Peraticos, Elias; (2021) Transport and interaction effects in low-dimensional semiconductor heterostructures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The work in this thesis is related to the strongly correlated electron interaction effects taking place within 2D and 1D electron systems in IIIV semiconductor devices. Initially the work conducted on GaAs/AlGaAs was to investigate the effects of incompressible/compressible strips on the transverse and longitudinal resistance. Effects due to these strips lead to resistance anomalies in the transverse resistance, when the incompressible strips are in the evanescent regime, as described by the screening theory. In the study described in this thesis, such anomalies are observed not just for integer states, but for fractional states as well, i.e v = 4/3, 3/2, 5/3, 8/3, 3, 10/3, 7/2 and 5, which have been predicted theoretically but not studied experimentally. Additionally, longitudinal resistance hysteresis was noticed which increases in size with lower-valued v and by increasing the constriction of the quasi-1D channel within the system. The relaxation times of the longitudinal resistance within these hysteretic areas were found to be linked to two mechanisms with τ_1 and τ_2 being in the order of a 102 s and 106 s. These hysteretic loops were discussed in terms of dynamic nuclear polarisation and Ising ferromagnetism and the screening theory. It is discussed that the latter provides a better t in the explanation of the hysteresis. Further studies were conducted using the GaAs/AlGaAs device, but instead the transverse voltage was measured while setting up the system in measuring the quasi-1D conductance. By varying the Magnetic fi eld it was found that various oscillations were measured indicating unusual features. These oscillations seem to be linked to the 's in the 2DEG regions and the change in the peaks height and position are linked to the constriction size within the quasi-1D channel. These effects seem to be related to the crossings of Landau levels. In addition when spin-polarisation is enhanced a set of peaks increase in size compared to others, which is why it is thought that they are linked to enhanced spin polarisation within the constriction. By applying source-drain bias it was found that peaks that seem to be linked to even fi lling factors tend to disappear with positive voltage bias but for negative voltage bias, peaks related to both odd and even filling factors seem to persist. This is explained by scattering being induced due to quasi-elastic inter-Landau-level scattering as well as through the spin gap model. The data from these two chapters will provide a better understanding on the physical phenomena taking place and how the Landau levels and electron-electron interactions affect the behaviour of the systems. Furthermore the oscillations observed at 3.25 T are thought that they could be linked to the Aharonov-Bohm effect. Finally an InGaAs/InAlAs device was used to study the interaction effects due to perpendicular magnetic field and spin-orbit interactions within a quasi-1D channel. While applying lateral voltage bias within the quasi-1D channel, the asymmetric voltage on the split-gates acts as a type of electric Stern-Gerlach apparatus inducing spin-splitting as well as Rashba spin-orbit-coupling (RSOC). As a consequence exotic phases occur which lead to fractional conductance states appearing within the system, which are enhanced by increasing the magnetic fi eld. Such states are the 5/2 and 12/5 fractional states which if proven to be non-Abelian can help in the creation of topological fault tolerant quantum computers. These fractional conductance states are thought to be a consequence of backscattering and umklapp scattering. Also some of the fractional conductance states noticed have been observed experimentally in other 2D systems, and the fact that fractional states like 3/2, 5/2 and 7/2 weaken and strengthen with certain perpendicular magnetic fields and asymmetric voltages within the system are explained by the RSOC inducing in-plane magnetic fi eld components in the system leading to similar behaviour observed in 2D systems in tilted B-fi eld setups. In the InGaAs/InAlAs system though more fractional states appear in higher sub-band levels, compared to the measurements conducted in other two-dimensional systems, which is thought to be due to the intrinsically stronger spin-orbit coupling InGaAs/InAlAs systems have. These fractional states would be highly valuable for spintronic devices and the construction of quantum computers by utilising lower perpendicular magnetic fi elds than what is required in conventional 2D systems.

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