Quantum Phase Transition in Superconducting Nanowires and Films

University Of Utah, Salt Lake City UT

Investigators

Abstract

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.