  Pure Topological Insulating materials preserve a unique electronic state
comprising of bulk insulating gap and conducting surface states. Here we use
bulk Bi2Se3 single crystals possessing Se vacancy defects as a prototype
topological insulator (TI) material for exploring the effect of non-magnetic
disorder on the conducting properties of TIs. We employ a sensitive,
non-contact, mutual inductance based technique for measuring the surface and
bulk contribution to electrical conductivity in the TI. We discern the
different contributions, by observing that predominant surface electrical
conduction produces linear frequency dependence of the pickup signal while bulk
conductivity gives rise to quadratic frequency dependence. We also see an
algebraic temperature dependent surface conductivity while an activated bulk
conductivity. Using the above we uncover an interplay between surface and bulk
contribution to electrical conductivity in the TI as a function of
temperatures. In the Bi2Se3 crystals the transformation from surface to bulk
dominated electrical transport is found to occur close to 70 K. This
temperature range matches well with our results from activated bulk electrical
transport results which shows an activation energy scale, delta which is in the
millieV range. The gap delta is much less than the bulk band gap in Bi2Se3, and
which we argue is associated with defect states in the TI material. To
understand our results, we propose a model of a TI comprising of an
inhomogeneous low electrically conducting medium (bulk) which is sandwiched
between thin two high electrically conducting sheets (surface). The
inhomogeneous TI state we argue is generated by Selenium vacancies defects in
Bi2Se3, which is responsible for producing an interplay between bulk and
surface conductivity.
