Quantum Phase Transition in one-dimensional superconductors

University Of Utah, Salt Lake City UT

Investigators

Abstract

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.