| Mesoscopic electron transport in semiconductor quantum dots is interesting from both a technological and purely physical perspective. They represent a new paradigm in device physics brought about by miniaturization. While the technological applications are still in their infancy, much work has been done to explore the physics behind the device. The semiconductor quantum dot can be fabricated and measured in many ways, and can be described as many things—from a tiny transistor or laser to an artificial atom. Here we present a specific description of a quantum dot as an open ballistic conductor, or waveguide for electrons, with a well-defined coupling to the environment through the leads.; The device studied for this work is a 0.8 μm square, shallow etched, ballistic quantum dot with a self-aligned Schottky gate. The gate acts to change both the size of the dot, and more importantly, the width of the quantum point contact leads, which couple the dot to the current reservoirs. By varying the magnetic field, we investigate the magnetoconductance fluctuations which characterize the density of states of the dot and its energy spectrum. Furthermore, by varying the gate voltage, a three-dimensional spectroscopic conductance image of the dot can be obtained.; Measurements of the dot at temperatures ranging from 4 K down to 10 mK indicate a saturation of the increase in the estimated phase coherence time with decreasing temperature. The nature of this saturation is explained by considering the finite lifetime broadening of the available spectrum in the dot. Opening the leads increases this broadening, and a subsequently lower phase coherence time is measured. Further evidence of the lead-induced broadening is found in the conductance spectral images. These images are essentially smeared versions of the expected zero-temperature result. Based on simulations, it is shown that the observed smearing is due to thermal and lead-induced broadening of the dot level spectrum.; Many studies of ballistic quantum dots have focused on the universal or generic aspects of the fluctuations in a semiclassical way, ignoring the inherently unique quantum nature of each dot in its environment. Experimental spectroscopic conductance images provide another useful tool for understanding the nature of fluctuations in ballistic quantum dots. These images appear similar to the energy level spectrum of a closed dot, but are fundamentally different due to the environmental coupling and the role of the leads in selecting states for transport. |
