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By a combined experimental and theoretical approach, we investigate normalstate thermoelectric transport in MgB2, as a probe of selective disorder anddoping in the sigma and pi bands. We calculate the temperature dependentdiffusive Seebeck coefficient Sdiff(T) with the Boltzmann equation resolved inrelaxation time approximation, taking into account the scattering with phononsand impurities, the effect of renormalization and the effect doping in a rigidband approximation. We show that selective disorder has a sizeable effect onthe Sdiff magnitude, as it tunes the relative contributions of sigma and pibands. Disorder also affects the Sdiff temperature dependences, eventuallyyielding a linear Sdiff(T) behavior in the dirty limit. We also show that bandfilling has opposite effects on S, depending on which band dominates transport.In parallel, we carry out Seebeck effect measurements on neutron-irradiatedMg11B2, and on two series of doped samples Mg1-xAlxB2 and Mg(B1-xCx)2. Fromcomparison of calculated Sdiff(T) and experimental S(T) curves, we demonstratethat diffusive and phonon drag terms give comparable contributions in cleansamples, but the phonon drag term is progressively suppressed with increasingdisorder. In C and Al doped samples we observe very different experimentalbehaviors in terms of sign, magnitude and temperature dependence. Indeed,notwithstanding the similar electron doping introduced by both substitutions, Cor Al doping yields disorder which mainly affects either sigma or pi bands,respectively. With the help of our ab-initio approach, we are able todisentangle the several effects and prove that Seebeck coefficient is a verysensitive probe of the kind of disorder.

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