Superconductor-(Metal)-Insulator Transitions: Understanding the Emergence of Anomalous Metallic States

非技术摘要：金属的导电性是由杂质散射决定的，由一个称为德鲁德公式的简单表达式给出。正如Bardeen-Cooper-Schriefer在他们著名的BCS理论中所解释的那样，在低温下，这种金属中的电子也可能形成一种奇特的状态，即玻色库珀对的凝聚体。然而，尽管有大量的实验数据支持上述“标准范式”，但实验数据中的反常现象，特别是在二维极限(2D)中的反常现象，一直在积累。很明显，在低温下存在一类新的金属状态，其中的电子性质不能根据现有的理论来理解。该项目的重点是研究在低温下出现的新的金属状态，主要是在超导态附近。该项目包括培养研究生和一名博士后学者。该项目还将寻求延续这一传统，积极招募女研究生和博士后加入该小组，同时保持强大的国际合作。技术摘要：金属状态的定义是，随着温度接近零度，导电性保持有限的状态。有一种非常成功的“费米液体理论”，即具有长平均自由程和相对较弱相互作用的清洁3D金属。在这个理论中，费米子激发(准粒子)在费米能级具有有限的态密度，而玻色子激发(如零声)在低温下作用较小，因为它们对电流的贡献可以忽略不计，因为它们的态密度为零。相对纯净的3d金属的低T电导率由杂质散射决定，并由Drude公式给出。另一个久负盛名的范例是超导性的BCS理论，它基于这样的想法，即在某些情况下，电子吸引可以主导电子排斥，因此在低温下，电子形成玻色子库珀对的凝聚体。正是这种凝析油携带着超强电流。当控制电子环境的参数改变时，系统可能会发生超导体到金属的转变，在零温下这是一个量子转变。传统的金属理论还预言，当系统从金属一侧接近BCS超导态时，其性质根本不能反映超导相的接近程度，系统的电导率受金属相中处处的费米子激发所控制。尽管有大量的实验数据支持上述“标准范式”，特别是在块状超导体上，但实验数据中的异常现象，特别是在二维极限中，已经积累起来。很明显，在零温度极限中存在一类新的金属态，其中的电子性质不能以费米液体和德鲁德理论为基础来远程理解。这一观察结果本身就令人震惊，因为它为金属的电子性质指出了一个新的范式。探索这种反常金属状态的性质是这个项目的重点。这将通过制备各种薄膜超导体来实现，并研究它们的输运特性。虽然过去这种研究只侧重于纵向传导性测量，但这个项目将强调横向传输，并将包括热传输，这可以进一步揭示金属状态的性质。电磁微扰的应用，如磁场和射频电场，将被用来调节这些金属状态，或者在某些情况下促进它们。该项目培养一到两名研究生，一名博士后学者。该项目还将寻求延续这一传统，积极招募女性研究生和博士后加入该组织，同时保持强大的国际合作。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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:Conductivity of metals is determined by impurity scattering, and is given by a simple expression known as Drude Formula. As explained by Bardeen-Cooper-Schriefer in their celebrated "BCS" theory, at low temperatures electrons in such metals may also form an exotic state known as a condensate of bosonic Cooper pairs. However, despite the vast experimental data supporting the above "standard paradigm", anomalies in experimental data, particularly in the limit of two-dimensions (2D), have been accumulating. It became clear that there exists a new class of metallic states at low temperatures, where the electronic properties cannot be understood on the basis of existing theories. The project focuses on the investigations of novel metallic states that emerge at low temperature, primarily in proximity to superconducting states. The project includes training of graduate students, and a post-doctoral scholar. The project will also seek to continue this tradition by actively recruiting women graduate students and post-docs to the group, while also maintaining strong international collaborations.Technical Abstract:A metallic state is defined as a state in which the conductivity remains finite as the temperature tends to zero. There is an extraordinarily successful "Fermi liquid theory" of clean 3D metals with long mean free path and relatively weak interactions. In this theory fermionic excitations (quasiparticles) have a finite density of states at the Fermi level, while bosonic excitations (e.g. zero sound) have lesser role at low temperatures, since their contribution to the current is negligible due to their vanishing density of states. The low-T conductivity of relatively pure 3D metals is determined by impurity scattering, and is given by the Drude formula. Another well-established paradigm is the BCS theory of superconductivity, which is based on the idea that under some circumstances electron attraction can dominate the electron repulsion so that at low temperatures electrons form a condensate of bosonic Cooper pairs. It is this condensate that carries the supercurrent. As parameters controlling the electronic environment change, the system may exhibit a superconductor to metal transition, which at zero temperature is a quantum transition. The conventional theory of metals also predict that as the system approaches the BCS superconducting state from the metallic side, its properties in no way reflect the proximity of the superconducting phase, and the conductivity of the system is controlled by the fermionic excitations everywhere in the metallic phase. Despite the vast experimental data supporting the above "standard paradigm", especially on bulk superconductors, anomalies in experimental data, particularly in the limit of two dimensions, have been accumulating. It became clear that there exists a new class of metallic states in the zero-temperature limit, where the electronic properties cannot remotely be understood on the basis of Fermi liquid and Drude theories. This observation by itself is astonishing because it points to a new paradigm for the electronic properties of metals. Exploring the properties of such anomalous metallic states is the focus of this project. This will be done by fabricating a variety of thin-films superconductors, and study their transport properties. While in the past such studies focused solely on longitudinal conductivity measurements, this project will emphasize transverse transport, and will include thermal transport, which can further shed light on the nature of the metallic state. The application of electromagnetic perturbations such as magnetic and RF electric fields will be used to tune through these metallic states, or in some circumstances promote them. The project trains one or two graduate students, one post-doctoral scholar. The project will also seek to continue this tradition by actively recruiting women graduate students and post-docs to the group, while also maintaining strong international collaborations.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.