Uncertainty analysis for ac-dc difference measurements with the AC Josephson Voltage Standard.

A detailed analysis of the uncertainties obtained in ac-dc difference measurements with an AC Josephson Voltage Standard (ACJVS) is presented. For audio frequencies and for voltages less than 200 mV, ac-dc transfers with the ACJVS may reduce the combined uncertainty by factors of 2 to 10, compared with conventional methods based on thermal converters. Type A uncertainties are predominantly limited by the thermal transfer standard (TTS), or the digital voltmeter used to acquire the output voltage from the TTS. In agreement with earlier work, the transmission line is the primary contributor to Type B errors for frequencies above 10 kHz. A Monte Carlo sensitivity analysis is used to demonstrate how the uncertainties of transmission line impedance and on-chip inductance impact the accuracy of the rms amplitude conveyed to the TTS.

Insensitivity of sub-Kelvin electron-phonon coupling to substrate properties.

We have examined the role of the substrate on electron-phonon coupling in normal-metal films of Mn-doped Al at temperatures below 1 K. Normal metal-insulator-superconductor junctions were used to measure the electron temperature in the films as a function of Joule heating power and phonon temperature. Theory suggests that the distribution of phonons available for interaction with electrons in metal films may depend on the acoustic properties of the substrate, namely, that the electron-phonon coupling constant Σ would be larger on the substrate with smaller sound speed. In contrast, our results indicate that within experimental error (typically ±10%), Σ is unchanged among the two acoustically distinct substrates used in our investigation.

A surface-sensitive UHV dielectric spectrometer for studies of nanoscale molecular systems on a planar surface.

We describe an apparatus for dielectric spectroscopy (DS) experiments on ultrathin (<100 nm) molecular films in an ultrahigh vacuum (UHV) environment. The apparatus is capable of measurements of ex situ prepared films, as well as in situ physisorbed or chemisorbed layers. Pressures of 5x10(-10) Torr are typical, and the audiofrequency DS measurements can be performed over a range of temperatures 7-500 K, using a continuous-flow cold finger. The combination of turbomolecular pump, differentially pumped quick-access door, and backfilling with dry N(2) affords vent-exchange-pump cycle times under an hour. The system pressure returns to the 10(-9) Torr scale within 48 h, without a high-temperature bake. Since the cold finger is mounted on a long-stroke translation stage, the apparatus can be attached to other analytical UHV equipment, and the sample may be transferred for additional in situ measurements. Film dielectric properties are probed via coplanar interdigital electrode capacitors, which are microfabricated on fused SiO(2) wafers. Besides permitting the usual equilibrium dielectric measurements, the system also provides a novel way to study adsorption/desorption phenomena and film growth.