Electron spin relaxation by hyperfine interaction in a double quantum dot

We use a pulsed-gate technique to measure singlet-triplet relaxation in a GaAs/AlGaAs few-electron double quantum dot at low magnetic field. Electrostatic pulses are applied to probe the time dynamics of the (1,1) to (0,2) charge state transition, while average dot occupation is measured by nearby quantum point contact charge sensors. In the (0,2) configuration only a spin singlet is allowed, blocking the transition from (1,1) if a triplet state is initially formed, but relaxation is strongly enhanced near zero magnetic field. We attribute this enhancement to different nuclear environments in each dot, and extract an average effective Overhauser field of ~3 mT. This implies a spin dephasing time of ~30 ns in this system.