EAGER: Local Structure and Dynamics in Tc-Enhanced Overdoped Cuprates: Beyond BCS?

非技术摘要：超导性是某些材料的一种特性，它们失去了电阻，使它们能够承载电流，而没有正常金属电导体中发生的损失。这使得用于MRI扫描的电磁铁能够运行数周甚至数月而无需补充，超导导线将节省传输中损失的大部分原始能量。问题是，可用于这些应用的超导性仅在非常低的温度下发生，需要昂贵且复杂的制冷设备。然而，这就是我们所知道的超导性，因为它？这是100多年前的发现。我们的团队发现，某种类型的氧化铜超导体可能遵循一套不同的规则，特别是一种化合物显示出与现有理论相矛盾的行为。第二种类型的超导性可以提供见解，甚至是手段，以延长超导性到更高的温度，以节省能源，大大提高效率的电气设备。技术摘要：回顾铜酸盐文献揭示了令人惊讶的事实，相图是“通用的”，因为YBa（2-x）Srx（CuO 2）2（Cu（1-y）My）O（7+ δ）类的“过掺杂”铜酸盐已被忽略。当使用氯酸盐或过氧化物通过高压氧（HPO）方法制备时，它们的转变温度在它们的50-80 K范围内连续增加，即使它们的O化学计量增加以在CuO 2铜上产生超过0.27值的过量电荷，其中常规材料中的超导性结束。由于HPO方法仅产生细粉末而不是晶体，因此只有少数技术适用于他们的研究，包括X射线吸收精细结构测量，这是该项目团队的主要能力，Cu相邻原子对表现出与c方向上的超导性相关的高度非谐行为，包括非简并Cu 2-顶端O双阱势。这种势的重整化导致其两个分量之间的能量差跟踪差距能量到团队测量的Tc-30 K下限。甚至更激进的是Sr2CuO3.3的结果，La 2CuO 4 + δ结构类似物。静态结构只包含部分CuO 2平面，在所有其他铜酸盐的完全有序的相反。这些被更复杂的三维Cu-O结构所取代。此外，超导转变伴随着结构转变发生，该结构转变是一组这些O离子的移位，其足够大以通过远大于差距的能量来修改费米表面。这将是传统BCS理论的一个实质性扩展，甚至可能预示着一种新的配对和凝聚机制。这个EAGER项目要解决的关键问题是与传统的铜酸盐直接比较，延伸到较低的温度，并在补充Sr边缘测量。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Non-technical Abstract:Superconductivity is a property of certain materials in which they lose electrical resistance, giving them the capability of carrying electric currents with none of the losses that occur in normal metal electrical conductors. This enables electromagnets such as the ones used for MRI scans to operate for weeks or even months without having to replenish them, and superconducting wires would save the large fraction of the original energy that is lost in transmission. The problem is that superconductivity that can used in these applications only occurs at very low temperatures, requiring expensive and complicated refrigeration equipment. This is, however, superconductivity as we have known it since it?s discovery just over 100 years ago. Our team has found that a certain class of copper-oxide superconductors may follow a different set of rules, and in particular one compound shows behaviors that contradict the existing theory. A second type of superconductivity could provide insights or even the means to extend superconductivity to higher temperatures to save energy by substantially increasing the efficiency of electrical devices.Technical Abstract:A review of the cuprate literature reveals the surprising fact that the phase diagram is "universal" because the YBa(2-x)Srx(CuO2)2(Cu(1-y)My)O(7+delta) class of "overdoped" cuprates has been neglected. When prepared by the High Pressure Oxygen (HPO) method using chlorate or peroxide their transition temperatures increase continuously within their 50-80 K range even as their O stoichiometry is increased to give an excess charge on the CuO2 coppers well past the 0.27 value where the superconductivity ends in conventional materials. Since the HPO method only produces fine powders and not crystals only a few techniques are amenable to their study, including the X-ray Absorption Fine Structure measurements that are the principal capability of this project team was that the Cu-neighbor atom pairs exhibit highly anharmonic behavior correlated with the superconductivity in the c direction, including a non-degenerate Cu2-apical O double well potential. Renormalization of this potential causes the energy difference between its two components to track the gap energy to the Tc-30 K lower limit of the team's measurements. Even more radical are results from Sr2CuO3.3, a La2CuO4+delta structural analog. The static structure contains only partial CuO2 planes, in contrast to the fully ordered ones in all other cuprates. These are replaced by more complex, three-dimensional Cu-O structures. In addition, the superconducting transition occurs concomitantly with a structural transformation that is a shift of a set of these O ions that is sufficiently large to modify the Fermi surface by an energy far larger than that of the gap. This would be a substantial expansion of conventional BCS theory, or possibly even indictative of a novel pairing and condensation mechanism. Crucial issues to be addressed by this EAGER project would be a direct comparison with conventional cuprates, extension to lower temperatures, and measurements at the complementary Sr edge.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.