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 electron-disorder and electron-electron 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.

Three-Dimensional 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 >
Two-Dimensional 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 three-dimensional electron system. The corrugation increases by a factor of twenty with respect to the three-dimensional system and is not related to single donors anymore. The data can be qualitatively reproduced within a single-particle calculation showing that the interaction with disorder is dominant. In simple terms, the patterns reflect the tendency of the two-dimensional electron system to weakly localize. read more >
		At larger disorder the system breaks up into droplets, which show s-like and p-like 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 >
One-Dimensional Electron Systems
		One-dimensional 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 g-factors as low as 0.7 and the electron-electron interaction strength is strong with respect to disorder, we do not find any indications for Luttinger properties. read more >
Zero-Dimensional 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/dV-curves. 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 non-locality of the exchange interaction. read more >