Discovery of a second family of bismuth-oxide-based superconductors
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Cited by 109 publications
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Oxygen-isotope effect has been investigated in a recently discovered superconductor Sr 0.4 K 0.6 BiO 3 . This compound has a distorted perovskite structure and becomes superconducting at about 12 K. Upon replacing 16 O with 18 O by 60-80%, the T c of the sample is shifted down by 0.32-0.50 K, corresponding to an isotope exponent of α O = 0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba 1−x K x BiO 3 with T c = 30 K. The very distinctive doping and T c dependencies of α O observed in bismuthates and cuprates suggest that bismuthates should belong to conventional phonon-mediated superconductors while cuprates might be unconventional supercondutors. 1The discovery of high-temperature superconductivity near 30 K in the nonmagnetic cubic perovskite oxide Ba 1−x K x BiO 3 (BKBO) [1,2] raises an interesting question of whether the layered structure and strong antiferromagnetic correlation in cuprates are essential for hightemperature superconductivity. In order to answer this question, it is important to find some common and distinct features in both systems. The band-structure calculations [3] suggest that the bare density of states at Fermi level in BKBO is at least 3 times smaller than that in cuprates. Since tunneling and extensive oxygen-isotope experiments on BKBO [4][5][6][7][8] seem to indicate that this material is a conventional phonon-mediated superconductor with an electron-phonon coupling constant λ ∼ 1, one might argue that 100 K superconductivity in cuprates could be understood even within the conventional theory with λ ∼ 3. If this were the case, the isotope effects in both cuprates and bismuthates would be similar. As a matter of fact, the oxygen-isotope exponent α O in BKBO has a maximum at optimal doping where T c is the highest [8], while α O in optimally-doped cuprates is the smallest [9][10][11][12]. Moreover, the isovalent substitution of Ca for Sr in the single-layer La 2−x Sr x CuO 4 system leads to a large decrease in T c , and to a large increase in α O [13]. Similarly, the isovalent substitution of Sr for Ba in YBa 2 Cu 3 O 7 gives rise to a strong suppression of superconductivity from 93 K to 60 K [14], and α O also increases with decreasing T c [15]. If the pairing mechanism in bismuthates and cuprates were the same, this unusual T c dependence of α O would also exist in bismuthates.The isovalent substitution of Sr for Ba in Ba 1−x K x BiO 3 cannot be realized using conventional solid-state reaction. Until recently, a new family of bismuth-oxide-based superconductor Sr 1−x K x BiO 3 (SKBO) has been synthesized by a high pressure technique [16]. This material has a distorted perovskite structure and exhibits a superconductivity at about 12 K, which is much lower than that in Ba 1−x K x BiO 3 . It appears that the isovalent substitution effects on T c in both YBa 2 Cu 3 O 7 and Ba 1−x K x BiO 3 are quite similar. Now the important question is whether α O in optimally-doped Sr 1−x K x BiO 3 remains the same or increases substantially comp...
