Quantum Phase Transition in one-dimensional superconductors

非技术摘要：该项目的总体目标是了解纳米线中的超导电性如何受到它们的几何形状、材料性质和外部磁场的存在的影响。了解这些因素如何影响超导纳米线的性质，是改进纳米线基单光子探测器以及缩小经典和量子超导电路规模所必需的。该项目对基础科学也很重要。我们的团队最近发现，与经典的水-冰相变类似，纳米线也经历了相变；然而，由于它发生在零温度，它属于量子相变(QPT)的一类。该项目的主要目标之一是揭示这一过渡尚不为人所知的本质。一维系统在物理学中扮演着特殊的角色，超导纳米线是其中的一个典型例子，因为它们往往比高维系统的对应系统更容易进行理论描述。因此，该项目有可能促进对超导薄膜和其他发生QPT的系统的物理理解，如磁性材料、液晶和冷原子气体。该项目的教育部分包括对学生进行纳米制造和精密电子仪器方面的培训，以及为参加犹他大学入学方案的本科生担任研究顾问。公众宣传包括在犹他大学和犹他州自然历史博物馆进行的关于超导和纳米科学的口头和展览演示，这是NSF资助的公共门户计划的一部分。技术摘要：该团队的研究集中在一维超导的四个问题上。(I)实验确定的对超导纳米线临界温度的抑制比目前理论预测的要强100倍。为了解决这个问题，该团队将系统地研究几何限制对MOGE和Al纳米线的临界温度(通过输运测量)、超导带隙(通过隧道传输)和超流密度(通过动力学电感测量)的影响。目标是确定抑制的主要机制，并与理论家合作，为纳米线和薄膜开发正确的费米子理论。(Ii)在磁场作用下，MOGe纳米线经历了性质尚不清楚的量子相变。为了揭示这种QPT的机制，我们将在磁场中对纳米线进行输运、隧穿和动力学电感测量。一系列纳米线的磁阻的组合标度将被用来寻找标度函数的约束条件。(Iii)将制备和研究一系列故意不均匀有序参数的纳米线。我们的目标是创建一个系统，其中QPT的物理过程强烈地被玻色子过程(相移)所主导。(4)将开发一种能够检测单个相移的高频技术。其目的是探索超低相位滑移率(低至1赫兹)的区域，并验证是否存在由相滑/反相滑相互作用和/或相滑雪崩引起的时间相关性。

Non-technical Abstract:The overall goal of the project is to understand how superconductivity in nanowires is affected by their geometry, material properties, and the presence of an external magnetic field. The knowledge of how these factors influence the properties of the superconducting nanowires is needed for improvement of nanowire-based single-photon detectors and for downscaling of classical and quantum superconducting circuits. The project is also important for fundamental science. Our team recently discovered that, similar to classical water-ice phase transition, nanowires also undergo a phase transition; because it occurs at zero temperature, however, it belongs to the class of quantum phase transitions (QPT). One of the primary goals of the project is to uncover the as yet unknown nature of this transition. One-dimensional systems, of which superconducting nanowires are a representative example, play a special role in physics since they often allow for easier theoretical description than their counterparts in higher dimensions. Hence the project has a potential to advance understanding physics of superconducting films and other systems where QPTs occur, such as magnetic materials, liquid crystals, and cold atomic gases. The educational component of the project includes training of students in nanofabrication and precision electrical instrumentation and service of a PI as a research adviser for undergraduate female students participating in the University of Utah ACCESS program. The public outreach includes oral and exhibit presentations on superconductivity and nanoscience, carried out both at the University of Utah and at the Natural History Museum of Utah, as a part of the NSF-funded Portal for the Public program.Technical Abstract:The team's research is focused on four problems in 1D superconductivity. (i) The experimentally determined suppression of critical temperature in superconducting nanowires is 100-fold stronger than the prediction from current theories. To resolve this problem, the team will systematically study the effect of geometrical confinement on critical temperature (by transport measurements), on superconducting gap (by tunneling), and on superfluid density (by kinetic inductance measurements) in MoGe and Al nanowires. The goal is to identify leading mechanisms of the suppression and, in collaboration with theorists, develop a correct fermionic theory for nanowires and films. (ii) Upon application of a magnetic field, MoGe nanowires undergo a quantum phase transition of yet unknown nature. To uncover the mechanism of this QPT, we will carry out transport, tunneling and kinetic inductance measurements on nanowires in a magnetic field. The combined scaling of the magnetoresistance of a series of nanowires will be used to find constraints on the scaling function. (iii) A series of nanowires with an intentionally inhomogeneous order parameter will be fabricated and studied. The objective is to create a system where the physics of the QPT is strongly dominated by bosonic processes (phase slips). (iv) A high-frequency technique capable of the detection of individual phase slips will be developed. The goal is to probe the regime of ultra-low phase slip rate (down to 1 Hz) and verify if there exist temporal correlations caused by phase-slip/anti-phase-slip interactions and/or by phase-slip avalanches.