Superconductivity and Proximity Effect in Two-Dimensional Films and Multilayers

非技术摘要量子相变继续吸引着人们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根本不同的量子基态的转变--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，比如量子计算的局限性。量子相变的一个范例是二维薄膜中的超导体到绝缘体的转变。根据所用材料的固有性质，这种转变有多种实现方式。控制薄膜的超导强度、无序或尺寸将影响转变的性质，这是通过改变外部参数如温度和磁场来实现的。该奖项支持一个探索非晶态硼化镁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的强度，控制无序，并制备双层和多层膜。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最低的，这为我们提供了新的机会来研究它对相变的影响。预计拟议的工作将影响几个领域的科学，包括揭示降维量子相变的性质，更好地理解非晶态超导薄膜的材料科学，以及使用新的测量技术开发新的方法。该项目涉及这一领域的研究生，他们对物理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密切相关。对于每个材质系统，会高亮显示一组不同的属性。多年来，NSF的支持使各种材料得以优化，用于研究超导薄膜，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-MgB_2薄膜，它除了与MOGE和Inox在参数上重叠外，还以控制自旋-轨道相互作用的形式提供了一个新的旋钮。一种新的可用于制备非晶态MgB_2/MgO多层膜的新技术允许进一步探索尺寸效应。该奖项支持一项探索该模型系统所提供的丰富物理知识的项目。实验将研究垂直磁场中不同类型的双层膜(如接近系统)和多层膜上的SiT_2。我们将特别关注SIT点附近的量子熔化效应。低自旋-轨道相互作用的表现形式将通过在平行磁场中的测量来探索，寻找一级跃迁和不可逆性的信号。除了输运测量外，还将使用包括扫描隧道电位法和克尔显微镜在内的新技术来探索相变附近的介观效应。这项拟议的工作有望为揭示降维量子相变的性质提供新的线索，并将影响对非晶超导薄膜材料科学的理解，以及使用新的测量技术开发新的方法。该项目涉及这一领域的研究生，他们对基础物理和未来的应用非常感兴趣。

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.