Metal bracket to hang stringer below joist?
Those are simple L-angle joist hangers. I'm not sure if it's appropriate to use them like that, though... especially if you plan to use the stringer to support something (other than itself).

In other words, the ground-support should be directly supporting the stringer and the joists should be resting on the stringer- unless you will not be using the stringer to support anything (significant)... but then why would you put it there?

edit- After some consideration, I think that this hanging step is interesting. I wouldn't make two steps like that, of course, and I wouldn't skimp on the stringers... the picture shows 7 stringers, each about 4 ft long, for one step/tread; it will be an expensive step, but part of the safety is a measure of how far you fall... maybe 8 to 10 inches.