The proposed research will look at issues in electron conduction in metals via two different routes. In the first project, the issue of boundary conditions in the superconducting proximity effect will be studied. There is a lack of clarity on how the superconducting transition temperature of a system with repeated layers of superconducting and non-superconducting materials will depend on the boundary conditions. The predictions of how surfaces affect the superconducting transition temperature will be compared against the results for bilayer, trilayer, and more complicated layered structures. The system to be considered will be sputtered layers of niobium and either tungsten or zirconium, with the layers being of order 60 nm or larger. The transition temperature will be characterized both by resistive and inductive methods. The second project will look at the behavior of thin chromium films as the thickness decreases to see if the material undergoes a standard metal-insulator transition which would be expected for a material that violates the Ioffe-Regel limit, or if it behaves as a "bad" metal which continues to show metallic resistivity in the regime where none would be expected. Amorphous Mo-Ge will be used as an example of a material that does undergo a metal-insulator transition for comparison. The desire is to look at electron conduction in the regime where Fermi liquid theory is assumed to not hold and yet a metallic behavior is still observed. Both projects will give insights > to the behavior of electrons in metals.
Laymen:
The proposed research effort will be looking at how the current carrying electrons in metals behave at low temperatures (approximately -440 degrees Fahrenheit). The first project deals with materials that loose all resistance to current flow (superconductors) and will deal with how this behavior changes due to electrical contact between the superconducting material and a metal that does not go superconducting. The two different metals (niobium and zirconium) will be layered as in a cake and observed to see how this affects the temperature at which they loose their resistance. This study is relevant to the use of superconductors, for at some point, they must all be put into electrical contact with non-superconducting metals at some point. The second effort is looking at the change in a metal's ability to carry current as the average distance the charge carriers can travel changes. At a certain value of this distance, (roughly the distance between the atoms in the metal) most metals change to insulators, in that they have very poor ability to carry current and that ability gets worse as the temperature gets colder. However, some materials do not change in that manner and become what physicists call "bad" metals. Instead they still behave like metals in their ability to carry current gets better as the temperature is lowered, but the overall ability is much smaller than before. This project will look at thin layers of the element chromium. For both projects the layers will be about the thickness of 1/1600 that of a human hair.
