G. Brunthaler

Low-density spin susceptibility and effective mass of mobile electrons in Si inversion layers.

We studied the Shubnikov-de Haas (SdH) oscillations in high-mobility Si-MOS samples over a wide range of carrier densities n approximately (1-50)x10(11) cm(-2), which includes the vicinity of the apparent metal-insulator transition in two dimensions (2D MIT). Using a novel technique of measuring the SdH oscillations in superimposed and independently controlled parallel and perpendicular magnetic fields, we determined the spin susceptibility chi(*), the effective mass m(*), and the g(*) factor for mobile electrons. These quantities increase gradually with decreasing density; near the 2D MIT, we observed enhancement of chi(*) by a factor of approximately 4.7.

Instability of the two-dimensional metallic phase to a parallel magnetic field

Magnetotransport studies of the unusual two-dimensional metallic phase in high-mobility Si-MOS structures are reported. It is found that a magnetic field applied in the 2D plane suppresses the metallic state, causing the resistivity to increase dramatically (by more than 30 times). The total existence range of the metallic state is found to contain three distinct types of magnetoresistance, related to the corresponding quantum corrections to the conductivity. The data suggest that the unusual metallic state is a consequence of both spin and Coulomb interaction effects.

Metal–insulator transition in two dimensions

The metal-insulator transition has been unexpectedly discovered in Si-MOS field effect transistors and has been verified in many other semiconducting materials afterwards. The transition caused much attention as it is in apparent contradiction to the scaling behavior of non-interacting two-dimensional electrons. This work gives an overview about recent experimental work and theoretical models on the metal-insulator transition and the metallic state in two dimensions.

Weak anisotropy and disorder dependence of the in-plane magnetoresistance in high-mobility (100) Si-inversion layers.

We report studies of the magnetoresistance (MR) in a two-dimensional electron system in (100) Si-inversion layers, for perpendicular and parallel orientations of the current with respect to the magnetic field in the 2D plane. The magnetoresistance is almost isotropic; this result does not support the suggestion of its orbital origin. In the hopping regime, however, the MR contains a weak anisotropic component that is nonmonotonic in the magnetic field. We found that the field, at which the MR saturates, varies for different samples by a factor of 2 at a given carrier density. Therefore, the saturation of the MR cannot be identified with the complete spin polarization of free carriers.

Lateral quantum dots in Si∕SiGe realized by a Schottky split-gate technique

Lateral quantum dots are formed in the two-dimensional electron gases of a high-mobility Si∕SiGe heterostructures by means of split Schottky gates. Palladium gates, defined by e-beam lithography and lift-off, show Schottky barriers with very well controlled leakage currents. At low temperatures we observe Coulomb-blockade and stability diamonds on lateral quantum dots containing a total charge of about 25 electrons. The experiments demonstrate that, in contrast to recent reports, Schottky gates are a feasible approach for the fabrication and integration of single electron transistors in the strained Si∕SiGe heterosystem.

Lack of universal one-parameter scaling in the two- dimensional metallic regime

The two-dimensional metallic state is studied in a number of Si-MOS structures with peak mobilities varying by a factor of 8.5. The data show a density dependence and disorder dependence of the major features of the scaling function and thus reveal the absence of universal one-parameter scaling over wide density range in the metallic regime.We show that the two-dimensional metallic state in Si-MOS samples persists over a wide range of temperatures (16 mK to 8 K), sample peak mobilities (varying by a factor of 8), carrier densities (0.8 to

Exclusion of Quantum Coherence as the Origin of the 2D Metallic State in High-Mobility Silicon Inversion Layers

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Molecular beam epitaxial growth and photoluminescence investigation of Si1-yCy layers

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Shubnikov-de Haas oscillations and negative magnetoresistance under hot-electron conditions in Si/SiGe heterostructures

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Breakdown of the anomalous two-dimensional metallic phase in a parallel magnetic field

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