Interband Transitions and Band Gap Measurements in Bloch Transistors

We have measured the switching current from the supercurrent branch of Bloch (superconducting single electron) transistors. Switching events occurring from the ground and first excited bands can be distinguished, allowing the band index of the system to be determined, thus giving the first direct evidence of the predicted excited bands. Both thermally assisted and microwave induced transitions have been observed. The latter provide a direct measurement of the band gap, which agrees well with theoretical predictions using independent estimates of the capacitance.