Aharonov-Bohm effect and local density of state in quantum rings

Staff : Hermann Sellier, Thierry Ouisse (now at LMGP), Vincent Bayot (chair position RTRA), Serge Huant
PhD : Frederico Martins (now at UCL, Belgium)
Postdoc : Benoit Hackens (now at UCL, Belgium)

In a scanning gate microscopy (SGM) experiment, the metallic tip of an AFM microscope is polarized and scanned above a semiconductor nanostructure, while its conductance is measured. The tip acts as a local gate and the conductance image obtained when the tip scans above the surface is a mapping of the local gate effect on the electronic device.

Principle of scanning gate microscopy (SGM) on a quantum ring.
Principle of scanning gate microscopy on a quantum ring

Using a cryogenic AFM microscope working under magnetic field and developped in the group, we are studying quantum rings by SGM microscopy, in order to image in real space the coherent electronic system of these mesoscopic devices.

AFM microscope working at low temperature and under magnetic field.
AFM microscope working at low temperature and under magnetic field.

The quantum rings are patterned by electron lithography in a high mobility two-dimensional electron gas (2DEG) burried 25 nm under the surface of the InGaAs/InAlAs heterostructure. The rings have external diameters ranging from 600 to 1000 nm, smaller than the electron mean free path which is larger than 2 µm, and the electron coherence length is larger than the micron at 4 K. The electron transport through the ring is therefore in the ballistic and coherent regime. Magneto-conductance curves show Aharonov-Bohm (AB) oscillations that demonstrate this coherent regime.

InGaAs/InAlAs heterostructure with the electron system 25 nm under the surface.
InGaAs/InAlAs heterostructure with the electron system 25 nm under the surface.

SGM images show conductance oscillations when the tip is scanned above the ring, but also around the ring. In the first case (inside), the fringes have a radial symmetry and a typical periodicity (100 nm) much larger than the Fermi wave length (25 nm). In the second case (outside), the fringes are concentric around the ring and shift with magnetic field with the same periodicity as the AB oscillations. A detailed analysis of conductance images for different external control parameters (tip voltage, temperature, magnetic field) allowed us to show that the concentric fringes outside the ring mark isophase lines for the AB interferences controlled electrostatically and magnetically, while the oscillations inside the ring can be associated to variations in the local electron density.

Image of conductance variation under local gate effect.
Image of conductance variation under local gate effect.

SGM microscopy appears therefore as a powerful technique to investigate in real space the effects of electron coherence which govern the transport in mesoscopic nanostructures burried under the surface of semiconductor heterostructures.



Publications :

Imaging and controlling electron transport inside a quantum ring
B. Hackens, F. Martins, T. Ouisse, H. Sellier, S. Bollaert, X. Wallart, A. Cappy, J. Chevrier, V. Bayot, and S. Huant
Nature Physics 2, 826 (2006)

Imaging electron wave functions inside open quantum rings
F. Martins, B. Hackens, M.G. Pala, T. Ouisse, H. Sellier, X. Wallart, S. Bollaert, A. Cappy, J. Chevrier, V. Bayot, and S. Huant
Phys. Rev. Lett. 99, 136807 (2007)

Local density of states in mesoscopic samples from scanning gate microscopy
M.G. Pala, B. Hackens, F. Martins, H. Sellier, V. Bayot, S. Huant, and T. Ouisse
Phys. Rev. B 77, 125310 (2008)

Scanning gate microscopy of quantum rings: effects of an external magnetic field and of charged defects
M. G. Pala, S. Baltazar, F. Martins, B. Hackens, H. Sellier, T. Ouisse, V. Bayot, and S. Huant
Nanotechnology 20, 264021 (2009)

Imager les interférences quantiques dans les semiconducteurs
H. Sellier, B. Hackens, M. Pala, and S. Huant
Images de la Physique 2009, p. 49, CNRS (2010)

On the imaging of electron transport in semiconductor quantum structures by scanning-gate microscopy: successes and limitations
H Sellier, B Hackens, M G Pala, F Martins, S Baltazar, X Wallart, L Desplanque, V Bayot, and S Huant
Semicond. Sci. Technol. 26, 064008 (2011)

Collaborations :

- Université Catholique de Louvain-la-Neuve (UCL), Belgique
- IMEP, Grenoble
- IEMN, Lille

Fundings :

- ANR PNANO MICATEC (2007-2010)


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