Noise Measurements
AC and DC techniques are being used to measure impedance and noise in thin-film and tunnel-junction structures. Measurements can be made in conditions of high vacuum, low-temperatures, and high magnetic fields.  These techniques utilize low-noise preamplifiers, lock-in amplifiers, and a spectrum analyzer.  As an example, we have recently found that the electrical impedance of such metal-insulating-metal (MIM) tunnel junctions is strongly affected by the presence of electronic states localized near the interface. We predict that the transfer of tunneling electrons into and out of the interface states should produce a unique noise signature in the measured voltage. We plan to correlate noise measurements with the frequency-dependent impedance at different temperatures and excitation levels for samples having a variety of  tunnel barrier heights.  In another project we are investigating noise properties of thin-films in which transport currents are confined to tortuous paths.

The tunneling of electrons through an ultrathin insulating barrier between two metals reveals a great deal about the quantum mechanical behavior of solids. We are studying the frequency-dependent impedance of tunnel-junction capacitors in the regime where interface capacitance dominates.  Our Solartron Impedance Analyzer allows measurements over the frequency range 10-4 -   10+5 Hz.   We also have the capability of measuring current-voltage characteristics, voltage-dependent differential conductance, and second derivatives for inelastic tunneling spectroscopy.

The laboratory is equipped with a Quantum Design Physical Properties Measurement System (PPMS).  This instrument allows transport measurements over a temperature range from 2K to 400K and to magnetic fields as high as 7 T.  It is also equipped with a sample rotator for orientation-dependent measurements and an insert for making ultra sensitive three-terminal capacitance measurements. This latter capability is presently being used to study magnetocapacitance in trilayer capacitor structures where one or both electrodes comprise a magnetic material. The PPMS and instruments used for related investigations are located in a radio-frequency (RF) screen room which provides a low-noise environment.

Atomic Force Microscope (AFM)
An Atomic Force Microscope (AFM, Park AutoProbe CP) is available for characterization of solid surfaces.  This instrument can be run in contact and non-contact modes.  In addition, it has been modified to acquire simultaneous images of topography and electric potential differences.  A nanotube work bench capability is being constructed so that carbon nanotubes can be attached to the AFM cantilever tips.

Skunk Works
In addition to normal well-defined investigations, opportunities are available to vent creative impulses.  Space is allocated for rapid set-up of equipment to pursue novel ideas in an unstructured environment.