  We analyze electron transport data through a Si/SiGe double quantum dot in
terms of spin blockade and lifetime-enhanced transport (LET), which is
transport through excited states that is enabled by long spin relaxation times.
We present a series of low-bias voltage measurements showing the sudden
appearance of a strong tail of current that we argue is an unambiguous
signature of LET appearing when the bias voltage becomes greater than the
singlet-triplet splitting for the (2,0) electron state. We present eight
independent data sets, four in the forward bias (spin-blockade) regime and four
in the reverse bias (lifetime-enhanced transport) regime, and show that all
eight data sets can be fit to one consistent set of parameters. We also perform
a detailed analysis of the reverse bias (LET) regime, using transport rate
equations that include both singlet and triplet transport channels. The model
also includes the energy dependent tunneling of electrons across the quantum
barriers, and resonant and inelastic tunneling effects. In this way, we obtain
excellent fits to the experimental data, and we obtain quantitative estimates
for the tunneling rates and transport currents throughout the reverse bias
regime. We provide a physical understanding of the different blockade regimes
and present detailed predictions for the conditions under which LET may be
observed.
