LCAO model for 3D Fermi surface of high-T_c cuprate   Tl_2Ba_2CuO_{6+delta}
  A simple analytical formula for three-dimensional Fermi surface (3D FS) of ${\rm Tl_{2}Ba_{2}CuO_{6+\delta}}$ is derived in the framework of LCAO approximation spanned over Cu~4s, Cu~3d$_{x^2-y^2}$, O~2p$_x$ and O~2p$_y$ states. This analytical result can be used for fitting of experimental data for 3D FS such as polar angle magnetoresistance oscillation. The model takes into account effective copper-copper hopping amplitude \tss between Cu~4s orbitals from neighbouring \cuo layers. The acceptable correspondence with the experimental data gives a hint that the \tss amplitude dominates in formation of coherent 3D FS, and other oxygen-oxygen and copper-oxygen amplitudes are rather negligible. For absolute determination of the hopping parameters a simple electronic experiment with a field effect transistor type microstructure is suggested. The thin superconductor layer is the source-drain channel of the layered structure where an AC current is applied. 
Martin Stoev, Todor Mishonov