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This paper details developments in the use of room temperature current comparators operating with low-frequency (

Gas sensor operating in part-per billion (ppb) range is required for environmental monitoring of nitrogen dioxide. Currently there is a lack of cheap sensors, which can operate in this concentration range. In this work, an ultrasensitive gas sensor based on epitaxial graphene on SiC has been proposed. The sensor exhibits a strong and reproducible response to nitrogen dioxide (NO2) for concentrations in air down to 1 ppb. A prototype of a portable device for environmental monitoring which utilizes a combination of sample gas exposure at room temperature and sensor's recovery at elevated temperature has been made. Prototype allows fast and reproducible measurements of NO2 concentration in the typical range for environmental pollution (5 ppb-50 ppb).

Gas sensor operating in part-per billion (ppb) range is required for environmental monitoring of nitrogen dioxide. Currently there is a lack of cheap sensors, which can operate in this concentration range. In this work, an ultrasensitive gas sensor based on epitaxial graphene on SiC has been proposed. The sensor exhibits a strong and reproducible response to nitrogen dioxide (NO 2 ) for concentrations in air down to 1 ppb. A prototype of a portable device for environmental monitoring which utilizes a combination of sample gas exposure at room temperature and sensor’s recovery at elevated temperature has been made. Prototype allows fast and reproducible measurements of NO 2 concentration in the typical range for environmental pollution (5 ppb–50 ppb).

Epitaxial graphene based quantum Hall resistance devices with four and eight series-connected Hall bars have been fabricated and tested for realization of quantum resistance standards with up-scaled values. Triple series connection with external bonding wires was used. Most of the tested devices had relatively low longitudinal resistance and showed half-integer quantum Hall effect at filling factor 2 starting from magnetic field of about 6 T, but the quantized Hall resistance deviated from the expected value much more than in our earlier experiments with four series-connected Hall bars.

Adjustment of the charge carrier density in quantized Hall resistance (QHR) devices based on epitaxial SiC graphene films can be performed using photochemical and chemical gating. Our experiments show that the carrier concentration in fabricated graphene devices can be reversibly varied by either treatment with UV light or using molecules from aqua ammonia. Treatment of SiC graphene QHR devices in such a way provides tuning of carrier concentration to optimum values, in the range (1-3) x 1011 cm-2, at which the dissipation in QHR regime is reduced and the full quantization of Hall resistance is expected.

Standards with toroidal windings are traditionally used for international comparisons of inductance. The achieved uncertainties of the comparisons are dominated by the rather high temperature coefficient of the traveling standards. Hence, thermoregulated enclosures were developed at Physikalisch-Technische Bundesanstalt (PTB) a few years ago. These have now been refined and their suitability for the transportation of standards has been proven in a bilateral comparison between PTB and VTT Technical Research Centre of Finland Ltd. The agreement was excellent, while the overall uncertainty of the comparison varied from 14 to 62 $\mu \text{H}$ /H in the frequency range 100 Hz–2kHz.

Improved magnetic materials open the perspective of operating room-temperature current comparators at lower frequency without compromising metrological accuracy. Two prototypes of low-frequency current comparator (LFCCs) have been successfully tested in precision resistance ratio measurements at drive frequencies of 0.5 Hz and 1 Hz using the BIPM's bridge setup with a recently realized frequency extension. These measurements have been performed using standard resistors of 10 kΩ, 1 kΩ and 100 Ω nominal values as well as the quantized Hall resistance (QHR) of GaAs and graphene devices.

We report preliminary results of precision measurements of the graphene quantum Hall resistance plateau value with respect to magnetic field, temperature and applied current. Resistance value of SiC graphene Hall device was modulated by chemical doping, heating, and UV irradiation at the ambient condition. Precision measurements for Rh (filing factor v = 2) were then performed with a cryogenic current comparator resistance bridge. Relative deviation from RH (v = 2) less than 30 parts in 109 can be achieved down to 7 T of magnetic field and up to 6 K of temperature with driving current of 19. 37 μA.

