Profile
The main task of the working group is the realisation of the capacitance unit farad, based on the quantised Hall resistance. This resistance is realised in so-called GaAs heterostructures and serves as a highly precise and stable fixed point whose numerical value depends only on fundamental constants. This quantised resistance is the base not only of the dc resistance unit ohm (Working Group 2.61), but also of the capacitance unit farad. For this purpose, the quantised Hall resistance measured with alternating current serves as the starting point of a measuring chain of coaxial bridges with the capacitance standards under calibration at the end. This method has been developed at PTB and allows an unexcelled relative uncertainty of 12·10 -9 (k = 2) for a 10 pF capacitance standard.
Beyond these lawful tasks, the working group aims at the optimisation of the measuring bridges and the ac properties of the GaAs heterostructures. Furthermore, the quantum Hall effect of graphene is investigated at dc and ac.
Research/Development
The R&D activities of the working group comprise
Optimisation and further development of coaxial measuring bridges
Optimisation of GaAs heterostructures for ac measurements in the frequency range of (1-5) kHz together with department 2.53 low-dimensional electron systems
Investigation of the quantum Hall resistance of graphene with dc and ac at frequencies up to 10 kHz. Aim is a future, more user-friendly resistance and impedance standard. This work is carried out together with department 2.53 low-dimensional electron systems in the frame of the European Metrology Research Programme „GraphOhm“.
Services
Our working group does not provide calibration services, but calibrates the secondary standards of department 2.1 Direct Current and Low Frequency which in turn carries out the customer calibrations. The nominal values of common secondary standards of capacitance are 10 pF and 100 pF. Furthermore, we calibrate the 10:1 ratio of inductive voltage dividers. International agreement of the unit is ensured by intercomparisons. Information on our calibration capabilities can be found in our database.
Information
The tasks of the working group are based on the quantum Hall resistance and allow the precise realisation of the capacitance unit farad. To get more information on these topics, please click at the particular link.