Exploring Quantum Effects in Two-Dimensional Multilayered Superconductors

****技术摘要****二维超导体相图的研究与寻找新材料密切相关。每个材料系统都有一组不同的属性。多年来，美国国家科学基金会的支持使各种材料的优化研究超导体薄膜，特别是超导体到绝缘体的转变（SIT）。其中包括可以发现“金属相”的非晶MoGe薄膜，可以研究强无序状态的非晶InOx薄膜，可以详细研究过渡附近霍尔效应的非晶TaNx薄膜，以及非晶MgB2薄膜，除了与MoGe， TaNx和InOx的参数重叠外，还提供了控制自旋轨道相互作用的新形式。这个项目探索了超导体-绝缘体过渡附近的丰富相图，重点是中间的金属态。特别是，我们将集中理解在金属背景（即耗散）存在下二维SIT的真实相图是什么；二维SIT系统的电子微观结构是什么；从输运效应、霍尔效应、能斯特效应等实验中可以推断出不同的长度尺度；并且，从由金属膜上有序的金属点制成的人造薄膜开始，当我们增加无序度时，这种方法的有效性范围是什么？这项工作有望揭示降维量子相变的本质，并影响对非晶超导薄膜材料科学的理解。****非技术摘要****量子相变继续引起强烈的理论和实验兴趣。这种转变——改变哈密顿量中的外部参数会导致从一个量子基态到另一个量子基态的转变，从根本上不同——已经被用来理解许多电子系统中的实验数据，以及分析更具挑战性的量子现象，如量子计算的局限性。量子相变的一个范例是二维薄膜中超导体到绝缘体的转变。根据所使用材料的固有特性，发现这种转变具有多种实现方式。控制薄膜的超导性强度、无序性或维度将影响转变的性质，这是通过温度和磁场等外部参数的变化来实现的。该奖项支持一个项目，探索非晶薄膜中超导体到绝缘体过渡附近的一系列独特量子态，特别是非晶moge， MgB2， InOx和TaNx。这组系统的独特之处在于能够在非常广泛的参数范围内控制超导性和无序性的强度，特别是允许访问我们小组首次确定的不为人知的普遍存在的金属态。该计划涉及的研究生在这一领域对物理学和未来的应用非常感兴趣。

****Technical Abstract****Studies of the phase diagram of two-dimensional superconductors has been intimately connected to finding new materials. With each materials system a different set of properties is highlighted. Over the years, NSF support enabled the optimization of a variety of materials for studies of thin superconducting films in general and the superconductor to insulator transition (SIT) in particular. These include amorphous-MoGe films that allowed the discovery of "metallic phases," amorphous-InOx films that allowed the studies of the regime of strong disorder, amorphous TaNx that allowed for the detailed study of the Hall effect near the transition, and amorphous- MgB2 films which in addition to the overlap in parameters with MoGe, TaNx, and InOx, also provide a new knob in the form of controlling spin-orbit interaction. This project explores the rich phase diagram near the superconductor-insulator transition with emphasis on the intervening metallic states. In particular we will concentrate on understanding of what is the true phase diagram of two-dimensional SIT in the presence of a metallic background (i.e. dissipation); what is the electronic microstructure of a 2D SIT system; can we infer the various length scales from experiments such as transport, Hall and Nernst effects, etc.; and, starting from artificial films made of orderly metallic dots on a metal films, what is the range of validity of this approach as we increase disorder. The proposed work is expected to shed new light on the nature of quantum phase transitions in reduced dimensions and impact the understanding of the material science of amorphous superconducting films.****Non-Technical Abstract****Quantum phase transitions continue to attract intense theoretical and experimental interest. Such transitions -- where changing an external parameter in the Hamiltonian induces a transition from one quantum ground state to another, fundamentally different one -- have been invoked to understand experimental data in many electronic systems, as well as to analyze more challenging quantum phenomena such as the limitations of quantum computing. A paradigm for quantum phase transitions has been the superconductor to insulator transition in two-dimensional films. This transition is found to have a variety of realizations depending on intrinsic properties of the materials used. Controlling the strength of superconductivity, disorder, or dimensionality of the films will impact the nature of the transition, which is accessed through the variation of external parameters such as temperature and magnetic field. This award supports a project to explore a series of unique quantum states near the superconductor to insulator transition in amorphous films, particularly amorphous-MoGe, MgB2, InOx and TaNx. The uniqueness of this set of systems is the ability to control the strength of superconductivity and disorder over a very wide range of parameters, and and in particular allow access to the poorly understood ubiquitous metallic states that have been first identified by our group. The program involves graduate students in this area of great interest to physics and future applications.