Electron tunneling and spin relaxation in a lateral quantum dot

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 149-158).We report measurements that use real-time charge sensing to probe a single-electron lateral quantum dot. The charge sensor is a quantum point contact (QPC) adjacent to the dot and the sensitivity is comparable to other QPC-based systems. We develop an automated feedback system to position the energies of the states in the dot with respect to the Fermi energy of the leads. We also develop a triggering system to identify electron tunneling events in real-time data. Using real-time charge sensing, we measure the rate at which an electron tunnels onto or off of the dot. In zero magnetic field, we find that these rates depend exponentially on the voltages applied to the dot. We show that this dependence is consistent with a model that assumes elastic tunneling and accounts for the changes in the energies of the states in the dot relative to the heights of the tunnel barriers. In a parallel magnetic field B the spin states are split by the Zeeman energy and we measure the ratio of the rates for tunneling into the excited and ground spin states of an empty dot. We find that the ratio decreases with increasing B. However, by adjusting the voltages on the surface gates to change the orbital configuration of the dot, we restore tunneling into the excited spin state. We also measure the spin relaxation rate W - TI-l between the Zeeman split spin states for a single electron confined in the dot. At B = 1 T we find that TI > 1 s. The dependence of W on magnetic field is a power-law, and the exponent is consistent with the prediction for the spin relaxation mechanism of spin-orbit mediated coup)ling to piezoelectric phonons. Since spin relaxation involves the orbital states of the (lot via the spin-orbit interaction, we can achieve electrical control over WI by using the surface gates to manipulate the orbital states.(cont.) We demonstrate that we can vary Wt by over an order of magnitude at fixed Zeeman splitting, and we extract the spin-orbit length, which describes the strength of the spin-orbit interaction in GaAs.by Sami Amasha.Ph.D