Superconductor-(Metal)-Insulator Transitions: Understanding the Emergence of Metallic States, A Continuation Proposal

摘要：最近越来越多的实验揭示了一种可能的“异常金属态”（AMS），它表现出在固态系统中电子的“标准范式”基础上无法理解的性质。该项目旨在探索异常金属相的出现程度及其与失效超导和/或绝缘状态的关系。此外，该项目探讨了AMS在多种情况下的可能变体。研究结果有望揭示一类新的物质金属态，并将对物理学这一学科、教育和社会产生影响。这表明人们对自然界的一种基本物质缺乏充分的了解，它应该会引发社会各阶层的好奇心，从而有助于提高我们的科学认识，并可能影响技术应用。技术摘要：最近在无序超导薄膜上进行的越来越多的实验，最初是寻找超导体-绝缘体转变（SIT），已经指出了从超导体到具有“失效超导体”特性的“异常金属态”（AMS）的零温度转变的可能性。这种状态表现出显著的超导相关性，然而系统即使在零温度下也不能全局凝聚，通过量子超导体到金属跃迁（QSMT）稳定在有限的电导率。此外，在研究强非均匀系统时，超导岛的充电能量通常会通过库仑阻塞导致绝缘状态，当电荷被削弱时，会经历电荷波动，从而阻碍了真正绝缘体的建立。随着对“标准范式”偏差的观察，特别是在二维极限中，已经迅速积累，很明显，当温度趋于零时，存在一类新的金属态，其中的电子性质无法根据费米液体和德鲁德理论来理解，从而指出了自然界金属态的新范式。本项目的重点是探索SIT和异常金属态的出现及其性质。这是通过关注使用高颗粒膜或非常规超导体（如超导体/铁磁体双层）增强相波动材料，以及低温磁输运测量（包括对外部扰动的灵敏度）和用于研究超流体响应的改进互感探头来实现的。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。

Nontechnical Abstract: An increasing number of recent experiments reveal a possible “anomalous metallic state” (AMS), which exhibits properties that cannot be understood on the basis of “standard paradigms” of electrons in solid-state systems. The project aims to explore the extent of the emergence of the anomalous metallic phase and its relation to a failed superconducting and/or insulating states. Furthermore, the project explores possible variants of the AMS in multiple cases. Results are expected to shed light on a new class of metallic states of matter and will have an impact on physics as a discipline, on education and on society. Pointing to a lack of full understanding of one of the fundamental types of substances in nature, it ought to trigger curiosity at all levels of society, thus helping to enhance our scientific understanding and possible impact technological applications.Technical Abstract: An increasing number of recent experiments on disordered superconducting films, initially searching for a superconductor-insulator transition (SIT), have been pointing to the possibility of a zero-temperature transition from a superconductor to an “anomalous metallic state” (AMS) that features properties of a “failed superconductor.” This state exhibits significant superconducting correlations, yet the system fails to globally condense even at zero temperature, settling at a finite conductivity through a quantum superconductor to metal transition (QSMT). Furthermore, where strongly inhomogeneous systems are studied, charging energy of the superconducting islands, which typically leads to an insulating state through Coulomb blockade, can experience charge fluctuations when weakened, preventing the establishment of a true insulator. As observation of deviations from the “standard paradigm,” particularly in the limit of two-dimensions, have been fast accumulating, it becomes clear that there exists a new class of metallic states as the temperature tends to zero, where the electronic properties cannot remotely be understood on the basis of Fermi liquid and Drude theories, thus pointing to a new paradigm for metallic states in nature. Exploring SIT and the emergence of anomalous metallic states and their properties are the focus of this projects. This is achieved through focus on enhanced phase fluctuations materials either using highly granular films or unconventional superconductors such as superconductor/ferromagnet bilayers, as well as low-temperature magneto-transport measurements including sensitivity to external perturbations and improved mutual-inductance probe for the study of superfluid response.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.