Dai P, Mook HA, Aeppli G, Hayden SM, Dogan F
Oak Ridge National Laboratory, Tennessee 37831-6393, USA.
Nature. 2000 Aug 31;406(6799):965-8. doi: 10.1038/35023094.
One of the most striking properties of the high-transition-temperature (high-Tc) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-Tc superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy. For YBa2Cu3O(6+x), where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6-11). Here we show that for underdoped YBa2Cu3O6.6, where x and Tc are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-Tc superconductivity. The persistence of a field effect above Tc favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors, awaiting transition to the superconducting state.
高温超导体最显著的特性之一是它们都源自绝缘反铁磁母体化合物。这些氧化铜材料中磁性与超导性之间的紧密关系从一开始就引起了研究人员的兴趣，因为在传统超导体中不存在这种关系。这种联系的证据来自中子散射实验，该实验表明高温超导体中明确存在短程反铁磁关联（激发）。即便如此，这种激发在超导配对机制中的作用仍是一个有争议的话题。对于YBa2Cu3O(6+x)，其中x控制空穴掺杂水平，磁激发谱中最显著的特征是一个尖锐的共振（参考文献6 - 11）。在此我们表明，对于欠掺杂的YBa2Cu3O6.6（其中x和Tc低于其最佳值），适度的磁场会显著抑制共振，对于大致垂直于CuO2平面的磁场，抑制作用比平行磁场更强。我们的结果表明，这种共振衡量了配对和相位相干性，这表明磁性在高温超导中起着重要作用。高于Tc时场效应的持续存在有利于这样的机制，即超导电子对在欠掺杂氧化铜超导体的正常态中预先形成，等待转变为超导态。
