Positive seebeck coefficient in highly doped La_2-xSr_xCuO₄(x = 0.33); Its origin and implication

We present a study of the thermoelectric (Seebeck and Nernst) response in heavily overdoped, non-superconducting La1.67Sr0.33CuO4. In spite of the electron-like curvature of the Fermi surface, the Seebeck coefficient is positive at low temperatures. Such a feature, previously observed in copper, silver, gold and lithium, is caused by a non-trivial energy dependence of the scattering time. We argue that this feature implies a strong asymmetry between the lifetime of occupied and unoccupied states along the zone diagonals and such an electron-hole asymmetry impedes formation of Cooper pairs along the nodal direction in the superconducting ground state emerging at lower doping levels.