Large electronic-density increase on cooling a layered metal: Doped Bi2Te3.

We have carried out optical and transport measurements of the free carriers and optical measurements of states above the semiconducting energy gap in the layered crystal ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$. We find that cooling causes a substantial amount of spectral weight to condense from the above-gap states into the metallic states.