A. Amar

Near‐field scanning microwave microscope with 100 μm resolution

We describe the operation of a simple near‐field scanning microwave microscope with a spatial resolution of about 100 μm. The probe is constructed from an open‐ended resonant coaxial line which is excited by an applied microwave voltage in the frequency range of 7.5–12.4 GHz. We present images of conducting structures with the system configured in either receiving or reflection mode. The images demonstrate that the smallest resolvable feature is determined by the diameter of the inner wire of the coaxial line and the separation between the sample and probe.

Magnetic microscopy using SQUIDs

SQUID-based magnetic microscopy involves scanning a sample closely past a low-noise SQUID. With the SQUID held in a flux-locked loop, a computer records the feedback output as a function of sample position and converts the resulting data into a false color image of magnetic field strength. Present systems have achieved spatial resolution down to about 5 /spl mu/m and flux resolution down to about 1 /spl mu//spl Phi//sub 0//Hz/sup 1/2/. They have been used to study the pairing symmetry of the high-T/sub c/ superconductors, for high-frequency imaging, and for a variety of applications related studies. Recently, microscopes have also been developed for high resolution magnetic imaging of room-temperature samples. We briefly describe the design, operation, and capabilities of these systems.

Application of single electron tunneling: Precision capacitance ratio measurements

A metrological application is reported of the single electron tunneling (SET) phenomena: a precise measurement of the ratio of two cryogenic capacitors. The measurement used a superconducting SET electrometer as the null detector for a capacitance bridge. A 3‐ppm level of imprecision has been achieved in the measurement of the capacitance ratio from 100 to 1000 Hz. Further improvements can be made in the attempt to obtain an imprecision of 10−8 at lower frequencies, sufficient for the metrological measurement of capacitance or the fine‐structure constant using a SET pump.

Advantages of superconducting Coulomb-blockade electrometers

We have measured the current-voltage gate charge characteristics and noise in superconducting Coulomb-blockade electrometers from 50 mK to 1.05 K. The Al-AlO/sub x/-Al junctions in our devices have capacitances which range from 0.3 fF to 1 fF and tunneling resistances from 20 k/spl Omega/ to 200 k/spl Omega/. We find that our normal and superconducting electrometers have comparable charge noise but that the latter have much larger modulation which persists to significantly higher temperatures. The maximum operating temperature in the superconducting state is close to T/sub c/. This behavior Is consistent with a simple model and makes the superconducting electrometer preferable for many applications.<<ETX>>

Distributed microwave damping filters for superconducting quantum interference devices

We describe a distributed thin-film filter for the broad-band damping of microwave resonances in thin-film superconducting strip lines and coils used in Nb–Al/AlOx–Nb superconducting quantum interference devices (SQUIDs). The filter consists of an array of 14 μm diameter thin-film Au–Cu dots which are insulated from the strip lines or coils by an SiO2 layer. Damping occurs via eddy current induced in the dots. Our measurements show that the filter removes resonance steps from the current–voltage curves of SQUIDs which are coupled to the coils and reduces white noise associated with the resonances.

Dynamics of a Charged Fluctuator in an Al–AlOx–Al Single-Electron Transistor

We report detailed observations of random-telegraph charge fluctuations in a two-junction Al–AlOx–Al single-electron transistor (SET). We measured the fluctuations from 85 mK to 3 K and observed that the SET switched between two states, causing charge shifts of ΔQo=0.1±0.025 e on the SETs island. The transition rate out of each state was periodic in the gate voltage, varied non-monotonically with the device bias voltage, and was independent of the temperature below about 0.3 K. We discuss two effects which could contribute to the behavior of the transition rates, including heating of the defect by the island conduction electrons and inelastic scattering between the defect and electrons flowing through the SET.

Electric field effect control of a superconducting YBa2Cu3O7 inductor

We discuss the design, fabrication, and testing of a thin‐film superconducting voltage‐controlled inductor which is made from a YBa2Cu3O7 (YBCO) superconducting field effect transistor. Applying voltage to an Au gate layer alters the areal carrier density, and hence the kinetic inductance, of an underlying 100‐nm‐thick YBCO channel layer. The channel is connected in series with an input coil to form a closed superconducting loop. We use a dc superconducting quantum interference device at 4.2 K to measure changes in loop inductance and find a fractional change in the kinetic inductance of about +2.6×10−4/V of applied gate voltage, close to the expected value.

Behavior of Al–Al2O3–Al single-electron transistors from 85 mK to 5 K

Using e-beam lithography and conventional double-angle evaporation, we have fabricated Al–Al2O3–Al single-electron transistors and studied their behavior from 85 mK to about 5 K. The total island capacitance CΣ of the devices ranges from 120 to 200 aF, with typical estimated junction overlaps of about 30 nm×30 nm. At 4.2 K, our devices display well-behaved periodic I–Vg characteristics with the maximum charge-transfer function ∂I/∂Q0 ranging from 4 to 130 pA/e. The electrical characteristics of these devices agree well with the predictions of the Orthodox Theory, with current modulation being observed up to a temperature T≃e2/(2CΣkB). Below 1 K small deviations occur, which are partly due to island self-heating effects.

Angular dependence of the symmetry of the order parameter in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

Using a 4.2 K scanning SQUID microscope, we examined twinned thin-film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO)-Ag-PbIn SQUIDs and measured the phase of the order parameter in the YBCO for 13 different tunneling angles. We have found that the order parameter in YBCO is time-reversal symmetric and shows a d(x/sup 2/-y/sup 2/)-pairing symmetry, provided the junctions are properly made. Detailed analysis of our data reveals that any time-reversal breaking component is less than 5%, which rules out states such as d(x/sup 2/-y/sup 2/)+ is or d(x/sup 2/-y/sup 2/)+ id/sub xy/.

Behavior of a charged two-level fluctuator in an Al-AlO/sub x/-Al single-electron transistor in the normal and superconducting state

We have studied the behavior of a charged two-level fluctuator in an Al-AlO/sub x/-Al single-electron transistor (SET) in the normal state over a temperature range from 85 mK to 3 K. The fluctuator caused the SETs island charge to shift by /spl Delta/Q/sub o/=0.1/spl plusmn/0.025 e with an escape rate out of each state which was periodic in the gate voltage. We compare our results to a model which assumes the fluctuator resides in one of the tunnel junctions and discuss model predictions for when the device is in the superconducting state.