Todd E. Harvey

Stable 1 volt programmable voltage standard

Several fully functional programmable voltage standard chips, each having a total of 32 768 Nb–PdAu–Nb Josephson junctions, have been fabricated and tested. The chips are based on a new design that provides fast programmability (1 μs) between voltages and stable voltage operation from −1 to +1 V. A comparison of the new standard with a conventional Josephson voltage standard is in agreement to 0.5±1.1 parts in 109. We demonstrate the utility of this standard by measuring the linearity of a digital voltmeter.

High‐TC superconductor‐normal metal‐superconductor Josephson microbridges with high‐resistance normal metal links

We have developed an in situ process for fabricating high transition temperature superconductor‐normal metal‐superconductor microbridges using a step edge to define the normal metal length. Critical current‐normal resistance products over 1 mV have been measured at low temperature in devices with high‐resistivity Ag‐Au alloy bridges. Results on samples with Ag bridges are compared with the alloy data as an initial test of recent theories of SNS Josephson junctions. Josephson effects have been demonstrated in these devices at temperatures higher than 80 K. Clearly defined rf steps have been observed, with power dependence qualitatively similar to theoretical predictions.

Ferroelectric thin film characterization using superconducting microstrip resonators

We describe a novel technique for characterizing the dielectric response of ferroelectric thin films at microwave frequencies. The method involves a microstrip resonator which incorporates a ferroelectric capacitor at its center. To demonstrate this method rye have fabricated a superconducting microstrip resonator from a laser-ablated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) film on a LaAlO/sub 3/ (LAO) substrate with a SrTiO/sub 3/ (STO) capacitor at its center. We report the observed dielectric behavior of the STO laser ablated film as a function of bias at liquid He and N/sub 2/ temperatures and at high and low frequencies. It is observed that the electrically tunable dielectric constant of the STO film is roughly independent of frequency up to 20 GHz (especially at high bias). The loss tangent of the STO/LAO capacitor decreases with increasing bias and is apparently independent of frequency between 6 and 20 GHz.<<ETX>>

Pulse-driven Josephson digital/analog converter [voltage standard]

The authors have designed and demonstrated a pulse-driven Josephson digital/analog converter. When used as a programmable voltage standard, this device can synthesize metrologically accurate ac waveforms as well as stable dc voltages. We show through simulations that Josephson quantization produces a nearly ideal quantization noise spectrum when a junction is driven with a typical waveform produced by a digital code generator. This technique has been demonstrated in preliminary experiments with arrays of 1000 junctions clocked at frequencies up to 6 Gb/s, where sine waves of a few millivolts in amplitude were synthesized at frequencies up to 1 MHz.

1 volt DC programmable Josephson voltage standard

NIST has developed a programmable Josephson voltage standard (JVS) that produces intrinsically stable voltages that are programmable from -1.1 V to +1.1 V. The rapid settling time (1 /spl mu/s), large operating current margins (2 to 4 mA), and inherent step stability of this new system make it superior to a conventional JVS for many dc measurements. This improved performance is made possible by a new integrated-circuit technology using intrinsically shunted superconductor-normal-superconductor (SNS) Josephson junctions. These junctions operate at lower excitation frequencies (10 to 20 GHz) than a conventional JVS and have 100 times greater noise immunity. The Josephson chip consists of a binary array sequence of 32 768 SNS Josephson junctions. The chip has been integrated into a completely automated system that is finding application in mechanical/electrical watt-balance experiments, evaluation of thermal voltage converters, electron-counting capacitance standards, and metrology triangle experiments.

AC and DC bipolar voltage source using quantized pulses

We have developed an accurate bipolar voltage source for AC and DC metrology, based on the quantized pulses of Josephson junctions. The output voltage is a factor of 4-6 higher than the amplitude of previous unipolar waveforms; this is achieved by generating bipolar waveforms where arrays of junctions are driven with both a broadband two-level digital code and a sinusoidal frequency. We experimentally demonstrate two bipolar waveforms: a 5 kHz sine wave with /spl plusmn/18 mV peak amplitude and quantization noise power eight orders of magnitude below the power in the fundamental frequency, and a 3.5 kHz square wave with /spl plusmn/13 mV peak amplitude and even harmonics at -100 dB (carrier).

MESFETs Made From Individual GaN Nanowires

In this paper, we demonstrate novel MESFETs based on individual GaN nanowires. The Pt/Au Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior. The average effective Schottky barrier height was 0.87 eV, with an average ideality factor of 1.6. In addition, the Schottky gates efficiently modulated the conduction of the nanowires. The threshold gate voltages required for complete pinch off were as small as -2.6 V, and transconductances exceeded 1.4 muS. Subthreshold swings approaching 60 mV/decade and on/off current ratios of up to 5times108 were achieved. These results show that the Schottky gate has the potential to significantly improve the performance of GaN nanowire field-effect devices.

Thermoelectric transfer difference of thermal converters measured with a Josephson source

We have measured the thermoelectric transfer difference of two thermal voltage converters using a Josephson source and compared the results to similar measurements made with a conventional semiconductor source. Both sources use the fast reversed DC method. The Josephson source is an array of 16 384 superconductor-normal-superconductor Josephson junctions that is rapidly switched between voltage states of +0.5 V, 0 V, and -0.5 V. A marginally significant difference is detected between measurements with the two different sources.

High‐Tc multilayer step‐edge Josephson junctions and SQUIDs

We have developed a reliable process to fabricate high‐quality YBa2Cu3O7−x (YBCO) superconductor‐normal metal‐superconductor (SNS) step‐edge junctions and SQUIDs over YBCO ground planes. These multilevel circuits employ thin films of SrTiO3 and NdGaO3 as the insulating layer between the active device and the ground plane and use Ag as the normal metal in the Josephson junction. The reproducibility and uniformity of the junctions are better than our single‐level devices grown directly on step edges cut into single‐crystal substrates. Here the critical current variation among junctions on a single wafer is less than a factor of 2. Junctions grown on thin‐film step edges of SrTiO3 have critical currents near 2 mA at 4 K, while those grown on NdGaO3 step edges have critical currents near 0.5 mA at 4 K.

Single-photon detection using a quantum dot optically gated field-effect transistor with high internal quantum efficiency

We investigate the operation of a quantum dot, optically gated, field-effect transistor as a photon detector. The detector exhibits time-gated, single-shot, single-photon sensitivity, a linear response, and an internal quantum efficiency of up to (68±18)% at 4K. Given the noise of the detector system, they find that a particular discriminator level can be chosen so the device operates with an internal quantum efficiency of (53±11)% and dark counts of 0.003 counts per shot.