  We present angle resolved photoemission experiments and scanning tunneling
spectroscopy results on the doped topological insulator Cu0.2Bi2Te3.
Quasi-particle interference (QPI) measurements, based on high resolution
conductance maps of the local density of states show that there are three
distinct energy windows for quasi-particle scattering. Using a model
Hamiltonian for this system two new scattering channels are identified: the
first between the surface states and the conduction band and the second between
conduction band states. We also observe that the real space density modulation
has a predominant three-fold symmetry, which rules out a simple, isotropic
impurity potential. We obtain agreement between experiment and theory by
considering a modified scattering potential that is consistent with having
mostly Bi-Te anti-site defects as scatterers.
