Superconductor-Insulator Transitions of Ultra-thin Films

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****Technical Abstract****This program will investigate superconductor-insulator transitions (SITs) of ultrathin films of elements and simple compounds. These transitions are the simplest examples of continuous quantum phase transitions, which occur at zero temperature, when the ground state of a system is changed in response to the variation of an external parameter. Quantum phase transitions, which are macroscopic manifestations of quantum mechanics, are studied through measurements of behaviors influenced by quantum fluctuations that persist at low, but nonzero temperatures. Investigations will include a study of the Nernst effect across the magnetic-field-tuned transition, and measurements of the penetration depth in the critical region with the goal of identifying the critical exponent of the order parameter and the universality class. In addition, the electronic compressibility near the critical point, which would be a thermodynamic signature of the SI transition, and the dissipation-tuned-SI-transition will be investigated. Electric double layer transistor configurations will be used to modify the electronic and superconducting properties of FeSe films. The broader impacts include training of graduate students in a traditional physics Ph.D. program and providing a venue for useful and positive research experiences for undergraduates. The work proposed will impact the understanding of known novel superconductors and the search for new ones. The primary outreach activity will be the teaching of freshman seminars by the PI on physics topics. ****Non-Technical Abstract****This award supports investigations of the superconductor-insulator transitions of elements and simple compounds. Superconductor-insulator transitions are among the simplest quantum phase transitions, and they are especially fundamental because of their connection to the Uncertainty Principle of quantum mechanics. Quantum phase transitions are an important paradigm in contemporary condensed matter science and understanding their nature is an important challenge in part because of their role in high temperature superconductivity. Quantum phase transitions, in contrast with conventional thermodynamic phase transitions, which are controlled by temperature, and occur at nonzero temperature, are transitions, which are controlled by external parameters such as magnetic field, pressure, or charge density, and occur at absolute zero. They have measurable signatures at low, but nonzero temperatures, which will be used to characterize them. The study of superconductor-insulator transitions requires advanced tools of experimental science such as nanofabrication, thin film growth, and ultra-low temperature cryogenic measurement techniques. This project supports the training of both Ph.D. and undergraduate students that is excellent preparation for a scientific career. The primary outreach activity will be the teaching of freshman seminars by the PI on contemporary physics topics.