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

摘要：超导性是某些材料的一种特性，它们失去了电阻，使它们能够在没有普通金属电导体损失的情况下携带电流。这使得用于核磁共振扫描的电磁铁可以工作数周甚至数月而无需补充，超导导线将节省传输过程中损失的大部分原始能量。问题是，超导只能在非常低的温度下使用，需要昂贵而复杂的制冷设备。然而，这就是我们所知道的超导性。这是美国100多年前的发现。我们的研究小组发现，一类氧化铜超导体可能遵循一套不同的规则，特别是一种化合物表现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，甚至可以通过大幅提高电子设备的效率，将超导性扩展到更高的温度，从而节省能源。技术摘要：对铜酸盐文献的回顾揭示了一个令人惊讶的事实，即相图是“通用的”，因为“过掺杂”铜酸盐的YBa(2-x)Srx(CuO2)2(Cu(1-y)My)O（7+delta）类被忽略了。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50-80 K范围内持续增加，即使它们的O化学计量量增加，使CuO2铜上的过量电荷远远超过0.27值，而在传统材料中超导性终止。由于HPO方法只产生细粉末而不是晶体，只有少数技术适合他们的研究，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邻居原子对表现出与c方向超导性相关的高度非调和行为，包括非简并的cu2 -顶O双阱势。该势的重整化导致其两个分量之间的能量差，以跟踪间隙能量到团队测量的tc - 30k下限。更激进的结果来自Sr2CuO3.3，一种La2CuO4+ δ结构类似物。静态结构只包含部分CuO2平面，而其他铜酸盐则是完全有序的。它们被更复杂的三维Cu-O结构所取代。此外，超导跃迁伴随着一种结构转变，这种结构转变是一组O离子的位移，其能量大到足以改变费米表面，其能量远远大于间隙的能量。这将是对传统BCS理论的实质性扩展，甚至可能预示着一种新的配对和凝聚机制。这个EAGER项目需要解决的关键问题是与传统铜酸盐的直接比较，扩展到更低的温度，以及在互补Sr边缘的测量。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.