  Direct current (DC) transport and far infrared photoresponse were studied an
InAs/GaSb double quantum well with an inverted band structure. The DC transport
depends systematically upon the DC bias configuration and operating
temperature. Surprisingly, it reveals robust edge conduction despite prevalent
bulk transport in our device of macroscopic size. Under 180 GHz far infrared
illumination at oblique incidence, we measured a strong photovoltaic response.
We conclude that quantum spin Hall edge transport produces the observed
transverse photovoltages. Overall, our experimental results support a
hypothesis that the photoresponse arises from direct coupling of the incident
radiation field to edge states.
