Electronic Interactions in Semiconductor Quantum Dots and Quantum Point Contacts.

摘要

We report several detailed experiments on electron transport through Quantum Point Contacts (QPCs) and lateral Quantum Dots (QDs), created in a Single-Electron Transistor (SET). In the experiment for QPCs, we present a zero-bias peak (ZBP) in the differential conductance, G, which splits in an external magnetic field. The observed splitting closely matches the Zeeman energy and shows very little dependence on gate voltage, suggesting that the mechanism responsible for the formation of the peak involves electron spin. We also show that the mechanism that leads to the formation of the ZBP is different from the conventional Kondo effect found in QDs. [1];In the second experiment, we present transport measurements of a QD in a spin-flip cotunneling regime and a quantitative comparison of the data to the microscopic theory by Lehman and Loss. The differential conductance is measured in the presence of an in-plane Zeeman field. We focus on the ratio of the nonlinear G at bias voltages exceeding the Zeeman threshold to G for those below the threshold. The data show good quantitative agreement with the theory with no adjustable parameters. We also compare the theoretical results to the predictions of a phenomenological form used for the determination of a heterostructure g-factor and find good agreement between the two.;In the third experiment, we report the magnetic splitting, Delta K, of a Kondo peak in G for a QD while tuning the Kondo temperature, TK, along two different paths in the parameter space: varying the dot-lead coupling at a constant dot energy, and vice versa. At a high magnetic field, B, the changes of DeltaK with TK along the two paths have opposite signs, indicating that Delta K is not a universal function of TK. At low B, we observe a decrease in DeltaK with TK along both paths, in agreement with theoretical predictions. Furthermore, we find DeltaK/Delta 1 at low B and DeltaK/Delta > 1 at high B, where Delta is the Zeeman energy of the bare spin, in the same system. [2];In the last experiment, we report the zero-bias differential conductance, of an SET in the Kondo regime as a function of temperature, T, and an in-plane magnetic field B. Scaled plots of both the Tand B-dependent data show universal behavior. At moderate and high B, the magnetoconductance data show good agreement with renormalization group calculations in the spin-1/2 Kondo regime. At very low B, we observe a non-monotonic behavior, which may due to the presence of multiple orbital dot levels with similar energies. Further study is required to confirm this assumption.