Superconductivity and Proximity Effect in Two-Dimensional Films and Multilayers

量子相变在理论和实验上一直引起人们的强烈兴趣。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。该奖项支持一个探索非晶MgB2薄膜和多层膜中超导体到绝缘体转变的项目。该系统的独特之处在于能够在很宽的范围内控制超导性的强度，控制无序性，并制造双层和多层。然而，更独特的是，由于镁和硼都很轻，自旋轨道相互作用可能是任何薄膜超导体中可以实现的最低的，这为我们研究其对跃迁的影响提供了新的机会。这项工作有望在几个领域对科学产生影响，包括揭示降维量子相变的本质，更好地理解非晶超导薄膜的材料科学，以及使用新测量技术开发新方法。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。技术摘要二维超导体相图的研究与寻找新材料有着密切的联系。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）。其中包括允许发现“金属相”的非晶MoGe薄膜，允许研究强无序状态的非晶InOx薄膜，以及最近的非晶MgB2薄膜，除了与MoGe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。一种新的、可用的、新颖的技术来制造多层无定形MgB2/MgO，允许进一步探索维度效应。该奖项支持一个探索MgB2模型系统提供的丰富物理的项目。实验将在垂直磁场下研究不同类型的MgB2薄膜双层（例如邻近系统）和多层的SIT。我们将特别关注SIT点附近量子熔化的影响。通过在平行磁场中寻找一阶跃迁和不可逆性特征的测量，探索低自旋轨道相互作用的表现形式。除了输运测量，介观效应附近的过渡将探索使用新的技术，包括扫描隧道电位法和克尔显微镜。这项工作有望对降维量子相变的性质提供新的认识，并影响对非晶超导薄膜材料科学的理解，以及使用新测量技术的新方法的发展。该项目涉及基础物理和未来应用领域的研究生。

Non-Technical AbstractQuantum 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 the superconductor to insulator transition in amorphous MgB2 films and multilayers. The uniqueness of this system is the ability to control the strength of superconductivity over a very wide range, control the disorder, and fabricate bilayers and multilayers. However, more uniquely, due to the lightness of both magnesium and boron, spin orbit interaction is probably the lowest that can be achieved in any thin film superconductor, providing us with new opportunities to study its effect on the transition. The proposed work is expected to impact science in several areas including shedding new light on the nature of quantum phase transitions in reduced dimensions, better understanding of the material science of amorphous superconducting films, and the development of new methodologies using novel measurements techniques. The program involves graduate students in this area of great interest to physics and future applications.Technical AbstractStudies 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, and more recently amorphous- MgB2 films which in addition to the overlap in parameters with MoGe and InOx, also provide a new knob in the form of controlling spin-orbit interaction. A newly available, novel technique to fabricate multilayers of amorphous MgB2/MgO allows further exploration of dimensionality effects. This award supports a project to explore the rich physics this MgB2 model system offers. Experiments will be performed to study SIT in perpendicular magnetic field on MgB2 thin films bilayers of various types (e.g. proximity systems) and multilayers. Specific attention will be given to the effect of quantum melting near the SIT point. The manifestation of low spin-orbit interaction will be explored through measurements in parallel magnetic field searching for signatures of first order transition and irreversibility. In addition to transport measurements, mesoscopic effects near the transition will be explored using novel techniques including scanning tunneling potentiometry and Kerr microscopy. The proposed work is expected shed new light on the nature of quantum phase transitions in reduced dimensions, as well as impact the understanding of the material science of amorphous superconducting films, and the development of new methodologies using novel measurements techniques. The program involves graduate students in this area of great interest to fundamental physics and future applications.