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Magnetotransport


 

He-3/He-4 dilution fridge

He-3 cryostat

Rotating sample holder



By means of magnetotransport measurements many spin phenomena in semiconductor structures can be investigated. Often the experiments are conducted at low temperatures. Depending on the required measurement temperature different types of cryostats are used. In order to obtain high magnetic fields, often superconducting magnets are employed.

For extremely low temperatures a He-3/He-4 dilution refrigerator with a base temperature of 30 mK is available. This cryostat is equipped with a 14 T magnet. By additional cooling of the coil the maximum field can be increased up to 16 T. In a dilution fridge a mixture of He-3 and He-4 is used for cooling. At low temperatures a phase separation into a He-3 rich and He-3 diluted phase occurs. The cooling power results from the transfer of He-3 from the He-3 rich into the He-3 diluted phase.

For measurements down to 300 mK a He-3 cryostat is available, which is equipped with a 10 T Magnet. For cooling down to base temperature previously condensed He-3 is pumped by a sorption pump.

In addition to the systems mentioned above it is also possible to measure samples in a He-4 continuous flow cryostat at temperatures from 4 K up to room temperature. Here a magnetic field of 0.5 T can be applied.

Important characterization methods of two-dimensional electron gases in semiconductor heterostructures are the measurements of the quantum Hall effect and the Shubnikov-de Haas oscillations. By these measurements relevant transport properties, i.e. mobility, conductivity or carrier concentration can be extracted. In addition, Shubnikov-de Haas measurements can also be used to gain information on spin-orbit coupling in semiconductor heterostructures, an important prerequisite for the realization of the spin transistor. The magnitude of the spin-orbit coupling can be obtained by analyzing the characteristic beating pattern observed in the Shubnikov-de Haas oscillations. By employing a rotating sample holder it is possible to gain information on the Zeeman spin splitting in the semiconductor.

At extremely low temperatures and low magnetic fields interference effects can be observed. Typical examples are weak localization (negative magnetoresistance) or Aharonov-Bohm effect in ring-shaped structures. Spin related effects are the occurance of weak antilocalization (positive magnetoresistance). By analyzing weak antilocalization information on spin scattering mechanisms can be gained.

 


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Contact Imprint 12.12.2012
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