Un-particle superconductivity in low-dimensional materials

超导现象是一个多世纪以前发现的。在20世纪的过程中，研究人员开始发现它的无数显着特性，包括无损，高功率电力传输，磁悬浮和约瑟夫森隧道（用于精确确定基本常数）。在21世纪，超导被广泛认为是量子计算前沿发展的关键参与者。在理论方面，超导的权威理论是由诺贝尔奖得主约翰·巴丁、利昂·库珀和鲍勃·施里弗在半个多世纪前发表的。BCS理论被证明是非常成功的，不仅解释了许多已知超导体的性质，而且还为寻找新的超导体提供了指导，即使是那些具有非常规或各向异性配对对称性的超导体。然而，随着时间的推移，出现了许多超导材料，它们似乎在挑战BCS模板。值得注意的是，它们的超导性能在许多方面都显得更为优越。BCS理论的基本概念是，库珀配对是由具有长平均自由程的相干电子态组成的“好”金属的不稳定性。然而，在过去的几十年里，在“坏金属”或“奇怪金属”（即不符合金属行为标准模型的金属）中也发现了超导性。坏金属的特点是电子平均自由程（在高温下）减少到原子间距离的一小部分，而奇怪的金属表现出电阻率随温度线性增长，有效地从绝对零度一直到它们的熔点，并且在磁场中的响应遵循完全不同的幂律依赖，这在传统金属中看到。现在的核心问题是BCS理论是否可以解释坏金属或奇怪金属中超导性的出现，或者是否需要一个全新的范式。坏金属和/或奇怪金属中的电子态位于相干/非相干边界的事实表明，这些材料中超导的凝聚能可能来自动能的节省，而不是像BCS超导体那样来自势能的节省，并且超流体凝聚可能来自非相干部分，而不是电子自能的相干部分。我们称这种替代模式为“非粒子超导”。该提案的目标是通过联合实验/理论研究计划探索候选材料中非粒子超导性的可行性，该计划旨在为电子谱函数的非相干部分形成的电子态配对建立理论框架，并通过精确测量其超流体密度和载流子密度（包括相干和非相干）来测试由此产生的预测。即非超导态。总的来说，三种不同的材料类别已被确定为实现非粒子超导的候选材料：氧化铜高温超导体，硫属铁和一维紫青铜。值得注意的是，铜酸盐的超导性在35年前就被发现了，尽管在凝聚态物质中已经受到了各种实验和理论技术的影响，但bcs型超导性的确凿证据仍然难以捉摸。此外，铜酸盐和硫属铁是唯一已知的在液氮沸点以上的环境压力下以单层形式超导的材料，这使得它们作为未来量子计算设备的平台非常有吸引力。最后，我们的研究目标的实现将导致（高温）超导的新范式，与原始的BCS模板相去甚远。

The phenomenon of superconductivity was discovered over a century ago. Over the course of the 20th century, researchers began to unearth its myriad of remarkable properties, including loss-less, high power electrical transmission, magnetic levitation and Josephson tunneling (used to determine fundamental constants with exquisite accuracy). In the 21st century, superconductivity is widely recognised as a pivotal player in the frontier development of quantum computation. On the theoretical side, the definitive theory of superconductivity was published by Nobel laureates John Bardeen, Leon Cooper and Bob Schrieffer over half a century ago. BCS theory proved to be remarkably successful, not only in explaining the properties of many known superconductors, but also in serving as a guide in the search for new superconductors, even those with an unconventional or anisotropic pairing symmetry. Over time, however, a number of superconducting materials have emerged that appear to challenge the BCS template. Significantly, their superconducting properties appear, in many respects, to be superior. Fundamental to BCS theory is the notion that Cooper pairing is an instability of a 'good' metal composed of coherent electronic states with long mean free path. Over the past few decades, however, superconductivity has also been discovered in 'bad' or 'strange metals', i.e. metals that do not conform to the standard models of metallic behaviour. Bad metals are characterized by an electron mean free path (at high temperatures) that diminishes to a fraction of the interatomic distance, while strange metals exhibit an electrical resistivity that grows linearly with temperature effectively from absolute zero right up to their melting point and a response in a magnetic field that follows an entirely different power law dependence to that seen in conventional metals.The core question now is whether BCS theory can account for the emergence of superconductivity in bad or strange metals or whether an entirely new paradigm is required. The fact that the electronic states in bad and/or strange metals lie at the coherent/incoherent boundary suggests that the condensation energy for superconductivity in these materials may derive from a saving in kinetic energy, rather than a saving in potential energy as is the case for BCS superconductors and that the superfluid condensate may emerge from the incoherent, rather than the coherent part of the electron self-energy. We call this alternative paradigm 'un-particle superconductivity'. The goal of this proposal is to explore the viability of un-particle superconductivity in candidate materials via a joint experimental/theoretical research programme that seeks to develop a theoretical framework for pairing of electronic states formed from the incoherent part of the electron spectral function and to test the resulting predictions with precise measurements of their superfluid density and carrier densities (both coherent and incoherent) in the normal, i.e. non-superconducting state. In total, three distinct material classes have been identified as candidate materials for the realization of un-particle superconductivity: copper-oxide high temperature superconductors, iron chalcogenides and one-dimensional purple bronze. Notably, superconductivity in the cuprates was discovered over 35 years ago, yet despite having been subject to the whole spectrum of experimental and theoretical techniques in condensed matter, smoking-gun evidence for BCS-type superconductivity remains elusive. Moreover, cuprates and iron chalcogenides are the only known materials to superconduct in monolayer form and at ambient pressures at temperatures above the boiling point of liquid nitrogen, making them highly attractive as platforms for future quantum computing devices. Finally, fulfillment of our research goals would lead to a new paradigm for (high temperature) superconductivity, one far-removed from the original BCS template.