30.11.2021
For the first time, graphene-based quantum Hall resistance (QHR) devices fabricated at PTB were used to realize and disseminate the ohm, unit of electrical resistance. It was verified at the highest precision level that the transition from the established material system (GaAs) to the new sample material (graphene) shows no difference within the measurement uncertainty.
Also, it was confirmed that the experimental effort to operate QH resistors made of graphene is significantly lower than for GaAs-based samples.
As part of the investigations, all devices were characterized according to the guidelines for reliable QHR measurements, established in resistance metrology. Graphene- and GaAs-based QH samples were promptly investigated in a cryomagnetic system using a high-precision resistance bridge, and the results were compared. Taking into account contributions of finite longitudinal resistivity, the quantized resistance value RK/2 was consistently realized by devices from both materials within an expanded measurement uncertainty of 1×10-9. RK is the von Klitzing constant, h/e2, where h is Planck's constant and e is the elementary charge.
The GaAs-based devices required operation at relatively high magnetic fields of typically 10.5 T to 11.0 T and low temperatures around 2.2 K. In contrast, the graphene QHR devices exhibited comparable performance already at magnetic fields below 6 T and at higher temperatures of 4.2 K. Thus, it was demonstrated that the graphene-based devices can be used as quantum resistance standards under relaxed measurement conditions as achieved with compact and economic cryogen-free table-top cryostats.
These results were made possible by recent improvements in the fabrication and preparation of graphene samples at PTB's clean room center, in particular with respect to the control of the charge carrier density. This aspect is crucial for the repeated usability of graphene-based QHR devices over longer periods of time.
The simplified quantum-based realization of the unit ohm opens up new perspectives for applications in industry and calibration laboratories.
Figure: Quantum Hall device made of graphene for use as a quantum resistance standard, electrically contacted in the sample carrier.
Department 2.6 „Electrical Quantum Metrology“