Quantum Phase Transition in Superconducting Nanowires and Films

非技术性抽象。 当一种物质或量子物体被冷却到零温度并受到压力和磁场的作用时，它可以从一种物质状态转变为另一种物质状态，例如从铁磁状态转变为非磁性状态。这种转变被称为量子相变（QPT）;它们在自然界中广泛存在，并且令人惊讶的是，它们可以在非常不同的物体中具有普遍的行为，例如原子核，恒星和地球上发现的普通材料。尽管如此，QPT的许多重要特征还没有得到很好的理解。本研究的对象，纳米线和薄膜，在磁场的作用下或通过尺寸减小，进行超导-正常金属QPT。该团队仔细研究了这些转变，目的是揭示QPT的普遍特征，以及控制超导系统转变的特定微观过程。除了提出有趣的基本问题外，超导纳米线还可以直接应用于天文观测和安全量子通信的探测器。该研究小组与美国国家标准与技术研究所的微弱光子学小组合作，改进这些探测器，并在探测器的设计中利用新的物理原理。在更广泛的范围内，从事该项目的学生在高精度电气仪器和纳米纤维方面接受了出色的培训。PI加入了犹他州访问计划，每年提供约40名大一女本科生在教育和研究的绝佳机会的大学。作为该计划的一部分，PI担任学生的研究顾问。PI和他的学生在犹他州科学奥林匹克和本科生研究研讨会上担任志愿者评委。PI的研究解决了纳米超导和QPT领域的几个突出问题。(i)实验观察到的超导纳米线临界温度的抑制比现有理论的预测强100倍。为了解决这个问题，该团队研究了几何约束对Tc（通过传输测量），超导间隙（通过隧道测量）和超流密度（通过动力学电感测量）的影响，在一系列MoGe和Al纳米线中，宽度可达10 nm。数据的组合体用于理论修正。(ii)为了揭示纳米线和薄膜中控制QPT的微观过程，该团队在MoGe和Nb薄膜和导线中磁场驱动和尺寸驱动转变的临界状态下进行了隧道和传输测量。其中一个目标是了解尺寸引起的障碍如何成为库珀配对的破坏者。(iii)我们制作了一系列具有内建强无序的纳米线，期望它们能以玻色过程为主，并呈现出虚幻的玻色绝缘体态。(iv)该团队对纳米线进行了宽带，高频研究，以解决单个相位滑移事件并表征其统计数据。这些统计数据预计将揭示相位滑移和“列车”效应之间的相互作用。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Non-technical abstract. When a material or a quantum object is cooled to ZERO temperature and is acted upon, say by pressure and magnetic field, it can transform from one state of matter to another, for example, from ferromagnetic state to non-magnetic state. Such transformations are termed quantum phase transitions (QPT); they are widespread in nature and, surprisingly, can have universal behavior in very diverse objects, such as nuclei, stars, and ordinary materials found on Earth. Still, many important characteristics of QPTs are not well-understood. The subject of this research, nanowires and thin films, undergo superconductor – normal metal QPT under the action of magnetic field or by size reduction. The team carefully studies these transformations with the goal to uncover both the universal character of QPT, but also specific microscopic processes that govern the transition in superconducting systems. Besides posing interesting fundamental questions, superconducting nanowires have immediate application in detectors used for astronomical observation and secure quantum communication. The research team collaborates with Faint Photonics Group of National Institute of Standards and Technology to improve these detectors and utilize new physical principles in detectors’ design. On a broader scale, students working on the project receive excellent training in high precision electrical instrumentation and nanofabrication. The PI has joined the University of Utah ACCESS Program that offers approximately 40 freshman female undergraduate students every year an excellent opportunity in education and research. As a part of that program, the PI serves as a research advisor to students. The PI and his students serve as volunteer judges at Utah Science Olympiads and Undergraduate Research Symposiums. Technical abstract The PI’s research addresses several outstanding problems in the field of nanoscale superconductivity and QPT. (i) The experimentally-observed suppression of critical temperature in superconducting nanowires is 100-times stronger than the prediction of existing theories. To resolve this problem, the team studies the effect of geometrical confinement on Tc (by transport measurements), on the superconducting gap (by tunneling measurement), and the superfluid density (by kinetic inductance measurements) in series of MoGe and Al nanowires with width down to 10 nm. The combined body of data is used for a theory revision. (ii) To uncover microscopic processes governing QPT in nanowires and films, the team carries out tunneling and transport measurements in the critical regime of magnetic-field-driven and size-driven transitions in MoGe and Nb films and wires. One of the goals is to understand how size-induced disorder can act as a Cooper pair breaker. (iii) A series of nanowires with the intentionally built-in strong disorder are fabricated with the expectation that they will be dominated by bosonic processes and display illusive Bose-insulator state. (iv) The team carries wide-band, high-frequency studies of nanowires to resolve individual phase slip events and characterize their statistics. These statistics are expected to reveal interaction between phase slips and “train” effects.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.