  We report on electronic transport measurements of dual-gated nano-devices of
the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all
devices the upper and lower surface states are independently tunable to the
Dirac point by the top and bottom gate electrodes. In thin devices, electric
fields are found to penetrate through the bulk, indicating finite capacitive
coupling between the surface states. A charging model allows us to use the
penetrating electric field as a measurement of the inter-surface capacitance
$C_{TI}$ and the surface state energy-density relationship $\mu$(n), which is
found to be consistent with independent ARPES measurements. At high magnetic
fields, increased field penetration through the surface states is observed,
strongly suggestive of the opening of a surface state band gap due to broken
time-reversal symmetry.

Mesoscale and Nanoscale Physics