Parallel field-induced universal conductance fluctuations in open quantum dots
Open quantum dots (OQDs) are now commonly used as an experimental tool for the investigation of a particular regime of quantum transport where the electron dynamics is both ballistic and coherent. In particular, the Universal Conductance Fluctuations (UCFs), observed in ballistic quantum dots, arise from the complex quantum interferences occurring between electron trajectories that bounce multiple times against the dot walls before escaping through its leads. Central to quantum interference phenomena is the presence of a magnetic field B that breaks the time-reversal symmetry and changes the phase experienced by electrons in the dot.
OQDs are typically patterned on top of two-dimensional electron gases (2DEGs). Interestingly, when confined to wide GaAs quantum wells (QWs), 2DEGs are known to exhibit a rich physics arising from the interplay of a strong in-plane magnetic field, multiple subband occupation, and the finite thickness of the electronic wavefunction.
In this thesis, we use 2DEGs, confined to wide (WQW) and narrow (NQW) quantum wells with one and two occupied subbands at B = 0 T, respectively, to study the parallel field-induced transport in open quantum dots as a function of the well width and the tilt angle of B with respect to the electron gas. Both the WQW and NQW dots feature a rich spectrum of UCFs at intermediate tilt angles and, quite unexpectedly, under a strictly parallel B. Combined with the observation, in the case of the WQW dot, of a reduction in UCFs amplitude at large parallel B, our data indicates that the finite thickness of the electron layer and the orbital effect are responsible for the in-plane field-induced UCFs.
In the second part of this work, we observe a saturation of the UCFs spectral distribution, expressed in terms of an effective tilt angle, as B approaches a strictly parallel configuration, along with the persistence of a limited number of frequency components in the case of the narrow quantum well dot. It is found that the saturation angle strongly depends on the width of the 2DEG confining well. Using the results of self-consistent Poisson-Schrödinger simulations, the magnetoconductance is rescaled as a function of the Fermi level E_F in the 2DEG. A power spectrum analysis of the parallel B UCFs in energy space and its good agreement with theoretical predictions suggest that such a B to E_F mapping is indeed relevant for the interpretation of parallel B-induced UCFs
School:Université catholique de Louvain
Source Type:Master's Thesis
Keywords:quantum transport dots magnetoconductance parallel field ballistic devices orbital effect nanotechnology phase coherence poisson schrödinger simulations
Date of Publication:03/15/2005