Delocalization by disorder in GaAs/AlGaAs heterostructures

An experimental study of quasiparticle and quasi-one dimensional properties of strongly correlated two-dimensional (2D) electron systems has been carried out. The samples were low disordered
GaAs/AlGaAs heterostructures. Measurements were performed at low temperatures, down to 24 mK, in a dilution refrigerator (MCK 50-100 TOF) equipped with a superconducting magnet.
The goal of our work was to study quasiparticle properties of 2DEG and to investigate the effect of local disorder on the conductance of the sample.
We have performed independent measurements of effective electron mass m*, transport and quantum scattering time at three different temperatures and effective g-factor. We used Shubnokov de Haas
effect , the oscillations in the longitudinal resistance in Hall effect to study the effective mass, scattering lifetimes and effective g factor. We found out that effective mass is unaffected and agrees well with
the typical value of GaAs/AlGaAs system i.e. 0.067 me, where me is the electron mass. The quantum scattering time was studied at three different temperatures, base temperature (24mK), 200 mK and
400 mK as function of electron density. We found out that the quantum scattering time at 24 mK is independent of electron density while at higher temperatures it decreases with decrease in density.
The ratio of two lifetimes τ_t and τ_q is more than 10 for all temperatures. It means that remote Coulomb centres play a dominating role in the scattering mechanisms of our sample.
More insight is needed to study the quantum scattering time at higher temperatures. The effective g-factor experiments were done with and without in-plane magnetic field. In both cases, the g-factor shows
dependence on the magnetic field.
The main purpose of this thesis work is to investigate possible breakdown of the Anderson localization in presence of local disorder. To implement the local disorder and create delocalization
we have used fine surface gates which tuned the potential barriers in the 2DEG. This was done in two different types of samples. In one sample the finger gates and top surface gate are isolated by
an insulating layer of SiO2 and in the other sample, the two gates are intercalated. The spacing in between the finger gates is determined by the mean free path of the system. The experiments are done in
absence of magnetic field. We found out that the sample with SiO2shows the effect of disorder with decrease in the resistance. But due to charge trapped in the SiO2layer, the effect was not repeated.
In the intercalated samples with two different finger gate spacings, the effect was not visible. The delocalization was not set in these sample even at high temperatures. More study is needed to prove this
effect in GaAs/AlGaAs heterostructures. A low mobility wafer can be considered as the future candidate for this experiment.