tanaka, S. (1987) Jpn. J. They involve the nonconventional electronic excitations like Appl. Phys. Lett. 26, L1-L2. excitons (15, 16), bipolarons (17), plasmon (18, 19), charge- 5. Chu, C. W., Hor, P. H., Meng, R. L., Gao, L. & Huang, Z. J. (1987) Science 235, 567-569. transfer fluctuations (20), spin fluctuations (21), or resonating 6. Cava, R. J., Batlogg, B., van Dover, R. B. & Rietman, E. A. (1987) valence bonds (22). At the present time, none of the above Phys. Rev. Lett. 58, 408-411. models can be ruled out unambiguously. 7. Wu, M. K., Ashburn, J. R., Torng, C. J., Hor, P. H., Meng, R. L., Finally, it should be pointed out that resistive indications Gao, L., Huang, Z. J., Wang, Y. Q. & Chu, C. W. (1987) Phys. Rev. Lett. 58, 908-910. of superconductivity at 120, 150, 180, and 240 K have been 8. Hor, P. H., Meng, R. L., Gao, L., Huang, Z. J., Wang, Y. Q., detected many times by us as shown in Fig. 2 as early as Chu, C. W., Wu, M. K., Ashburh, J. R. & Torng, C. J. (1987) Phys. Rev. Lett. 58, 911-912. (La, Ba 9. Hazen, R. H., Ringer, L. W., Angel, R. L., Prewitt, C. T., Mao, )2Cu04 La Ba2Cu3Oi+b H. K., Hadidiacos, C. G., Hor, P. H., Meng, R. L. & Chu, C. W. (1987) Phys. Rev., preprint. 10. Hor, P. H., Meng, R. L., Gao, L., Huang, Z. J., Wang, Y. Q., Bechtold, J., Forster, K., Chu, C. W., Mao, H. K., Hazen, R. H., Finger, L. W., Angel, R. L., Prewitt, C. T., Ross, N. L. & Hadidiacos, C. G. (1987) Phys. Rev., preprint. 11. Hor, P. H., Meng, R. L., Chu, C. W., Huang, C. Y., Zirngiebl, E. * 0 * & Thompson, J. D. (1987) Phys. Rev., preprint. 12. Hor, P. H., Meng, R. L., Chu, C. W. & Huang, C. Y. (1987) Appl. Phys. Commun., preprint. 13. Chu, C. W., Hor, P. H., Meng, R. L., Wang, Y. Q., Gao, L., --W-C Huang, Z. J., Bechtold, J. & Forster, K. (1987) Phys. Rev. Lett., preprint. 14. Hor, P. H., Meng, R. L., Wang, Y. Q., Gao, L., Huang, Z. J., Bechtold, J., Forster, K. & Chu, C. W. (1987) Phys. Rev. Lett., -*- 0-0-ton-o-*-0*L *O preprint. 15. Allender, D., Bray, J. & Bardeen, J. (1973) Phys. Rev. B 7,1020-1028. 16. Cohen, M. L. & Louie, S. G. (1976) Superconductivity in d- andf- Band Metals, ed. Douglas, D. (Plenum, New York), p. 7. 17. Ting, C. S., Xing, D. Y. & Lai, W. Y. (1987) Phys. Rev., preprint. 18. Kresin, V. (1987) Phys. Rev., preprint. 19. Ruwald, J. (1987) Phys. Rev., preprint. 20. Varma, C., Shmitt-Rink, S. & Abraham, E. (1987) Solid State *Cu 00 OBaorLa 0Bo *La * 0or empty Commun., preprint. 21. Leggett, A. J. & Wheatley, J. C. (1975) Rev. Mod. Phys. 47, 331-345. 22. Anderson, P. W. (1987) Science, preprint. unit cell -CuO layer 23. Moodenbaugh, A., Suenaga, M., Asano, T., Shelton, R. N., Ku, H. C., McCallum, R. W. & Klavins, P. (1987) Phys. Rev. Lett. 58, FIG. 1. Structure of (LaBa)2CuO4-_ and LaBa2Cu306+1. 1885-1887. Downloaded by guest on October 1, 2021