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Local probe of electronic states in high mobility quantum Hall samples Samani Nasab, Mohammad


The discovery of the integer quantum Hall effect (IQHE) and the fractional quantum Hall effect (FQHE) in a 2-dimensional electron gas (2 DEG ) have created a new and rich field in condensed matter physics in low dimensions. Almost 35 years after these discoveries, there are still several unanswered questions regarding the nature of various electronic phases formed in such systems. The 2 DEG in ultra-high mobility quantum well (QW) samples in large magnetic fields and millikelvin temperatures are studied in this thesis. We developed a reproducible recipe for enhancing the quality of the very fragile FQHE states reliably, which can be used to reset an electrically shocked sample in-situ at low temperatures. We then developed a protocol for measuring the local electronic density on QW samples using a single-electron transistor (SET). We also developed a technique for modulating the temperature of the sample at about 10Hz by about 10mK. We used the electrometer and the fast temperature modulator to obtain a measure of changes in chemical potential as a function of temperature oscillations. This quantity can reveal the existence of an enhanced entropy in the state of the electrons. We investigated theories that predict the non-Abelian state of matter, that follows neither Fermionic nor Bosonic statistics. Non-Abelian quasi-particles are expected to form as collective excitations in the fractional quantum Hall regime at filling factor ν=5/2. The experimental results were incompatible with the non-Abelian theory under investigation. We also studied the nature of localization of electrons in the bulk of the sample when the system is in one of the incompressible states near an IQHE plateau. The noise characteristics detected by the ultra-sensitive charge sensor implanted on the surface of the sample revealed new behaviours not observed in the past. The results can be explained by telegraph noise arising from charge carriers jumping from one localized potential pocket to another, evolving into 1/f noise, as the filling factor shifts away from the centre of the integer state.

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