Superconductor-Insulator Transitions in Disordered Ultrathin Films

Technical Abstract

This project will pursue experiments that probe superconductor?insulator (SI) transitions in simple two-dimensional metallic systems. These transitions are believed to be continuous quantum phase transitions (QPTs). The QPT is an important paradigm in contemporary condensed matter physics. A QPT, in contrast with a thermally driven phase transition, occurs at zero temperature with the ground state being changed in response to the variation of an external parameter. The fluctuations associated with QPTs are quantum mechanical rather than classical. Measurements at nonzero temperature in the critical regime can reveal these quantum fluctuations and can be used to characterize them. Other systems exhibiting QPTs include two-dimensional electron gases, compounds with strongly correlated electrons, and certain cold atom systems. The experiments that will be pursued will answer open questions regarding the nature of physical models governing SI transitions tuned with external parameters such as disorder, dissipation, charge density, and magnetic field, the nature of the insulating state, the influence of material properties, and the nature of the critical region, where new physics often emerges. The investigations will include measurements of transport and magneto-transport, electrical noise, the Nernst effect, and the electronic compressibility at the transition. The work requires advanced tools of experimental science including nanofabrication and ultra-low temperature physics. This project will support the education of PhD students and undergraduates in a manner that is excellent training for scientific careers from academia to high technology industries.

Nontechnical Abstract

Quantum mechanics is the theory describing the sub-microscopic world. Of contemporary interest is the nature of its role in the macroscopic world. Thermodynamic phase transitions are transitions between disordered and ordered phases. Their microscopic mechanisms are always quantum mechanical. In such transitions the macroscopic behavior, in particular the spatial and temporal fluctuations of the degree of order found near the transition, is classical. Quantum phase transitions are transitions between disordered and ordered phases, occurring at absolute zero. Instead of temperature, they are controlled by changes in external parameters such as magnetic field, pressure or charge density. In contrast with thermodynamic transitions, their fluctuations are quantum mechanical, and they persist at nonzero temperatures thus allowing their study. This project is focused on the investigation of the simplest quantum phase transitions, superconductor-insulator transitions in two-dimensions. The experiments that will be pursued will answer open questions regarding the macroscopic quantum fluctuations associated with these transitions and explore new physics that emerges near the transitions. The work has implications for other areas of research as quantum phase transitions occur in two-dimensional electron gases, compounds with strongly correlated electrons, and certain cold atom systems. The proposed experiments require advanced tools of experimental science including nanofabrication and ultra-low temperature physics. This project will support the education of PhD students and undergraduates in a manner that is excellent training for scientific careers from academia to high technology industries.