Effects of Valley-Orbit Coupling on the Exchange Interaction in a Si/SiGe Double Quantum Dots

We investigate the effects of the phase and magnitude of the valley orbit coupling on the tunnel coupling matrix elements and the exchange interaction in a Si double quantum dot. Our results show that a difference in the phase between the two quantum dots modifies the single-electron tunnel coupling between different valley eigenstates. In particular, valley-blockade between the ground valleys of individual dots could be obtained when the valley phase difference between the dots is π. For the two-electron case, exchange interaction at the symmetric point (zero detuning) is suppressed when valley phase difference is finite, and reaches its minimum value when the phase difference is 180 degrees. The energy spectrum at finite detuning also depends on the magnitudes and phases of the valley orbit couplings of the two dots. Our results shed new lights on the controllability of Silicon based spin qubit for its application in quantum information processing.