= Band convergence
<Band convergence> brings distinct extrema of an <electronic band structure> close enough in <energy> that several valleys contribute to <charge carrier> transport at the operating <temperature>. Increasing the effective <valley degeneracy> can improve the balance between <Seebeck coefficient> and <electrical conductivity>, rather than merely increasing the number of carriers in one valley. Composition and <semiconductor doping> can tune this <energy> separation. A primary example is the lead telluride-selenide alloy study that combined measured transport with a multiband model.<Convergence of electronic bands for high performance bulk thermoelectrics by Yanzhong Pei and others>[Abstract and pp. 66–69: composition/doping control and convergence of valence-band valleys]{ref}
The <thermoelectric power factor> is not determined by <valley degeneracy> alone. Additional inter-valley <scattering> can reduce <charge carrier mobility>, so a larger number of populated valleys need not improve transport. The spacing of the valleys in both <energy> and <wave vector>, and the relevant <electron>-<phonon> interactions, matter.https://www.nature.com/articles/s41467-021-23839-w[When band convergence is not beneficial for thermoelectrics, abstract and discussion of inter-valley scattering]{ref}
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