Answer Happy

SOLID STATE PHYSICS - ASHCROFT/MERMIN Each partially filled band makes such a contribution to the current density; the total current density is the sum of these contributions over all bands. From (13.22) and (13.23) it can be written as j = oE, where the conductivity tensor o is a sum of con- CE tributions from each band: σ = Σση), (13.24) n ت % ) در جاده اهر - dk olm e2 Senat - » e.com (E,(k))v,(k),(k) (13.25) E=E/) 2. Deduce from (13.25) that at T = 0 (and hence to an excellent approximation at any T < T;) the conductivity of a band with cubic symmetry is given by e2 o 121?h T(E)US, (13.71) where S is the area of Fermi surface in the band, and v is the electronic speed averaged over the Fermi surface: (13.72) ſas pras). (Note that this contains, as a special case, the fact that filled or empty bands (neither of which have any Fermi surface) carry no current. It also provides an alternative way of viewing the fact that almost empty (few electrons) and almost filled (few holes) bands have low conductivity, since they will have very small amounts of Fermi surface.) Verify that (13.71) reduces to the Drude result in the free electron limit.