Acquisition of a Dilution Refrigerator and Superconducting Magnet System

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Matching funds are provided for the acquisition and the construction of a unique facility for the study of superconducting films and structures in magnetic fields at ultralow temperatures. The instrument consists of a high cooling power dilution refrigerator, capable of achieving temperatures below 5 millikelvin, a 7-tesla horizontal/8.5- tesla vertical vector magnet system, and associated pumps, power supplies, and gas handling system needed for the operation of the refrigerator and magnet system. Since many of the investigations involve the study of films prepared by deposition onto substrates held at low temperatures, the equipment will be built into an existing ultrahigh vacuum growth chamber. This will be done in a manner which permits insitu deposition of films onto cold substrates, but which also retains the option of free-standing operation of the refrigerator and magnet. Funds also are provided for the purchases of a nuclear orientation thermometer which will serve as a primary temperature standard, and a dc SQUID system which will be configured in a voltage-measuring mode to gauge transport properties at very low current excitations. The equipment would be used to extend the measurement of properties of superconducting films and structures in high magnetic fields (7 to 14 tesla) to ultralow temperatures (below 0.005 kelvin). The facility would be used to investigate the quantum properties of selected electronic systems in the limits of ultralow temperatures and two dimensions. For such systems, under appropriate conditions, quantum fluctuations are more important than thermal fluctuations, and many systems exhibit quantal rather than thermal phase transitions. The particular interests are in insulator-to-superconductor transitions in both ultrathin films and junction arrays, the macroscopic quantum tunneling of vortices in superconductors, and the critical fields of reduced transition temperature superconductors. These studies have important implicati ons and applications in developing and characterizing new technologically significant materials.