Browsing by Author "Dahlem, Franck"

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Adiabatic transport in the quantum Hall regime : comparison between transport and scanning force microscopy investigations
(2008) Dahlem, Franck; Von Klitzing, Klaus
In this PhD work, the local potential distribution has been measured in high mobility 2DES under quantum Hall conditions. The 2DES embedded in a GaAs-AlGaAs heterostructure designed in a small Hall bar geometry shows intrinsic adiabatic transport features. Usually presented in the literature with the edge state picture, these features are the disappearance of peaks in the Shubnikov-de Haas oscillations, the extension of quantum Hall plateaus to lower magnetic fields and the existence of non-local resistances. Our local potential measurement via cryogenic scanning force microscopy presents another microscopic explanation of such adiabatic transport. The new picture is based on compressible and incompressible strips. An incompressible strip is a region in which the Fermi energy is located inside the energy gap (the electron density is constant and the electrostatic potential is changing) whereas a compressible strip occurs if the Fermi energy is pinning inside a Landau level (the electron density is changing and the electrostatic potential is screening). In previous work, the compressible and incompressible strips model has been successfully used to describe the quantum Hall effect. The present work demonstrates that the strips distribution accounts also for the adiabatic transport features observed on high mobility samples in the quantum Hall regime. Our research shows that in adiabatic situations, compressible regions with an unusual difference of electrochemical potential are found to coexist along the same edge due to an insulator-like incompressible strip in between and due to the lack of impurities scattering. Due to the high mobility and small size of the Hall bar, such non equilibrium survives along the complete length of the sample and determines the transport features. The insulator properties of incompressible strips in front of the alloyed ohmic contacts are found to be anisotropic with a dependency on the orientation of the contact borderline with respect to the crystal direction. The incompressible strips are broader -so more insulating- if they are located close to contact with an interface perpendicular to the [01-1] direction than if they are in front of contact with an interface parallel to the [01-1] direction. This finding gives a physical meaning to the term "non ideal contact" in the case of low resistive and ohmic contacts. Finally our results advertise that every 2DES is inhomogeneous. A 2DES is never a flat distribution of electron but it owns border with gradient of electron density even in front of metal contacts. These "Regular inhomogeneities" at the edges of the mesa and in front of contacts determines the insulator properties of the incompressible strips in high magnetic field and therefore the transport.