Martin Götz

Validation of a quantized-current source with 0.2 ppm uncertainty

We report on high-accuracy measurements of quantized current, sourced by a tunable-barrier single-electron pump at frequencies f up to 1 GHz. The measurements were performed with an ultrastable picoammeter instrument, traceable to the Josephson and quantum Hall effects. Current quantization according to I = ef with e being the elementary charge was confirmed at f = 545 MHz with a total relative uncertainty of 0.2 ppm, improving the state of the art by about a factor of 5. The accuracy of a possible future quantum current standard based on single-electron transport was experimentally validated to be better than the best (indirect) realization of the ampere within the present SI.

Improved Cryogenic Current Comparator Setup With Digital Current Sources

We have realized an improved resistance calibration setup based on a cryogenic current comparator (CCC). The new comparator with 18 windings and a total of 4647 turns suits well for all the necessary comparisons of a 100-Omega standard resistor with the quantum Hall effect (QHE) and within the range of standard resistance values from 1 Omega to 1 MOmega. The new state-of-the-art setup is equipped with a dc superconducting quantum interference device (SQUID) and a digital double current source, allowing a reduction in the ramping time by at least one order of magnitude and significantly improving the dynamic stability of the system. The latter also benefits from the internal wideband feedback that we present in this paper.

Radio-frequency based monitoring of small supercurrents

We consider the applicability of the established rf readout technique, which allows to obtain the supercurrent-phase relation of a Josephson element from impedance measurements in the phase-biased regime. In experiments on Nb-based single and double tunnel junctions, we demonstrate that this method holds even if the Josephson coupling energy is smaller than the thermal energy. Compared with conventional current–voltage measurements, we evaluate the rf technique to be favorable in particular for investigations of small supercurrents in low-capacitance Josephson elements.

Improving the Traceable Measurement and Generation of Small Direct Currents

We present the latest improvements in the traceable measurement and generation of small electric currents. A central tool in our traceability chain for small direct currents is a new binary cryogenic current comparator (CCC) with a total of 18276 turns. This 14-bit CCC is well suited for the calibration of highvalue resistors and current amplifiers, but also for the direct amplification of small currents. A noise level of 5 fA/√Hz at 0.05 Hz is routinely achieved. The systematic uncertainty due to noise rectification was exemplarily investigated in a ratio-error test configuration, showing that a total uncertainty of about one part in 106 can be achieved at 100 pA. For further improvement, a new instrument was developed, the ultrastable low-noise current amplifier (ULCA). Its transfer coefficient is highly stable versus time, temperature, and current amplitude within a full dynamic range of ±5 nA. The ULCA is calibrated with the 14-bit CCC at high current amplitude, and allows the measurement or generation of 100-pA direct current with an uncertainty of one part in 107. The novel setup was successfully used to investigate the uncertainty of the established capacitor charging method. A quantum metrology triangle experiment based on the presented instruments is proposed.

Comeback of epitaxial graphene for electronics: large-area growth of bilayer-free graphene on SiC

We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultra-smooth defect- and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of deposited polymer adsorbate which acts as a carbon source. Unique to this method are the conservation of mainly 0.25 and 0.5 nm high surface steps and the formation of bilayer-free graphene on an area only limited by the size of the sample. This makes the polymer-assisted sublimation growth technique a promising method for commercial wafer scale epitaxial graphene fabrication. The extraordinary electronic quality is evidenced by quantum resistance metrology at 4.2 K showing ultra-high precision and high electron mobility on mm scale devices comparable to state-of-the-art graphene.

Precision quantization of Hall resistance in transferred graphene

We show that quantum resistance standards made of transferred graphene reach the uncertainty of semiconductor devices, the current reference system in metrology. A large graphene device (150 × 30 μm2), exfoliated and transferred onto GaAs revealed a quantization with a precision of ( − 5.1 ± 6.3) · 10−9 accompanied by a vanishing longitudinal resistance at current levels exceeding 10 μA. While such performance had previously only been achieved with epitaxially grown graphene, our experiments demonstrate that transfer steps, inevitable for exfoliated graphene or graphene grown by chemical vapor deposition, are compatible with the requirements of high quality quantum resistance standards.

Aspects of Application and Calibration of a Binary Compensation Unit for Cryogenic Current Comparator Setups

We have developed a binary compensation unit for balancing measurement bridges with cryogenic current comparators (CCCs) commonly used in resistance metrology. We present the basic design ideas for the new microcontroller-operated module. It offers balance over a gapless range of resistance ratios of the resistors to be compared. The binary setup allows for convenient calibration when being combined with a binary CCC. The calibration can be performed using the CCCs control electronics and software. Examples of measurements demonstrate the units high flexibility and stability of resistance calibration well below one part in 109 over a period of 21 hours.

Settling Behavior of the Bridge Voltage in Resistance Ratio Measurements With Cryogenic Current Comparators

The settling of the bridge voltage in resistance comparisons based on cryogenic current comparators is investigated starting 0.2 s after current reversal and sampling at a rate of 16 Hz. For certain configurations, the result obtained for a current reversal frequency of 0.1 Hz deviates from the result when waiting tens of seconds for the decay of transients by two parts in 109, only.

Validation of the ultrastable low-noise current amplifier as travelling standard for small direct currents

An interlaboratory comparison of small-current generation and measurement capability is presented with the ultrastable low-noise current amplifier (ULCA) acting as travelling standard. Various measurements at direct currents between 0.16 nA and 13 nA were performed to verify the degree of agreement between the three national metrology institutes involved in the study. Consistency well within one part per million (ppm) was found. Due to harsh environmental conditions during shipment, the ULCAs transfer accuracy had been limited to about +/-0.4 ppm. Supplemental measurements performed at PTB indicate that further improvements in accuracy are possible. Relative uncertainties of 0.1 ppm are achieved by applying on-site calibration of the ULCA with a suitable cryogenic current comparator.

A compact 14-bit cryogenic current comparator

A cryogenic current comparator with an overall of 17,252 turns and windings including numbers of turns ranging from 20 to 213 has been realized. This system offers the possibility of resistance calibration at increased magnetic flux levels and allows for a direct calibration of 1 Ω to 100 MΩ normal resistors against the quantum Hall effect. Besides that, the very low-current measurements at PTB will benefit from the availability of this additional setup. Such applications include the use of this comparator as a current amplifier as well as for calibration of highly-stable transimpedance amplifiers.