Series connection of four quantum Hall effect (QHE) devices based on epitaxial graphene films was studied for realization of a quantum resistance standard with an up-scaled value. The tested devices showed quantum Hall plateaux RH,2 at filling factor i = 2 starting from relatively low magnetic field (between 4 T and 5 T) when temperature was 1.5 K. Precision measurements of quantized Hall resistance of four QHE devices connected by triple series connections and external bonding wires were done at B = 7 T and T = 1.5 K using a commercial precision resistance bridge with 50 microA current through the QHE device. The results showed that the deviation of the quantized Hall resistance of the series connection of four graphene-based QHE devices from the expected value of 4*RH,2 = 2h/e^2 was smaller than the relative standard uncertainty of the measurement (< 1*10^-7) limited by the used resistance bridge., 11 pages, 5 figures

Cryoresistors based on Pd thin films were designed and investigated in the temperature range 50 mK-300 K. The resistors in the range 100 kΩ-1.3 MΩ were fabricated by thermal evaporation technique. Resistivity of Pd film at 293 K varies in (21-53)·10 8 Ωm range for 15-40 nm films. Temperature co efficient of resistance at low temperatures depends on the thickness of the film. The lowest temperature coefficient in the range 3 K-4 K of about few parts in 10 6 /K was obtained for 20-nm-thick film. Current coefficient measured on 20-nm-thin film 1.002-MΩ resistor does not exceed 2·10 5/μA in 0.7 K-1 K temperature range.

Metal-alloy (Ni-Cr-Cu-Al-Ge) thin-film resistors were coated with alumina (Al2O3) using the atomic layer deposition (ALD) technique. The electrical properties of the thin-film resistors were studied in the temperature range of 4.2-300 K. It was experimentally demonstrated that the protective dielectric alumina coating improves the long-term stability and repeatability of high-value, thin-film resistors (100-500 kOmega). The drift rate of the resistance due to the native oxidation at room temperature was reduced from -2.45 times10-6 h-1 for a nonaged uncoated resistor to 0.03 times10-6 h-1 for an alumina-coated resistor. It was shown that the additional 15-nm-thick alumina coating does not significantly change the thermoelectrical properties of the metal-alloy, thin-film resistors.

The method for realization of the kilogram using 'superconducting magnetic levitation' was re-evaluated at MIKES. The realization of the kilogram based on the traditional levitation method is limited by the imperfections of the superconducting materials and the indefinable dependence between supplied electrical energy and the gravitational potential energy of the superconducting mass. This indefiniteness is proportional to the applied magnetic field and is caused by increasing losses and trapped magnetic fluxes. A new design of an electromechanical system for the levitation method is proposed. In the proposed system the required magnetic field and the corresponding force are reduced, as the mass of the body (hanging from a mass comparator) is compensated by the reference weight on the mass comparator. The direction of the magnetic force can be upward (levitation force, when the body is over the coil) or downward (repulsive force, when the body is under the coil). The initial force to move the body from the coil is not needed and magnetic field sensitivity is increased, providing linearization of displacement versus applied current. This new construction allows a lower magnetic induction, reduces energy losses compared with previous designs of electromechanical system and reduces the corresponding systematic error.

A cryogenic dual compensated calorimeter was constructed for energy loss measurements in superconducting materials. The losses induced by slowly varying periodic magnetic fields (amplitudes

We report about technology of fabrication and optimization of a gas sensor based on epitaxial graphene. Optimized graphene/metal contact configuration exhibited low contact resistance. Complementary annealing of graphene sensor after each gas exposure led to significant improvement in the sensing performance. The response of the annealed sensor to the nitrogen dioxide (NO2) was tenfold higher than that of an as-fabricated graphene sensor. NO2concentration as low as 0.2 parts per billion (ppb) was easily detectable. Devices have high signal-to-noise ratio. The detection limit of the graphene sensor was estimated to be 0.6 ppt (parts per trillion). The present technology with additional annealing improves the performance of the graphene based sensor and makes it suitable for the environmental nitrogen dioxide gas monitoring.

