Band filling and disorder effects on the normal state thermoelectric behavior in MgB2

By a combined experimental and theoretical approach, we investigate normal state thermoelectric transport in MgB2, as a probe of selective disorder and doping in the sigma and pi bands. We calculate the temperature dependent diffusive Seebeck coefficient Sdiff(T) with the Boltzmann equation resolved in relaxation time approximation, taking into account the scattering with phonons and impurities, the effect of renormalization and the effect doping in a rigid band approximation. We show that selective disorder has a sizeable effect on the Sdiff magnitude, as it tunes the relative contributions of sigma and pi bands. Disorder also affects the Sdiff temperature dependences, eventually yielding a linear Sdiff(T) behavior in the dirty limit. We also show that band filling has opposite effects on S, depending on which band dominates transport. In parallel, we carry out Seebeck effect measurements on neutron-irradiated Mg11B2, and on two series of doped samples Mg1-xAlxB2 and Mg(B1-xCx)2. From comparison of calculated Sdiff(T) and experimental S(T) curves, we demonstrate that diffusive and phonon drag terms give comparable contributions in clean samples, but the phonon drag term is progressively suppressed with increasing disorder. In C and Al doped samples we observe very different experimental behaviors in terms of sign, magnitude and temperature dependence. Indeed, notwithstanding the similar electron doping introduced by both substitutions, C or Al doping yields disorder which mainly affects either sigma or pi bands, respectively. With the help of our ab-initio approach, we are able to disentangle the several effects and prove that Seebeck coefficient is a very sensitive probe of the kind of disorder.