Oxygen-isotope effect has been investigated in a recently discovered superconductor Sr 0.4 K 0.6 BiO 3 . This compound has a distorted perovskite structure and becomes superconducting at about 12 K. Upon replacing 16 O with 18 O by 60-80%, the T c of the sample is shifted down by 0.32-0.50 K, corresponding to an isotope exponent of α O = 0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba 1−x K x BiO 3 with T c = 30 K. The very distinctive doping and T c dependencies of α O observed in bismuthates and cuprates suggest that bismuthates should belong to conventional phonon-mediated superconductors while cuprates might be unconventional supercondutors. 1The discovery of high-temperature superconductivity near 30 K in the nonmagnetic cubic perovskite oxide Ba 1−x K x BiO 3 (BKBO) [1,2] raises an interesting question of whether the layered structure and strong antiferromagnetic correlation in cuprates are essential for hightemperature superconductivity. In order to answer this question, it is important to find some common and distinct features in both systems. The band-structure calculations [3] suggest that the bare density of states at Fermi level in BKBO is at least 3 times smaller than that in cuprates. Since tunneling and extensive oxygen-isotope experiments on BKBO [4][5][6][7][8] seem to indicate that this material is a conventional phonon-mediated superconductor with an electron-phonon coupling constant λ ∼ 1, one might argue that 100 K superconductivity in cuprates could be understood even within the conventional theory with λ ∼ 3. If this were the case, the isotope effects in both cuprates and bismuthates would be similar. As a matter of fact, the oxygen-isotope exponent α O in BKBO has a maximum at optimal doping where T c is the highest [8], while α O in optimally-doped cuprates is the smallest [9][10][11][12]. Moreover, the isovalent substitution of Ca for Sr in the single-layer La 2−x Sr x CuO 4 system leads to a large decrease in T c , and to a large increase in α O [13]. Similarly, the isovalent substitution of Sr for Ba in YBa 2 Cu 3 O 7 gives rise to a strong suppression of superconductivity from 93 K to 60 K [14], and α O also increases with decreasing T c [15]. If the pairing mechanism in bismuthates and cuprates were the same, this unusual T c dependence of α O would also exist in bismuthates.The isovalent substitution of Sr for Ba in Ba 1−x K x BiO 3 cannot be realized using conventional solid-state reaction. Until recently, a new family of bismuth-oxide-based superconductor Sr 1−x K x BiO 3 (SKBO) has been synthesized by a high pressure technique [16]. This material has a distorted perovskite structure and exhibits a superconductivity at about 12 K, which is much lower than that in Ba 1−x K x BiO 3 . It appears that the isovalent substitution effects on T c in both YBa 2 Cu 3 O 7 and Ba 1−x K x BiO 3 are quite similar. Now the important question is whether α O in optimally-doped Sr 1−x K x BiO 3 remains the same or increases substantially comp...
There is a widely-held belief that the preparation of new solid-state compounds based on rational design is not possible. Herein, we present a concept that points the way towards a rational design of syntheses in solid-state chemistry. The foundation of our approach is the representation of the whole material world, that is, the known and not-yet-known compounds, on an energy landscape, which gives information about the free energies of these compounds. From this it follows that all chemical compounds capable of existence are present on this landscape. Thus the chemical synthesis always corresponds to the discovery of compounds, not their creation. Consequently, the first step in planning a synthesis can and must be to identify a synthesizable compound. Up to now, materials capable of existence are discovered in the course of an experimental exploration of the energy landscape; however, an a priori identification of a synthesis goal requires an exploration using theoretical methods. In contrast to those computational approaches currently employed for structure determination for fixed composition and already known unit cells, our aims clash with such restrictions and full global optimizations have to be performed on the landscape. Although for reasons of computational feasibility the accuracy of the energy calculations is not yet as high as one would wish, our approach proves to be surprisingly robust. One always finds the already known compounds of a given chemical system, and, in addition, further plausible structure candidates are discovered. The second step of a rational planning of syntheses is the design of feasible synthesis routes. Modeling such routes requires highly accurate computations for realistic thermodynamic conditions, however this is usually beyond our current capabilities. Thus, we have not seriously pursued such a deductive approach; instead we have attempted, to reproduce the "computational annealing" employed during our structure predictions in the experiment. Educts, generated by vapor deposition methods, that are disperse on an atomic level are found to react with surprisingly low activation energies to give highly crystallized products. However, even this technique does not yet provide the possibility to selectively synthesize a specific solid compound. For this final step, modeling and experimental control of nucleation processes will be the key ingredient. Only when viewed superficially, our goal of a "rational design" of solid-state syntheses and the "high-throughput" syntheses are in contradiction. But an exhaustive exploration of the unimaginably large combinatorial diversity of chemistry remains beyond our capabilities, even with an exceedingly high throughput. The future of solid-state synthesis will be found in a union of these two conceptual approaches.
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