Quantum Hall effect (QHE) devices based on epitaxial graphene films grown on SiC were fabricated and studied for development of the QHE resistance standard. The graphene-metal contacting area in the Hall devices has been improved and fabricated using a double metalization process. The tested devices had an initial carrier concentration of (0.6 - 10)*10^11 1/cm^2 and showed half-integer quantum Hall effect at a relatively low (3 T) magnetic field. Application of the photochemical gating method and annealing of the sample provides a convenient way for tuning the carrier density to the optimum value. Precision measurements of the quantum Hall resistance (QHR) in graphene and GaAs devices at moderate magnetic field strengths (, 6 pages, 9 figures, 3 tables

We present first ac measurements of the quantized Hall resistance in graphene. Remarkably, the first results are very similar to that of conventional, unshielded GaAs devices. The future exploration of the detailed loss mechanisms in graphene by established ac analysis techniques thus gives a good chance to understand and to eliminate the ac losses, enabling a future graphene-based impedance standard.

This paper describes a low-frequency AC resistance bridge based on a Cryogenic Current Comparator (CCC). A low-noise dc SQUID is used as a null detector. The bridge is the main tool to trace resistance to 10 k/spl Omega/, 100 /spl Omega/ and 1 /spl Omega/ levels from the quantum Hall resistance (QHR). The ratio of the resistors ranging from 1 /spl Omega/ to 10 k/spl Omega/ can be measured with an uncertainty of the order of 1 ppb. The bridge is typically operated with a PC generating a sinusoidal ac signal ranging from 0.1 Hz-2 Hz. Using a vector analyzer as a signal source the bridge is used up to 1 kHz. Both sinusoidal and pseudorandom signals have been used to verify the frequency dependence of the standard resistors against the QHR. At present, the bridge can be used to perform accurate calibrations only up to 10 Hz.

Quantum Hall effect (QHE) devices based on epitaxial graphene films grown on SiC were fabricated and studied for development of QHE resistance standard. The graphene-metal contacting area in the Hall devices has been improved and made using double metallization process. The tested devices has initial carrier concentration of 3 1011 cm-2 and showed half-integer quantum Hall effect at relatively low (4 T) magnetic field.

Two prototypes of a current comparator for operation at low frequencies in the 0.1 Hz to 1 Hz range and at room temperature have been realized. Experimental tests include the determination of the frequency-dependent sensitivity and a comparative measurement of the ratio of two standard resistors with nominal values of 1 kΩ and 100 Ω. The realized current comparator is the main ratio standard in a precision ac resistance bridge operated without need of liquid helium which will be one of the main measurement instruments for a future graphene-based quantum Hall resistance standard.

A new joint research project (JRP) integrating metrology institutes and universities from nine countries is aimed at realization of a new generation of standards for quantum resistance metrology. The project exploits graphene's properties to simplify operation of standards without compromising the unprecedented precision delivered by semiconductor quantum Hall devices. Higher operating temperatures (above 4.2 K, and up to 8 K) and together with lower magnetic fields (below 5 T, and potentially down to 2 T) will lead to a significantly improved and cost-saving dissemination of intrinsically referenced resistance standards to all end-users relying on electrical measurements.

Precision measurements of the quantum Hall resistance with alternating current (ac) in the kHz range were performed on epitaxial graphene in order to assess its suitability as a quantum standard of impedance. The quantum Hall plateaus measured with alternating current were found to be flat within one part in 107. This is much better than for plain GaAs quantum Hall devices and shows that the magnetic-flux-dependent capacitive ac losses of the graphene device are less critical. The observed frequency dependence of about -8?×?10-8/kHz is comparable in absolute value to the positive frequency dependence of plain GaAs devices, but the negative sign is attributed to stray capacitances which we believe can be minimized by a careful design of the graphene device. Further improvements thus may lead to a simpler and more user-friendly quantum standard for both resistance and impedance.