Scanning Tunneling Spectroscopy on Semiconductors
Interacting electron systems in different dimensions
We use the simple, isotropic and largely parabolic conduction band of InAs to study the influence of electrondisorder and electronelectron interactions on the appearance of the local density of states. Dimensionality and magnetic field are systematically varied, while electron density and the disorder potential are independently determined which gives full access to the input parameters of the Schrödinger equation. Since, on the other hand, the local density of states is directly linked to the output of the Schrödinger equation, we get access to the fascinating quantum world of interacting electrons.
ThreeDimensional Electron Systems 

At B=0 T, we find simple Bloch states which are scattered at ionized dopants. The atomic structure of the Bloch states can be reproduced by a calculation within the local density approxiamtion (FLAPW). The long range part of the scattering states is reproduced within the WKB model. read more > 

In magnetic field, in particular in the extreme quantum limit, we see a transformation into drift states which is not complete up to B=6 T. It is accompanied by the development of a quadratic Coulomb gap at the Fermi level. read more >  


TwoDimensional Electron Systems 

At B=0 T and relatively low disorder, we find a much more complicated and much more strong standing wave pattern than in the threedimensional electron system. The corrugation increases by a factor of twenty with respect to the threedimensional system and is not related to single donors anymore. The data can be qualitatively reproduced within a singleparticle calculation showing that the interaction with disorder is dominant. In simple terms, the patterns reflect the tendency of the twodimensional electron system to weakly localize. read more >  
At larger disorder the system breaks up into droplets, which show slike and plike quantum dot states. Percolation at higher energy is observed. read more >  
In magnetic field, drift states are formed at low disorder. As expected they run along equipotential lines of the sample. These states are clearly localized at the edge of the Landau levels. The particularly interesting extended state in the center of the Landau level has been measured for InSb. read more >  
The preparation of a 2DES appropriate for STS measurements is described read more >  


OneDimensional Electron Systems 

Onedimensional systems containing one or two subbands have been found below charged step edges. Their local density of states shows nearly 100 % corrugation pointing to weakly localized states. Alignement with the disorder potential is directly observed. Although the system exhibit gfactors as low as 0.7 and the electronelectron interaction strength is strong with respect to disorder, we do not find any indications for Luttinger properties. read more > 



ZeroDimensional electron systems 

Quantum dots are induced by using the tip as a local gate with respect to the sample. Quantized states are observed as peaks in dI/dVcurves. Since the quantum dot can be moved with the tip impurities can be palced into the quantum dot and the response of the energy spectrum on the disorder is probed. In magnetic fields the states are identified as spin polarized Landau states. Their interaction with impurities, in particular the response of the spin splitting to the disorder indicates nicely visualizes the nonlocality of the exchange interaction. read more >  