I had read some reference about this topic. I only found experiments using magnetic traps or at the first stage using a magnetic trap then transferring to an optical dipole trap. I guess the reason is when we do evaporative cooling for 87Rb atoms we also loss 40K atoms in the optical dipole trap. But if we use a magnetic trap, we can selectively evaporate 87Rb atoms.
But, is there any experiment really just using optical dipole traps for these 87Rb-40K mixture?
I think it is just as you say -- the forced evaporation with rf- or microwave in a magnetic trap is more species-selective, so all experiments that I know of start there. Typically the initial number of K atoms is much less than Rb, due to the very low abundance (even with enriched sources it is only a few percent) and the less-effective magneto-optical trapping. So it is preferred to selectively evaporate Rb and sympathetically cool K. As far as I know all Rb-K experiments start this way, with evaporation in a magnetic trap before loading into an optical trap to finish the evaporation.
In fact, relatively few optical trapping experiments (even single species) go straight from the MOT to the optical trap. There is usually a magnetic trap in between. This is because evaporation in the magnetic trap is more efficient (the trap spring constant is not reduced during evaporation), and also it is easer to match the size and shape of the MOT cloud.
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