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[转载] 日本发现电子自旋是高温超导发生关键
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Superconductivity: A magnetic isotope effect Michael Norman1 Michael Norman is in the Materials Science Division, Argonne National Laborato ry, Argonne, Illinois 60439, USA. e-mail: norman@anl.gov AbstractThe role of phonons in conventional superconductivity — first determi ned by isotope substitution — has been known for over half a century. But ide ntifying the mechanism in unconventional superconductivity is a much more chal lenging affair. In 1950, two key experiments identified the source of superconductivity in con ventional superconductors1, 2, the results of which Fröhlich predicted ar ound the same time3. By substituting one isotope for another, researchers obse rved a change in the transition temperature. This clearly pointed to an involv ement of the lattice in the superconductivity. Seven years later, based on the se pioneering results, Bardeen, Cooper and Schrieffer conceived the correct mi croscopic theory4. On page 27 of this issue, Terashima et al. demonstrate a di fferent type of isotope effect that relies on substitution of one kind of magn etic ion for another5. Consequently, they suggest that high-temperature superc onductivity results from magnetic, rather than lattice, interactions. Copper oxide superconductors are composed of metallic CuO2 layers (see Fig. 1) separated by non-metallic spacer layers. The electronic structure near the Fe rmi energy consists of a single energy band that is a mixture of copper d and oxygen p orbitals. It is well known that replacement of the copper ions by oth er ions (zinc, nickel, cobalt) has a strong influence on the superconducting p roperties. But we know that impurities break Cooper pairs in unconventional su perconductors, so this is not an unusual result. On the other hand, this subst itution also has a large impact on the magnetic properties of these materials. For instance, a very prominent spin-1 resonance seen in inelastic neutron sca ttering experiments is strongly altered when zinc is substituted for copper6. Figure 1 - CuO2 plane in high-temperature superconductors. The blue atoms are copper ions; the green atoms, oxygen. In the experiment, a magnetic copper ion is replaced by either a non-magnetic zinc ion or a magneti c nickel ion (red atom). This influences the spectrum of magnetic excitations measured by inelastic neutron scattering, and also anomalies in the energy–mo mentum relation of the electrons observed by angle-resolved photoemission. The implication is that magnetic interactions may indeed be responsible for high- temperature superconductivity in these materials. Full size image (4 KB) Figures & tables index To appreciate the significance of this result, we return to affairs half a cen tury ago. The same electron–lattice interaction that gives rise to supercondu ctivity also affects the dispersion of the energy bands (that is, their energy versus momentum relation). In the early 1960s, tunnelling measurements exhibi ted this 'strong coupling' effect that nailed down the theory for conventional superconductors. Now, fifty years later, a debate has emerged concerning simi lar strong-coupling effects seen in the high-temperature superconductors, in p articular with regard to angle-resolved photoemission spectroscopy that direct ly measures the energy–momentum relation of the electrons7. Several photoemis sion groups advocate that these strong coupling features are indeed due to lat tice interactions, as for conventional superconductors. But others argue for m agnetic interactions. In particular, they believe the spin-1 resonance is resp onsible for the observed 'kink' in the energy dispersion. The lattice interaction hypothesis has been tested in an experiment published last year8, in which the kink shifted on substitution of 16O for 18O. One woul d think this would have settled the debate, but those measurements have genera ted a lot of controversy. This has led researchers to consider the second — m agnetic — possibility. But how to test it? As mentioned above, one way to do this is to replace the magnetic copper ion by another type of ion, and see whe ther there is a correlation between the change in magnetic behaviour and that of the energy dispersion of the electrons. This is indeed what Terashima and co-workers observe5. Their substitution of z inc in place of copper influences the dispersion kink in much the same way tha t the spin-1 resonance is affected6 (as seen in neutron scattering measurement s). And a somewhat different influence is found on the kink upon nickel substi tution, again correlating with a similar difference in the neutron data. As su ch, the experiment reported by Terashima et al. provides strong evidence that the kink is caused by the spin-1 resonance. If so, this implies that magnetic interactions are indeed the source of high-temperature superconductivity. In s upport of these conclusions, similar results have been reported recently by an other photoemission group9. As with the experiment reported in ref. 8, there are other possible interpreta tions of the data. Some years ago, the Stanford group observed an influence on photoemission spectra due to zinc substitution for copper10. Their view, howe ver, was that zinc substitution enhances charge inhomogeneity, which gives ris e to the observed changes. Regardless, the 'magnetic isotope effect' reported by Terashima et al. will probably generate much debate in the coming years, an d has brought us one step closer, perhaps, to the microscopic theory of these fascinating superconductors. |
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