  Spin-orbit coupling is relatively weak for electrons in bulk silicon, but
enhanced interactions are reported in nanostructures such as the quantum dots
used for spin qubits. These interactions have been attributed to various
dissimilar interface effects, including disorder or broken crystal symmetries.
In this Letter, we use a double-quantum-dot qubit to probe these interactions
by comparing the spins of separated singlet-triplet electron pairs. We observe
both intravalley and intervalley mechanisms, each dominant for [110] and [100]
magnetic field orientations, respectively, that are consistent with a broken
crystal symmetry model. We also observe a third spin-flip mechanism caused by
tunneling between the quantum dots. This improved understanding is important
for qubit uniformity, spin control and decoherence, and two-qubit gates.
