Tunneling and Transport in Ordered and Disordered Superconductors

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The main focus of this program is on the superconductor-insulator (SI) transition in ultra-thin films, an example of a quantum phase transition (QPT). Such transitions occur at zero temperature in response to the tuning of an external parameter of a system that alters its quantum mechanical ground state. They are studied through measurements at nonzero temperature of properties influenced by quantum fluctuations associated with the transition. Although many systems exhibit QPTs, including two-dimensional electron gases, and strongly correlated electron materials including high temperature superconductors, the SI transition is most fundamental because of its connection with the phase-number uncertainty relation of quantum mechanics. The nature of the physics of the SI transition has become uncertain, as in some work evidence of an intermediate metallic regime separating the superconducting and insulating ground states has emerged. It is the goal of this program to resolve the nature of the underlying physics through careful measurements of thermodynamic and electrical transport properties of films at ultra-low temperatures. Research and education are integrated in this program through the training of Ph.D. students, and research experiences for undergraduates and high school students. Some of the sample structures studied have a potential for technological significance. %%%The main focus of this program is on the investigation of the superconductor-insulator (SI) transition in ultrathin films of metals. This transition is a quantum phase transition (QPT). In contrast with ordinary phase transitions, which are tuned by temperature, QPTs are tuned by external parameters, such as magnetic field or charge density, and are found at absolute zero. They are studied through measurements at nonzero, but low temperatures, of properties influenced by proximity to the transition both in temperature and in the value of the tuning parameter. Other systems exhibiting QPTs include two-dimensional electron gases, and correlated electronic materials, including high temperature superconductors. The SI transition is one of the most fundamental of QPTs because of its connection with the uncertainty principle of quantum physics. It is the goal of this program to resolve the nature of the underlying physics of the SI transition through measurements of thermodynamic and electrical transport properties at ultra-low temperatures. Research and education are integrated in his program through the training of Ph.D. students in physics as well as by providing research experiences for younger students. Some of the configurations studied could be important in electronics technology.