Acquisition of a Compact Dilution Refrigerator

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9317417 Lee This compact dilution refrigerator used in experiments probing the electronic and electrodynamic response of advanced materials and novel solid-state structures at very low temperatures at University of Virginia. These research projects focus on electron tunneling, noise, and transport studies of the fundamental ground-state and elementary excitation properties of interacting electron systems. Because full spin, Coulomb, and quantum correlation effects often do not become manifest until temperatures of ~ 0.1K, attainment of temperatures below those achievable by pumped liquid 3H2(~0.3 K) will be required, making necessary the use of a dilution refrigerator which can achieve a base temperature of approximately 0.02K in a rapid and convenient manner. It is anticipated that normal use of the dilution refrigerator will be divided among three principal investigators: Prof. Mark Lee, who will be the principal user responsible for the maintenance of the facility, Prof. Joseph Poon, and Prof. Elias Towe. Dr. Lee seeks to elucidate aspects of the ground state in variable range hopping (VRH) conductors such as Si:P and PrBa2Cu3O7 in the strongly interacting regime around 0.1 K by tunneling and noise experiments. He will also use the dilution refrigerator to investigate large scale quantum coherency at low- temperature in high mobility superconductor-semiconductor structures, and how the coupling between novel superconductors and normal metals can shed insight into possibly unconventional superconducting mechanisms. Dr. Poon is interested in highly ordered new materials, composed entirely of metal atoms, that show semiconducting or insulating behavior at low temperature as a result of unusual symmetries, as occurs in quasicrystals, or low dimensionality. For this work it is important to have the ability to systematically measure resistivities down to ~ 0.03 K in a convenient manner in order to confirm and study the existence of non-metallic behavior. Dr. Towe currently grows high purity semiconductor heterostructures by molecular beam epitaxy. Quantitative characterization effective masses and mobilities is typically done by measuring Shubnikov-de Haas oscillations and of the Hall effect quantization at very low temperatures. Dr. Towe's investigation will include a study of mobilities in heterostructures of varying crystallographic orientation and development of nitride-based semiconductor heterostructures. This instrument is straightforward to maintain and operate, has a relatively rapid cool-down and sample turn- around time, and will thus serve as an excellent teaching tool for graduate and undergraduate students in solid-state and low- temperature experimental physics. The dilution unit will be housed in an existing 4He dewar equipped with a compensated 10 T superconducting solenoid and will share pumping and gas support facilities with an operational 4He variable temperature cryostat. ***