Torque Magnetometry Studies of Two-Dimensional Electron Systems
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[S.l. : s.n.]
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RU Radboud Universiteit Nijmegen, 6 juli 2004
Promotor : Maan, J.C. Co-promotor : Christianen, P.C.M.
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Condensed Matter Science (HFML)
SubjectCorrelated Electron Systems / High Field Magnet Laboratory (HFML)
This thesis describes a study of the magnetization two-dimensional electron gases (2DEGs). To detect the typically small magnetization, a sensitive magnetometer with optical angular detection was developed. The magnetometer uses a quadrant detector to measure the rotation of the sample. By mounting a current-coil and using a feedback system, the magnetization is directly determined quantitatively. An additional advantage is the active damping of mechanical vibrations. The magnetometer has a sensitivity of 1 pJ/T in a Bitter-magnet and 0.2 pJ/T at 15 T in a superconducting magnet. First a single 2DEG is investigated. The measured magnetization can be well described by a simple model: a sharp, 1/B-periodic sawtooth with an amplitude of 1 Bohr magneton per electron. Contrary to this model, the magnetization steps have a finite width attributed to a background density of states. The density of electrons in the 2DEG can be increased far enough to occupy a second electronic subband. In contrast to the single 2DEG, the magnetization of such a 2DEG in a heterojunction shows non-1/B-periodic, triangularly shaped oscillations with a significantly smaller amplitude. A field-dependent self-consistent model quantitatively explains all three effects. At 1 K additional minima become visible at magnetic fields where Landau levels of the two subbands cross. Finally, the magnetization of bilayer 2DEGs in a double quantum well is determined. This system has an extra energy gap, determined by the inter-well barrier. Although it is purely electronic in nature, a magnetization step is clearly visible when a transition across this gap takes place. This observation is quantitatively explained by thermodynamics. Surprisingly, the apparent size of the magnetization oscillations at Landau-level transitions is significantly less than 1 Bohr magneton per electron. This reduction suggests the magnetization has an in-plane component.
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