Superconductor-Insulator Transitions

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Non-Technical Abstract: This research supports the experimental investigation of superconductor-insulator transitions of ultrathin films of elements and compounds. Understanding their nature is important, in part because of their role in high temperature superconductivity. They are the simplest examples of an important paradigm of contemporary condensed matter physics, continuous quantum phase transitions, which in contrast with thermal transitions, occur at zero temperature, but have measurable signatures at nonzero temperatures, which are used to characterize them. In the case of the superconductor-insulator transition, the ground state is changed in response to the variation of an external parameter of the system, such as parallel or perpendicular magnetic field, disorder, or carrier density. Among other systems exhibiting quantum phase transitions are 4He adsorbed on random substrates, two-dimensional electron gases, numerous complex strongly correlated electron materials, and configurations of cold atoms, which can be effectively used to model many condensed matter phenomena. The study of superconductor-insulator transitions requires advanced tools of experimental science such as film growth, nanofabrication and ultra-low temperature 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 of the PI is the teaching of freshman seminars. Technical Abstract: The research is directed at identifying the mechanisms associated with a variety of superconductor-insulator transitions involving different materials and tuning parameters. It involves four distinct investigations. The first investigation is an attempt to see whether a superconductor-insulator transition in a disordered thin film can be systematically tuned by dissipation provided by a tunable, proximate two-dimensional electron gas. The second investigation is the measurement of the charge-density dependence of films of the super-electron density using an AC technique, over a range of carrier concentrations of films of the oxygen-enriched compound, La2CuO4+x (LCO) which are electrostatically doped using ionic liquids. The third thrust is the measurement of the out-of-equilibrium, current noise spectral density near the superconductor-insulator transition of quench-deposited metal films, and the fourth direction of research is the study of the perpendicular-field tuned superconductor-insulator transition of quench-deposited metal films with the goal of determining whether the limiting resistance at the quantum critical point is the quantum resistance for pairs, and whether there is a Hall insulator in these materials, as is apparently present in the case of compounds such as indium oxide.