The quantum Hall effect has been the basis of resistance metrology for a number of years. Metrology laboratories around the world use it routinely as a practical standard, although the ultimate knowledge of resistance continues to be based on a calculable capacitor. Quantum Hall devices produce very precise values of resistance of the order 10 parts per billion.
Even though the quantum Hall effect was discovered more than two decades ago, there has not been a focused effort to produce optimized quantum Hall devices for metrology. The worldwide lack of a fabrication effort aimed specifically at quantum Hall devices has resulted in a lack of technological progress similar to major advances with the Josephson array voltage standard over the same period.
The Quantum Devices Group is therefore initiating a program to remedy these limitations.
With the support of the EEEL a program was initiated in 2001. The program will consist of two staff members and the capability of fabricating two-dimensional electron gas (2DEG) samples using a dedicated molecular beam epitaxy (MBE) machine, followed by characterizing them at appropriate temperatures and magnetic fields.
The approach to the program will range from relatively straightforward tasks to the development of new fundamental knowledge that will enhance future electrical metrology. Following that order, the following are the objectives we foresee at the present time:
Fabrication of optimized conventional quantum Hall samples – We will undertake a collaborative program with EEEL’s Fundamental Electrical Measurements Group to produce optimized quantum Hall samples for that Division’s use in resistance calibrations. Using an iterative approach we will optimize the samples for this application. We hope to reduce occasional unintended noise, to increase the operating temperature, and to reduce the required magnetic field strength. Ultimately we hope to produce metrology-quality samples that operate at 4.2 kelvin in magnetic fields of one tesla or less. At this level the systems would be inexpensive enough that they could be used by many more laboratories than can afford quantum Hall resistance standards at present. Series arrays will be particularly important in EEEL’s pursuit of closing the metrology triangle (described elsewhere in this publication.)
Quantum Hall integrated circuits – We will fabricate quantum Hall integrated circuits for use in metrology or physics studies. Inspired by the report of a French group at the 2002 Conference on Precision Electromagnetic Measurements, we will produce integrated circuits of quantum Hall devices. That group fabricated a parallel/series array of devices to enable lower or higher resistance than the standard resistance of h/e2 = 25812.807 ohms. Their significant accomplishment was to reduce the contact resistance between samples to a low enough value that the overall resistance was accurate to better than 100 parts per billion. In addition we will explore the possibility of combined quantum Hall and superconducting circuits that might permit miniature, or even on-chip, magnetic field generation to eliminate the need for a bulky external magnet, further reducing the cost of the overall system. If this field could be confined to a small region of the chip, it might be possible to fabricate a Josephson array voltage standard on the same chip as the quantum Hall device, creating a monolithic quantum multimeter for the first time. The device would achieve a voltage and resistance having quantum accuracy, with the current derived as the ratio of the two.
New quantum Hall physics for metrology – Experiments performed over the last few years at the California Institute of Technology (Caltech) by Professor James P. Eisenstein, his students and collaborators demonstrate new physics having potential importance to metrology. In these experiments quantum Hall samples are fabricated with two 2DEGs spaced by less than 100 nm to permit tunneling between the layers. These remarkable experiments demonstrate the quantum Hall effect with a filling factor of 1/2 in each layer, which is not observed in a single layer. Moreover with a current through only one of the layers, a voltage is generated in the other layer that is proportional to the product of the current and the quantum Hall resistance in the first layer. While the metrological utility of the second layer is yet to be explored, this effect suggests opportunities important to practical quantum Hall metrology.
Taking advantage of the refocusing of another program in EEEL, we have acquired an MBE system of suitable cleanliness to fabricate conventional quantum Hall samples. Based on the experience of researchers at Bell Laboratories, it would also be capable, with modification, of producing very high mobility samples similar to those presently used in the Caltech bilayer experiments. We are also searching for two world-class staff members to initiate and carry out this work.
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| Molecular beam epitaxy machine prior to shipment to Quantum Devices Group. |
Since this program is still being initiated at NIST, the relevant background is in publications from the work at Caltech. Two are listed below:
M.Kellogg, J.P. Eisenstein, L.N. Pfeiffer and K.W. West, “Double layer two-dimensional electron systems: probing the transition from weak to strong coupling with Coulomb drag,” cond-mat/0211502 (2002).
M.Kellogg, I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer and K.W. West, “Observation of quantized Hall drag in a strongly correlated bilayer electron system,” Phys. Rev. Lett. vol. 88, pp. 126804 (2002).