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

Abstract

By a combined experimental and theoretical approach, we investigate normal state thermoelectric transport in MgB 2 , as a probe of selective disorder and doping in the  and  bands. We calculate the temperature dependent diffusive Seebeck coefficient S diff (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 S diff magnitude, as it tunes the relative contributions of  and  bands. Disorder also affects the S diff temperature dependences, eventually yielding a linear S diff (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 Mg 11 B 2 , and on two series of doped samples Mg 1-x Al x B 2 and Mg(B 1-x C x ) 2 . From comparison of calculated S diff (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  or  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.

Special issue on novel superconducting and magnetic materials

“…Moreover, the abundant experimental phenomenology allows for the development of novel interpretative framework. Following this direction, Pallecchi et al [15] come up with a rationale to control the Seebeck coefficient by inducing disorder or varying the electron filling of the characteristic π and σ bands of MgB 2 . Indirectly, this can provide useful insights on the strength of the electron-phonon coupling.…”