Aharonov-Bohm Oscillations in Semiconductor Quantum Rings

Quantum-interference of electrons confined to a small ring has fascinated physicists for a long time, because it is directly related to the quantum-mechanical phase of electrons. One famous example is the so-called Aharonov-Bohm effect, originating from the periodic dependence of the electron phase on the magnetic flux through the ring, leading to oscillatory persistent ring-currents. Until present experimental evidence of Aharonov-Bohm oscillations was found in metallic and semiconducting rings in the mesoscopic regime, containing many electrons. Now an international research team from the Nijmegen High Field Magnet Laboratory (the Netherlands), the Eindhoven University of Technology (the Netherlands), the University of Antwerp (Belgium), the University of Moldova (Moldova) and the the Institute of Microelectronics in Madrid (Spain) has succeeded to detect oscillatory currents carried by single electron states in a semiconductor quantum ring. These findings were published in the journal Physical Review Letters.

The persistent ring-currents are measured by detecting the magnetic moments of electrons using an ultrasensitve magnetometer (Fig. A). The electrons are confined to 23 nm diameter Indium Arsenide quantum rings, embedded in a Gallium Arsenide host (Fig. B). By optimizing the growth parameters a sample with a highly homogeneous ensemble of nearly-identical quantum rings was prepared, each containing on average 1.5 electrons. The magnetization signal exhibits a clear Aharonov-Bohm oscillation around 14 Tesla (Fig. C). Surprisingly, this magnetic field is much larger than that expected for an ideal ring of 23 nm diameter (5 Tesla). Detailed characterization by cross-sectional scanning tunneling microscopy (X-STM) reveals however that the self-assembled quantum rings more resemble nano-volcanoes than ideal rings (Fig. B). Inserting the realistic quantum-ring potential (inset Fig. D) in a theoretical model calculation the remarkable shift of the first Aharonov-Bohm oscillation from 5 to 14 Tesla is explained (Fig. D).

These results demonstrate the possibility to design and fabricate nonmagnetic semiconductors with magnetic properties, which can be controlled by tuning the size and shape of self-assembled nanostructures.

Figure: A - Schematic lay-out of the ultrasensitive torsional magnetometer with optical readout. B - Characterization of the quantum rings using cross-sectional scanning tunneling microscopy (X-STM). C - Measured magnetization curves of an ensemble of identical quantum rings. D - Calculated magnetization curves using a realistic quantum ring potential (inset) based on the X-STM characterization.

This work was published in:

N.A.J.M. Kleemans, I.M.A. Bominaar-Silkens, V.M. Fomin, V.N. Gladilin, D. Granados, A.G. Taboada, J.M. García, P. Offermans, U. Zeitler, P.C.M. Christianen, J.C. Maan, J.T. Devreese and P.M. Koenraad,
Oscillatory Persistent Currents in Self-Assembled Quantum Rings
Physical Review Letters 99, 146808 (2007)